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Monday, December 30, 2013

Emulator Version 0.18 Released

Nigel and I are pleased to announce that version 0.18 of the retro-B5500 emulator has been released. All changes have been posted to the Subversion repository for our Google Code project GitHub project. The hosting site has also been updated with this release, and for those of you running your own web server, a zip file of the release can be downloaded from Google Drive.

This is a bug-fix release. The only significant new feature is the ability to dump the system processor and memory state to a window in the B5500Console and B5500SyllableDebugger interfaces, as described below.

Processor Bugs Fixed

The following corrections were made in B5500Processor:
  1. The TRN (Transfer Numerics) syllable was not clearing zone bits in the destination character string as it should have. In addition, it was not always setting the True-False Flip Flop (TFFF, also known as MSFF) properly.
  2. The TRW (Transfer Words) syllable was incorrectly throwing an Invalid Address interrupt when the source string ended at an address which did not have a valid next address (e.g., the highest memory address, @77777).
  3. A call on cc.fieldIsolate() in the ISO (Field Isolate) syllable had an extraneous parameter. Thanks to Peter Grootswagers for identifying this.

More Corrections for Tape Drive Oscillation

We thought we had all of the issues with tape drives oscillating between rewind and ready states when tape was at BOT resolved in release 0.17, but alas, that proved not to be the case. Part of the problem was that the I/O Unit did not have tape writes implemented yet, but the rewind and interrogate functions are degenerate forms of tape write. As a result, result descriptors for Mod III I/O Units were not being properly constructed in some cases.

We also discovered, under Sid McHarg's advice, that the MCP is capable of detecting Mod III I/O Units automatically, but the problems mentioned above with the way that result descriptors were being constructed prevented that. With these fixes in place, that automatic detection now works properly, and the "SO USE OPTN 2" command described in the blog post for the 0.17 release is no longer necessary.

Dumping the System State

We have not had a good way to look at the complete system state up to now. The SyllableDebugger script provides some means to do this, but it is awkward to use when having to look at many widely-dispersed memory locations, and it's of no use at all when running the standard B5500Console user interface.

With this release, you can now dump the complete system state. In the SyllableDebugger, there is a Dump button near the top of the page. In the B5500Console interface, you can click the MEMORY CHECK indicator. Both methods will open a separate window and format both the processor state and the contents of core memory in that window. You can examine the dump in the window, save the window's contents, or copy/paste the contents into another application (such as a text editor) for further analysis. When you are finished with the dump, simply close its window.

These dumps can be requested any time the emulator is in a powered-on state. You can generate multiple dumps and have multiple dump windows open at a time. The emulator will pause while the dump is taking place and then resume operation. Thus, you can easily take a snapshot of the system state while the MCP is running, and then continue.

Further Work

We are continuing to see occasional flag-bit and invalid-address errors when running the MCP under heavy load. At present, we are working under the assumption that these are due to emulator bugs, and are pursuing them as a priority. The system state-dump feature mentioned above was implemented to facilitate this.

The next major feature scheduled for implementation is writing to tape, along with providing a means to persist tape volume data outside the emulator environment. After that we are considering implementing a more dynamic and user-friendly mechanism for controlling the system configuration, and implementing (finally) the double-precision numeric syllables.

Monday, December 16, 2013

Emulator Version 0.17 Released

Nigel and I are pleased to announce that version 0.17 of the retro-B5500 emulator has been released. All changes have been posted to the Subversion repository for our Google Code project GitHub project. The hosting site has also been updated with this release, and for those of you running your own web server, a zip file of the release can be downloaded from Google Drive.

This release contains one really big thing and a number of smaller things.

P2 Lives!

The really big thing is that the second processor, P2, is now functional. Getting this to work is especially gratifying to me, as it is an important feature of the B5000/B5500 and I have wanted it to work from the beginning. It has been very frustrating trying to get this going, as there are only a few places in the processor design where it needs to know whether it's P1 or P2, and it should have just worked. It didn't.

I had made several serious runs at this problem over the past six months, but made very little progress until one morning a few weeks ago when I awoke early. Lying in bed, I started thinking about an intermittent problem we have seen with the system under load and how to trap it. In the middle of those thoughts, it suddenly came to me why P2 wasn't working. It wasn't so much something to do with the design of the emulator's Processor object as with the way we interleave the activities for all of the processors, I/O units, peripheral devices, and console display on one Javascript thread. Without going into a whole lot of details, the way that interleaving was being done interfered with the way we need to start and stop P2. Changing less than ten lines of code fixed the problem.

The processors on a B5500 are physically identical, with a manual switch in Central Control determining which one is P1 (the control processor) and which is P2. Unlike the B6500/B6700 that followed it (and most multiple-processor systems today), the processors on the B5500 do not operate symmetrically. P1 is the control processor. Only P1 can run in Control State, service interrupts, and initiate I/Os. Much of the MCP runs only on P1.

P1 can also run in Normal State, which is used for user tasks. P1 enters Normal State during an Initiate P1 (IP1) syllable, which effectively assigns the processor to a user task by automatically loading state from a few words in the task's stack. P1 reverts to Control State when any interrupt occurs, automatically storing the task's state in the stack and PRT in the form required by IP1 for reactivation.

P2, on the other hand, can run only in Normal State, and thus can be used only to run user tasks. The MCP (running in Control State in P1) assigns P2 to a user task by means of the Initiate P2 (IP2) syllable, which functions similarly to the IP1 syllable. P2 runs until it detects one of its internal interrupts or the MCP running in P1 executes a Halt P2 (HP2) syllable. When one of those events occurs, P2 stores its state in the stack and PRT as P1 does and then idles, waiting for the next IP2 to occur. P2 does not sense external interrupts (such as the timer and I/O completions) and is not directly affected by them. They are handled by P1 instead.

P1 never idles until the system is halted -- its equivalent of idling is for the MCP to continuously execute its NOTHINGTODO loop, waiting for some interrupt or change in system status to occur.

The design of the MCP is intended to assign user tasks preferentially to P2 if it is available. If there is more than one user task ready to run and P1 does not have any Control State duties at the  moment, then both processors can simultaneously run user tasks. When running a single program in the mix, we have seen the program bounce between the processors, but if there is enough processor-bound work in the mix, then P2 will stay busy. Every time P2 senses an internal interrupt or needs some MCP service (which it signals by triggering a COM interrupt), P2 stores its registers and idles. If there is another task waiting for a processor, the MCP will generally initiate P2 again right away on it.

Compared with today's sophisticated SMP system designs, the multiple-processor mechanism on the B5500 probably seems a little clunky and inefficient, and in hindsight it was. But then, even multi-tasking was rare and controversial in the early 1960s, and for the B5000/B5500 to support both that and multi-processing was so far out that Burroughs was often accused of faking it when the capability was demonstrated. It did work, though, and was quite effective if you had a sufficiently processor-bound workload and the memory to support that workload. The two main reasons P2 can be underutilized are insufficient memory (generally resulting in MCP overhead and I/O to overlay memory to and from disk) and high levels of requests for MCP services, such as I/O, both of which can handled only by P1.

You can enable second-processor functionality for the emulator by a setting in the emulator/B5500SystemConfiguration.js script. Set both PA and PB to true, and set PB1L to indicate which processor is P1. The other will be P2. At present, the second processor is disabled by default in the standard release.

Those of you using a hosted web site are not able to modify that configuration script, so we have implemented a temporary workaround to allow you to enable the second processor:

  • Clicking the NOT READY lamp on the Console display will toggle the availability of Processor B in your local copy of the system configuration. When enabled, it will run as P2.
  • When P2 is enabled in this way, the version level to the left of the "retro-B5500" logo will be yellow. When P2 is disabled, the logo will be white as usual.
  • The availability of P2 is evaluated only when the POWER ON button on the Console is clicked, so it is best to change the status of P2 when the emulator is in a powered-off state.
When the system is running and P2 is disabled, the P2BF annunciator (P2 Busy flip-flop) on the Console will be constantly lit. This lamp reflects a flip-flop in Central Control that prevents P1 from initiating P2, and generates a P2 Busy interrupt if P1 attempts to do so.

When the system is running and P2 is available, you should see P2BF turn on and off in concert with the B NORMAL lamp (assuming Processor B is configured as P2) as the MCP assigns user tasks to the processor. You may also see the HP2F (Halt Processor 2 flip-flop) annunciator flash occasionally as the MCP stops P2, usually to assign another task to it after the current task's time slice has expired.

You will need a fairly fast system to run both processors simultaneously at full speed. If the P1 Slack indicator on the emulator console is not at least 50% when running one processor, then your system may not be powerful enough. My one-year old, 3GHz, quad-core Dell Optiplex 390 running 64-bit Windows 7 Pro handles this with ease, as does a six-year old Mac Mini with an Intel Core 2 Duo processor running OS X Snow Leopard. An eight-year old Dell Optiplex GX 520 with a 2GHz Pentinum P4 running Windows XP is borderline. All of these tests were done using Firefox 24.0. Dual B5500 processors will still work on less powerful hosts, but the entire emulation will run slower than it otherwise would.

Tape Drive Oscillation at Load Point

Since the initial magnetic tape drive implementation was released in 0.15, a few people have been bedeviled by some odd behavior when a drive is made ready after loading a tape image, or after a tape is rewound by the MCP. There were some problems in the way that the driver was handling the tape image and reporting status to the MCP at load point (also known as Beginning of Tape or BOT). Those were mostly fixed in release 0.16, but a couple of people were still having significant problems. I couldn't reproduce them.

After about two weeks of collective hair-pulling, we finally determined that the problem was not the tape drive module, but an MCP configuration problem. Paul Cumberworth and Tim Sirianni were the ones still having the problem on 0.16, but they were also the ones working on cold-starting the emulator from card decks. In fact, they had that working, and had each cold-started their emulator instances from the Mark XIII cold start deck instead of the ColdLoader web script.

For some reason that we still have not run to ground, that version of the cold-start program does not properly set bit #2 (MOD3IOS) in the MCP option word. The emulator's I/O Control Units were written to behave like Mod-III I/O units, but that setting in the option word was telling the MCP that the I/O units did not have Mod-III capabilities. As a result, the MCP was handling the reading of tape labels entirely differently, and that difference was resulting in an endless cycle of the MCP reading the tape label and then rewinding the tape.

Paul Cumberworth reported this past week that the problem does not occur when cold-starting using the Mark XVI deck.

Fortunately, we found an easy workaround for systems that have been cold-started using the Mark XIII card deck. After the MCP has halt/loaded and you have entered the time of day, simply enter this command on the SPO:

That will set bit #2 in the option word, and that setting will persist across halt/loads and emulator/browser restarts. Note that you will not need to do this if you have cold-started using the ColdLoader script. Also note that the SPO TO command does not display the status of bit #2 in its output.

If anyone feels left out and wants to experience the tape oscillation problem for themselves, you can manually reset bit #2 in a similar way and see what was driving Paul and Tim batty:

Then load a tape image and make the drive ready. When you are tired of watching the tape drive misbehave (it probably won't take long), just use the first command above to set things right again.

Other Changes in this Release

  1. The Character Mode syllables FAD (Field Add) and FSU (Field Subtract) would complement the wrong operand in certain cases. This was due to the initial compare of the operands being done in an alphanumeric mode instead of a numeric mode.
  2. The Character Mode syllables TRN (Transfer Numerics), TRZ (Transfer Zones) and TBN (Transfer Blank for Non-numeric) did not work properly if the transfer spanned more than two words of memory. Words other than the first and last in the destination string were not being fetched into the B register.
  3. If you have access to the B5500SystemConfiguration.js script, you may now configure up to 16 tape drives on the system.
  4. The flip-flop latching mechanism in Central Control that is used by B5500Console for displaying annunciators was completely redone.
  5. The routine that cleared interrupts in Central Control was reworked for more efficient operation.
  6. New algorithms were implemented to compute average slack and delay in the Processor.schedule() routine.
  7. Some Character Mode syllables were optimized by substituting local variables for Processor object "this" properties.
  8. The Processor single-precision divide syllables were leaving the stack in an incorrect state after a divide by zero in Control State. This has been corrected.
  9. A few further minor tweaks to performance throttling were implemented.
  10. References to this.cc in the Processor.run() routine were optimized.
  11. A few improvements to the tape drive module were implemented to eliminate oscillation at load point and improve the timing of rewind operations. The bulk of the tape drive oscillation problem was caused by the configuration problem discussed above.
Finally, I want to remind everyone that we now have a web/email forum on Google Groups linked to this project. If you have a question or a problem, this is the first place you should go to look for answers or post a request for help. Membership is open to everyone. To join the group, go to:

Wednesday, November 20, 2013

Emulator Version 0.16 Released

Nigel and I are pleased to announce that version 0.16 of the retro-B5500 emulator has been released. All changes have been posted to the Subversion repository for our Google Code project GitHub project. The hosting site has also been updated with this release, and for those of you running your own web server, a zip file of the release can be downloaded from Google Drive.

This is a bug-fix release. Our community of enthusiasts found some rather serious problems in the new datacom interface, and some minor ones in the new magnetic tape drive interface. The most serious problem was that entering a zero-length message on the datacom terminal when would result in a "DCA ERROR ... READ BOUNCED BACK" error on the SPO, followed by a complete hang of datacom.

These problems have been fixed, and both new interfaces now appear to be working well. We have also made a few corrections and enhancements to the wiki pages.

We have also seen and had other reports of major instability when running the Timesharing MCP (TSMCP) and CANDE -- Invalid Address and Flag Bit faults, which typically result in either a program being DS-ed (aborted) by the MCP, or a complete system hang. Another report earlier today suggests that doing a fresh cold-start may resolve the problem, but this has not yet been confirmed. Obviously, we are concerned about this, and have not ruled out that it may be a problem in the emulator. Investigation is underway, but you may want to wait a bit before getting too excited about running TSMCP and CANDE.

In other news, Paul Cumberworth and Tim Sirianni are currently working on the set of card-load-select decks necessary to cold- and cool-start a B5500. Up to the present, we have had to rely on the ColdLoader script, a custom web page built for the emulator that initializes the IndexedDB disk subsystem, creates initial disk directory structures, and optionally loads files from Sid McHarg's B5500 Mark XIII tape image files. The card-load programs are the official way to do this type of system initialization, but could not be used until the card-load-select mechanism and support for tape drives became available. Once Paul and Tim have these decks working, we will make them available to everyone.

Finally, communication among the members of the retro-B5500 emulator community to date has been accomplished largely through email. As the number of interested parties has grown, this has become burdensome, so this week we created a web/email forum on Google Groups and linked it to this project. Membership is open to everyone. To join the group, go to
 The primary purpose of this group is to provide a vehicle for discussion, questions, answers, and support for those using the emulator, and for discussion on the Burroughs B5500 system and software in general. We hope you will join us.

Saturday, November 16, 2013

Emulator Version 0.15 Released

Nigel and I are pleased to announce that version 0.15 of the retro-B5500 emulator has been released. All changes have been posted to the Subversion repository for our Google Code project GitHub project. The hosting site has also been updated with this release, and for those of you running your own web server, a zip file of the release can be downloaded from Google Drive.

In the past six months, the emulator has transitioned from a state where we were astonished if it did anything, to one where now we are astonished if it doesn't do everything -- not that it is exactly in a state of perfection, mind you. The 0.15 release, in addition to numerous minor corrections and improvements, supports two significant new features: datacom and tape drives.

Datacom and Terminal Interface

With the emulator operating within a web-browser, understanding how to implement a data communications interface for it has been a bit of a thorny problem for us. To support external terminals, it would have been nice (and perhaps even necessary) for us to use the modern networking capabilities we have all come to rely on. The TCP/IP protocol upon which most network-based technologies are built, however, requires a network connection to have two ends -- a "client," which initiates the connection, and a "server," which listens for new client connections and reacts to them.

The problem is that the emulator would need to act as a server in this arrangement, but web browsers are capable of acting only as clients. There just isn't any way -- that we've been able to find -- for an application running within a web browser to accept incoming network connection requests and act on them. There are some approaches involving a third party that would serve as sort of a client-client "gender changer," but we chose to avoid those as being too complex.

Recognizing that an application running in a browser is very much a personal environment to begin with, we finally decided to abandon the idea of network-based datacom altogether. Our goal at present is for you to be able to access datacom-enabled applications running within the emulator, not to implement the type of multi-terminal, dial-in timesharing network one would have seen at a university or commercial site in the late 1960s. We hope that for most of you, being restricted to a single terminal instance will not prove to be too limiting.

Therefore, this initial implementation of datacom for the B5500 supports a single terminal interface operating within the emulator itself. Internally, we are emulating a B249 Data Transmission Control Unit (DTCU) with a single B487 Data Transmission Terminal Unit (DTTU). The DTTU is configured to have a single teletype-like terminal adapter, with a 112-character buffer, operating on buffer address 01/00. All of that is buried within the emulator and pretty well hidden from view.

What you see when running the emulator is a new window for the terminal itself. This window attempts to emulate something similar to a dial-in Teletype Model 33 KSR station. Along the top of the window is a Connect button to establish a connection to the B5500, and a set of indicators that show the status of the buffer in the B487. These indicators were originally implemented to debug the interface and may be removed at some point in the future.

The terminal works somewhat like the current SPO device, but without the need for input request to get the system's attention for input. You just key in messages and the terminal types out the responses from the system. Here are the basics:
  • To initiate a session with the B5500, click the green Connect button. The button will light. The system should respond with its identification within a few seconds. Click this button again to disconnect.
  • To enter a message, simply key it into the window. Terminate the message with a left-arrow (represented by the tilde, "~", in this emulator). As a convenience, you may also press the Enter key, which will echo as a tilde.
  • To backspace to correct a mistake, enter a less-than ("<"). Enter this character multiple times to backspace the corresponding number of characters in the buffer. As a convenience, you may also press the Backspace key, which will echo as a "<".
 There are more keyboard capabilities, which are described in the wiki, Using the Datacom Terminal.

The B5500 had two separate operating system environments that supported datacom, the Datacom MCP (DCMCP) and the Timesharing MCP (TSMCP). If you followed the standard getting-started instructions, you probably cold-started with the DCMCP.

Both environments require some setup in order to use datacom. For DCMCP, see Appendix D and E in the B5500/B5700 Operation Manual. For TSMCP, see the Timesharing Terminal User Guide, Timeshare System Operation Manual, and Operating the B5500 Timesharing System. All of these documents are available at http://bitsavers.org/pdf/burroughs/.

We would welcome someone preparing a guest post for this blog on how to set up and configure the TSMCP and CANDE environment. If you are interested, post a comment to this blog or contact me through our Google Code project.

Magnetic Tape Drive

I started working on the tape drive interface because I needed to load more files from the Mark XIII system release tape image in order to test the datacom interface. The standalone B5500ColdLoader.html script has been able to load files from the Mark XIII tape images since the beginning, but in order to use that you needed to shut down the emulator, and that was inconvenient. In addition, the ColdLoader's handling of the MCP disk directory structures was not that robust. It could load files only to the first disk unit (EU0), and attempting to replace an existing file often corrupted the directory.

This initial implementation for tape drives is read-only, and works only with the ".bcd" format that is used for the Mark XIII tape image files. We plan to implement support for writing to tape and for other image formats in the near future.

Library/Maintenance is the portion of the MCP that manages disk files and maintains the disk directory. Among its features is the capability to dump disk files to and load disk files from magnetic tape. This was typically used for file backup, archiving, and transferring files among systems.

Files can be loaded from Library/Maintenance volumes such as the SYSTEM, SYMBOL1, and SYMBOL2 tape images using the LOAD and ADD control card commands, e.g.,
A control card command can be continued across multiple card images by terminating the card with a hyphen ("-") wherever a word or other punctuation character might appear. The continuation card(s) that follow must not have an "invalid character" in column 1.

All files having the same first or last identifier in their file name may be loaded by specifying MFID/= or =/FID. All files from a tape can be loaded by specifying =/=. Library/Maintenance will not overwrite certain critical system files, such as the currently running MCP and Intrinsics.

The ADD command works the same as LOAD, except that it loads only files that are not already in the disk directory.

See the wiki, Using the Magnetic Tape Drive, for details on the .bcd image format and operating the tape drive. Also see Section 4, and in particular page 4-15 ff, in the B5500/B5700 Operation Manual for information on the LOAD and ADD control-card commands used to load files from B5500 Library/Maintenance tapes.

Miscellaneous Changes and Enhancements

In addition to the two major features above, this release has the following changes and enhancements:
  1. A serious bug was fixed in the B5500SetCallback.js module, in preparing arguments for the call-back function. Among other things, this prevented the SPO from working properly in Google Chrome. Thanks to Hans Pufal for the information that led me to isolate and fix that bug.
  2. Device drivers can now be optionally specified in the base web page hosting the emulator (e.g., B5500Console.html, B5500SyllableDebugger.html). Previously, all drivers known to the B5500CentralControl module had to be specified as <script> tags in the base web page, or initialization of CentralControl would fail during "power on." Peripheral devices will now be created only if they are both configured in the B5500SystemConfiguration.js script and have a <script> tag for their driver module specified in the base page. This will make it easier to implement alternate user interfaces for the emulator in the future.
  3. I am finally caught up with documentation in the wiki pages. There are new pages for the card reader, card punch, line printer, tape drive, and datacom devices, along with numerous corrections and additions to existing pages. You can access these through the wiki directory on our Google Code project, or from the Help menu on our hosting site.
  4. The way the SPO times and outputs characters, especially when echoing keyboard input, has been significantly improved. The SPO window is now slightly narrower, so that it takes less space on your monitor. The most significant change you will see, however, is that the SPO window is no longer given the focus when the MCP sends it a message. I liked that feature a lot, but it became very annoying to have the SPO suddenly get the focus when you were in the middle of entering a message into the datacom terminal window.
  5. Some minor changes have been made to tune the performance of the Processor module, directed towards trying to get it to match as closely as possible that of a real B5500. We are still not there yet, but this is the first release where I have seen the emulator run faster than some timings I have for B5500 runs from the 1970s.
  6. Additional work has been done towards getting P2, the second processor, to work. A little progress has been made, but P2 still craps out after only a short time running. I would really like to get this to work, but it's not a very high priority at present.
  7. Finally, the standalone B5500DiskDirList.html script (in the tools/ directory) has been modified to dump the first ten words of each disk header in raw octal.

Other News

I am pleased to report that Hans Pufal in France and Fausto Saporito in Italy have finished their joint transcription effort for Gary Kildall's B5500 APL interpreter, and are now industriously proofing and debugging their work. With the initial datacom interface working, we hope to have APL alive once again in the near future.

Paul Cumberworth in Australia has been transcribing patches for the Mark XVI source files that were previously transcribed, and building compile decks for both those patches and the ones on the Mark XIII release tapes. We have encountered a couple of small problems with the base Mark XIII release files (e.g., the effect of a SPO MC command does not persist across halt/loads) that are addressed by these patches, and hope to make these decks available soon so that everyone can upgrade to the "latest" (i.e., October 1971) Mark XIII updates.

I am starting to accumulate a number of test and sample programs for the B5500, some from my own files and some sent to me by others. Presumably some of you out there have programs that you are (or would like to be) running in the emulator, and there are plenty of listings on bitsavers.org that could be transcribed for use with the emulator. We need a way to collect all of this in a central place so that it can be shared and maintained for the benefit of everyone who is interested in the B5500.

I have started discussing this with a few participants in the project, but we have not yet come up with a good idea of how this material should be organized, stored, and made available. We are very interested in any ideas that other participants and readers of this blog may have. We are also very interested in any B5500 software or other materials you may have in whatever form or condition it may be in. Please share. You can comment on this subject on the blog or contact Nigel or me through the Google Code project.

Another idea, if people are interested, is to set up a forum where everyone can interact independently, ask questions, share information, and otherwise act as a community. We have this capability available for the project through Google Groups, but just have not turned it on. If there is sufficient interest in having a common forum, I'll set it up. In the interim, you can post comments on this blog. You can also post entries in the issue tracker that is part of our Google Code project. You will need a (no-cost) Google account in order to do so, however.

I want to express again Nigel's and my appreciation for the interest and enthusiasm that many people are showing for this project and for the Burroughs B5500, plus all of the effort that several are putting into making the emulator and the software for it better. Please let us know any ideas or suggestions you may have, and any problems you encounter in trying to use the emulator.

Tuesday, October 22, 2013

On-going Progress with the Emulator

Another four months has come and gone since the last blog post for this project. There is much to report. We had made a lot of progress through July, then I was pulled away by other commitments for most of August and September, and have just reengaged with the project over the past few weekends.

In the work leading up to my two-month hiatus, the emulator became substantially more stable and more capable on almost a weekly basis. We are now at Release 0.14, which was pushed out in early October. There are still problems, and more features to implement, but the emulator is now in a very usable state.

The last blog post ("It's Alive...," 3 June 2013) seemed to strike a chord. Several people are now using the emulator, others have contacted us with comments, and we have received offers of additional B5500 material. There is more on this subject below.

Significant Changes and Improvements

Shortly before the last blog post, we resolved a very nasty problem with the so-called "R+7" aspect of subroutine stack linkage. That one fix made the emulator about an order of magnitude more stable than it had been prior to that point. It enabled us to begin using the system as you would a real B5500 under the control of its MCP operating system. Since then, the following major enhancements and fixes have been implemented:

Card Reader
Initially, all that we had were the SPO and Head-per-Track disk peripheral units. This made it impossible to run anything but programs we could load from the Mark XIII system tape images using the ColdLoader utility. A high priority was to implement card input. The current driver emulates the Burroughs B129 1400-card/minute reader. Card decks are ordinary ASCII text files. You load one or more files into the reader using a standard file picker dialog, then press the reader's START button. The MCP senses the reader's change in status, and starts reading cards, just as it worked on a real B5500.
"Dummy" Line Printer
Nigel started working on the implementation for a line printer peripheral unit, and ran into problems getting a prototype to work. It turned out the problems were mine in the way that printer I/Os were being initiated and terminated in the IOUnit and CentralControl modules. In the process of fixing those, I literally threw together a very basic diagnostic printer driver out of pieces of the SPO and card reader implementations. After getting my IOUnit problems fixed, I took out the diagnostic stuff, and well, we're still using that. It works fine for simple output, but at some point will need to be replaced by something with a better user interface and more complete functionality.
Card Punch
After getting the card reader and preliminary line printer units to work, it was a straightforward task to clone a card punch peripheral driver out of those. Besides, I was beginning to work on the Card-Load-Select mode of loading the system, and needed a way to output card decks for programs like the COOL and COLD loaders.
Improved Console Display
The B5500 had a very minimalist operator console -- just a few buttons and lights. There were lots more lights on the maintenance panels in the Distribution and Display unit, but those were usually hidden from view behind the "skins" of the mainframe cabinets. With the emulator, though, it was often difficult to see from the console what was happening with the system (or whether anything was happening at all), so I have added some annunciators to the console that show the activity of the I/O Units, external interrupts, and the individual peripheral devices. These are really helpful to gauge the activity of the system, and they make a nice show, besides. The extra lights can be disabled if you are a purist and want the console to look like it did on a real B5500.
Smaller User Interface Windows
The initial design of several of the windows for the peripheral devices were just too large. They worked fine on the 23-inch monitor I typically use for development and testing, but on other systems, particularly laptops, the windows crowded each other out and made it difficult to see what was happening overall with the system. The windows for the SPO, card reader, and card punch have all been reduced in size to better accommodate smaller displays.
RTS Presence-Bit Bug
RTS is the Return from Subroutine instruction. It is typically used to exit from what Burroughs termed "accidental-entry" procedures,  but what the rest of the world refers to as "thunks." This type of subroutine is used to implement the semantics of Algol Call-by-Name parameters. It turns out that such subroutines can return data descriptors, and a requirement of the RTS instruction is to throw a Presence Bit interrupt (page fault) if the returned descriptor points to an absent memory segment.

That requirement was poorly-documented, and we weren't checking for descriptor absent status in the emulator. The result was that the emulator could use the address field of an absent descriptor as if that field contained a valid memory address. This error only showed up while running some FORTRAN programs, and it proved to be very difficult to trace the symptoms back to the cause. It took several long, frustrating days of tracing and debugging before I finally found an obscure reference to the P-bit requirement, after which the fix was obvious and simple -- as is often the case with such problems.
Floating-Point Arithmetic Bugs
One of the first things I tried once the card reader and line printer were working was an Algol number-crunching program from my student days, for which I still had listings of source and output from a B5500 run in 1970. The program does orthonomalization of vectors to compute rheological parameters for two-phase flow in a round pipe (and before you ask, no, I don't understand what that means anymore).

Getting the program to run was no problem, but the results from the emulator were not even close to those on the 1970 listing. I was getting at best one digit of agreement between the two. I have a few other programs and listings from that era, and they were showing similar problems for cases involving complex calculations. Programs doing simpler calculations showed quite good agreement, however, so that smelled like some sort of rounding or normalization problem in the emulator.

After being deviled by this for months and frustrated in a couple of attempts to find the problems, earlier this month I wrote an Algol program for the B5500 that generated a variety of numeric-word bit patterns, computed all of the combinations of those bit patterns for the add, subtract, multiply and floating-divide operators, and dumped the results in octal. I then converted that program to the modern Unisys MCP architecture (which uses the same numeric format) and generated an equivalent set of results.

Comparing the two sets of results indeed revealed a number of cases of off-by-one differences in mantissa values, and a few cases where the differences were even worse. Knowing the bit patterns that generated these differences, I was able to trace the evaluation of those specific patterns in the emulator, and found several problems with rounding and normalization. All but one of the problems were in add/subtract, which internally is the same operation with some sign manipulation. The remaining problem was due to rounding when multiplying two integers -- which by their nature should never have their product rounded.

After correcting these issues, the results from the emulator now match -- to the digit -- the results in all of my listings from 1970 that I've been able to check thus far.
Card Load Select Bug
By default, the B5500 booted from disk. A push-button switch on the operator console would cause it to load from cards, however. Loading the MCP from disk has been working for months, but attempting to load from cards would read the binary boot loader card plus the first card of the program being loaded, then hang. After previously making several runs at the problem, earlier this month I finally found the cause -- hardware load proceeds much like any other I/O, but it is initiated as a special mode of the I/O Unit. It generates a result descriptor, but not a completion interrupt.

The problem turned out to be that the emulator was not suppressing the completion interrupt. Load from disk worked, either because of the timing involved, or more likely because the MCP's KERNEL bootstrap was smart enough to ignore the extraneous interrupt. In contrast, the binary one-card loader is pretty dumb, and apparently became confused by the pending interrupt left by the hardware load mechanism. Booting the system from cards now works.
Hosting Site and Wiki
While the emulator runs entirely within a web browser, it requires a web server from which it can be loaded into the browser. Not everyone has the wherewithal to set up and operate their own web server, so we have set up a web site to support the following:
  • Host the current release of the emulator.
  • Make available the Mark XIII tape images containing the Burroughs system software. 
  • Make available releases of the emulator source code for downloading.
  • Serve as a central source for emulator utilities and information. 
You are welcome to visit and use this site at http://www.phkimpel.us/B5500/.

We have also created a number of wiki pages on the project's Google Code site (http://code.google.com/p/retro-b5500/) GitHub site describing how to set up and use the emulator and its components. There are links to these wiki pages under the main Help link on the hosting site

Browser Status and Performance

One of the goals of this project has been to have the emulator execute programs at the speed of a real B5500, or as close to that as can be practical in its browser-based environment. Throughout the development of the emulator, especially in the Processor module, we have been concerned about the emulator's potential performance, and have tried the keep the coding as lean as possible. With the emulator becoming reasonably stable and the ability to compile and run various programs through the card reader, we are starting to get a feel for its performance.

Based on timing statistics in some of my listings from the 1970s, the emulator initially appeared to be running about 50% slower than a real B5500. I do most of my development and testing on a relatively new Dell Optiplex 390 with a quad-core, 3.3GHz Intel Pentium i3-2120 processor running 64-bit Windows 7. Monitoring performance in the Windows Task Manager while the emulator was running in Mozilla Firefox showed that raw processor power was not the problem -- the emulator may have been running slower than desired, but the core1 running that Javascript thread was loafing. So why was the emulator appearing to run slow?

The problem has turned out to be two-fold. First, the emulator attempts to throttle its performance by estimating for each instruction the number of 1MHz clock cycles that instruction should take to execute. It accumulates those cycle counts and periodically compares the number of microsecond cycles it has accumulated against the amount of real time that has elapsed. If the number of microseconds accumulated is greater than the elapsed time, then the emulator is running too fast, so it pauses using the Javascript setTimeout() function to allow real time to catch up to emulated time. We were obviously accumulating more clock cycles than we should have, especially in the number of cycles that memory access consumed. Simply lowering the number of clock cycles accumulated for each read or write memory access brought the emulator to within about 15% of the timing statistics from 1970.

Second, we found that the Javascript setTimeout() and time-of-day facility (using new Date().getTime()) were not nearly as granular as we were counting on. Some research revealed that (as of June of this year) most browsers on Windows had a timing resolution of about 15 milliseconds, and that the emerging HTML5 DOM standards call for a minimum resolution of 4ms. The emulator was requesting delays below 4ms, with the result that when the throttling mechanism tried to delay, say, 3ms, the real delay might be 15ms, resulting in the appearance that the emulator was running slow. It was actually running amazingly fast, but throttling too much.

The throttling mechanism has another important role besides regulating the speed of the Processor module. Everything in the emulator runs synchronously on one Javascript thread -- the Processor, the I/O Units, the peripheral devices, the interval timer in Central Control, even the second Processor, once we get that working. If the Processor does not yield control of the thread periodically, I/Os will not be initiated and completed, external interrupts will not be serviced, and in general the system just won't work. Thus, while one approach to dealing with the Javascript timing granularity would be to increase the amount of time the Processor runs before throttling, hogging the thread has a negative impact on the system's ability to do I/O and service interrupts.

We could see this in the difference of the emulator's performance when running in Google Chrome compared to Mozilla Firefox. Earlier this summer, Chrome was ahead in implementing the 4ms HTML5 DOM standard for setTimeout(), and the effective speed was much closer to the B5500 than with Firefox. Apparently Firefox made a change to its timer granularity in version 22, and the emulator performance is now better with Firefox that it is with Chrome.

Resolving this problem in the emulator has turned out to be an adventure. The main improvement has been to change the throttling mechanism so that it could better tolerate long setTimeout() delays without interfering with access to the Javascript thread by the other components. What we needed was a way to yield control of the thread without introducing any additional delay -- if other components were scheduled to run on the thread, they would, but control could return to the Processor as soon as everyone else had their turn.

There is a proposed setImmediate() function for Javascript that does just that, but only Microsoft Internet Explorer implements it, and it does not appear that this proposal will become a standard. Some freely-available "shims" have been written to implement the behavior of setImmediate() using existing DOM features, however, and we found one by Dominic Denicola (https://github.com/NobleJS/setImmediate) that works quite well. The throttling mechanism now computes a delay time and compares it to some threshold value (currently 4ms). If the delay is greater than the threshold, it uses setTimeout(); otherwise it uses the pseudo setImmediate() implementation.

In that latter case, the Processor will resume sooner than it should (and not actually throttle the performance very much), but the throttling mechanism does its computations based on total cycles accumulated vs. total elapsed time, so at the end of the next throttling cycle, it will typically compute an even larger delay. Eventually the delay will grow to the point that it exceeds the threshold, and setTimeout() will be called to do some real throttling. This approach generates jitter in the execution of the Processor, but the delays and non-delays average out, and it happens fast enough (15ms is approximately the refresh rate on most monitors), that the jitter usually is not noticeable.

With this new approach to throttling in place, the emulator is still running a little slower than a real B5500, but only by 7-8%. Further improvements in apparent performance will probably require detailed tuning of clock accumulation in the individual instructions. That can wait. In any case, B5500 instruction timings are difficult to model, because the Processor overlapped execution and memory access whenever it could, and the crossbar memory access mechanism in Central Control could generate random delays due to conflicting access to a memory module by multiple Processors and I/O Units.

Another outstanding problem in performance tuning is that I/O times appear to be four to eight times longer in the emulator that my listings from 1970 indicate they should be. I/O time is essentially channel time -- the MCP records the time when an I/O request when is initiated against an I/O unit, and again when the I/O complete interrupt is serviced. The difference between the two times is accumulated as I/O time for the requesting job. I suspect that this may be another problem with setTimeout() granularity, or possibly the IndexedDB mechanism we are using for disk I/O is just plain slow. [Correction as of 2013-10-23] Oops -- I am completely wrong about this. The program I have been using in timing tests outputs results at several points during its execution. I had thought that the times were differential between the output cases, but it turns out they are cumulative. When I compute the differences between the cases, the emulator's I/O times are actually lower than those on the 1970 listing by almost half. The exception is the first output case, where the I/O time is substantially higher. That might be due to the overhead of the AUTOPRINT spooler printing the output of the program's compilation during the time that first case is running.

Performance of the emulator also depends somewhat on the underlying platform. Firefox and Chrome remain the two browsers we have found that support the features the emulator needs to run. Apple Safari through 6.0 does not yet support IndexedDB, although the emulator should work in Firefox on a Mac. It does not work on Microsoft Internet Explorer through IE10. We have not yet tried Opera.

I have tried the emulator on a variety of Windows systems using Firefox, and it runs everywhere I have tried. It runs well, if slightly slower than on my quad-core Optiplex 390, on a five-year old Dell D830 with a 2 GHz Pentium Core Duo T7250 under 32-bit Windows 7, and also slightly significantly slower on an eight-year old Dell Optiplex GX520 with a 3 GHz Pentium P4 under Windows XP. [Clarifications added 2013-10-26]  It even runs acceptably on a four-year old HP Mini netbook with an Atom processor, also running under XP. The big problem with the Mini is not performance, but that the screen is too small to see much more than one of the windows at a time. On laptops, the best performance has been observed when they are on external power. The performance is noticeably slower when running on battery, or on a travel charger with a lower wattage rating than the standard charger.

Other Participants

One of the gratifying things about this project is the interest that other people have shown in it. We have been somewhat surprised at the number of people who have picked up the emulator and started using it, without much apparent difficulty, and only then let us know what they were doing. A few of those people have become more intensely involved with the project:
  • Fausto Saporito of Naples, Italy has been an early user of the emulator, and has contributed a number of FORTRAN benchmarks, including Whetstone and an arctangent program that appears to do a good job of measuring floating-point loss of significance for a processor. Fausto has also single-handedly transcribed the Mark XVI FORTRAN source from the listing on bitsavers.org. The current version is in the project's Subversion repository on Google Code.
  • Tim Sirianni of Eureka, California, US, stunned us by reporting that he had the TS (timesharing) MCP running and was using the CANDE timesharing editor, sort of. We don't have datacom working yet in the emulator, which is where the "sort of" comes in. Tim found a way to use the SPO as a CANDE terminal, but it is awkward to use, and not an approach for the less-than-determined.
  • Paul Cumberworth of Adelaide, Australia has transcribed the patches for our Mark XVI ESPOL compiler source and gotten those to compile with the base source. These are also available in our Subversion respository.
Out of the blue, Ed Vandergriff of Chaska, Minnesota, US, wrote me in late August to say that he had a listing of the APL interpreter for the B5500, created by Gary Kildall (of CP/M fame) and others at the University of Washington in the early 1970s. He asked if we were interested in it. I had not been aware of the existence of this interpreter, but Nigel had been looking for a copy of it for some time without success. Ed generously sent me the listing, which is actually a first-generation photocopy taken from a line-printer listing. We have scanned it for our use, and will eventually donate the original to a museum for long-term preservation.

A copy of the scanned listing is available on our hosting site at http://www.phkimpel.us/PickUp/APL-B5500-Listing-19710111.pdf. It is about 44MB in size.

Fausto and Hans Pufal of Angouleme, France, have volunteered to transcribe the APL listing. Hans has helped us before, having previously transcribed the Mark XVI source code we used to create our ESPOLXEM cross-compiler. Fausto is starting from one end of the scanned APL listing and Hans from the other. At last report, they had only 20 pages to go until they meet in the middle, à la the Mont Blanc tunnel. Their progress to date is available in the Subversion repository. Once their transcription is complete, it will need to be proofread and corrected before it can be used. We will also need to get datacom working in the emulator.

Current Efforts

We have some known problems and a couple of high-priority features requiring attention.
  1. A proper datacom interface will be required to run the TSMCP and CANDE, as well as the APL interpreter. I am currently working on a very basic, one-terminal implementation of the B249 Data Transmission Control Unit and B487 Data Transmission Terminal Unit. Supporting external terminals in a browser environment is extremely difficult (browsers are quite determined be be clients, not servers), so this initial implementation will simply host a single terminal as a user interface to the B249/B487, somewhat similar to the way the SPO currently works. That should be adequate for most users. We hope to have this feature available soon.
  2. After datacom, the next priority is support in the emulator for magnetic tapes. We think we know how to approach this, but detailed design work has not yet begun.
  3.  The B5500 would support two processors, but our attempts to get the second processor working have thus far been a failure. This has been especially frustrating, because the differences between P1 (the control processor, which is currently working) and P2 (which could only run Normal-State user programs under control of P1) are very minor. In fact, the two processors on a real B5500 were physically identical, and either one could be designated as P1 by means of a mechanical switch. I have made three serious runs at this problem, most recently last weekend, and come up short each time. I made some progress this last time, finding a problem in the way P2 was handled by the SFI (Store for Interrupt) instruction. With that change, P2 now runs for a few seconds before somehow failing. The problem is obviously subtle, and is proving difficult to trap, even with special code inserted into the emulator to do so. Getting P2 to work is a relatively low priority, so this problem has been set aside for now.
  4. The other major deficiency in the Processor implementation at present is that the double precision arithmetic operators have never been finished. Their single-precision equivalents are currently standing in for them. One of Fausto's benchmarks requires double precision, and the compilers require double-precision in order to properly compile double-precision literals, so the priority of this issue is rising.
We look forward to hearing from everyone who is using the emulator, or a least trying to. Please let us know why you are interested in the B5500 and what you are doing with the emulator. We are anxious to hear your comments and suggestions, and we especially want to hear about any problems or bugs that you encounter. Either comment on the blog, or contact me privately at paul (dot) kimpel (at) digm (dot) com. If you have a Google account, you can also post issues on our Google Code project GitHub project site at http://code.google.com/p/retro-b5500/issues/list.

1 Has anyone else noticed that in the days of the B5500, "core" meant memory, but now it means processor?

Monday, June 3, 2013

It's Alive...

After almost six months of silence, this blog is back. You can code or you can blog, and since the last post in December, we have been coding -- and then, of course, debugging what we have been coding. The central components of the B5500 emulator are essentially finished and are starting to work. There is more to be done for I/O, we have ideas for a richer user interface and display of system state, and there are certain to be more bugs that have not yet been discovered, but in a technical sense, this project is over the hump -- the emulator is running, it has successfully halt/loaded (i.e., booted) a version of the B5500 Datacom MCP, and it is able to run a few programs natively, including the Algol compiler.


A Brief Background

As we have reported in previous posts, Nigel Williams and I started this project in early 2012, after having talked about it for several months previously. When we first started talking about it, I thought we would program the emulator in Java, or perhaps something like Python. Nigel stunned my by suggesting that we implement the emulator within a web browser, and do the programming in Javascript. To be honest, I initially didn't think this would be possible, but Nigel pointed to some existing browser-based emulators, and eventually won me over. The emulator is 100% Javascript and currently runs within a couple of standard web browsers.

We started with pretty good references for the hardware architecture, but no machine-readable source or object code for the Burroughs system software. Having an emulator for a system but no software to run on it is not much fun. There were, however, a number of scans of listings of B5500 source code available on the bitsavers.org web site, so it appeared that our only option was to transcribe the source code manually from those scans and then somehow figure out how to bootstrap that into the emulator environment.

Nigel had hand-transcribed the Mark XVI Extended Algol compiler source from such a listing the year before we started, and Hans Pufal in France had made a similar transcription of the Mark XVI ESPOL compiler and given it to Nigel. A little more than a year ago, I decided that I needed to do my part, so started transcribing the Mark XVI Datacom MCP source. I'm still at it.

Thinking that the only way we were going to be able to have system software for the B5500 was to transcribe the source from listings, we were going to need a way to bootstrap object code from that source. Thus, last year I took the manually-transcribed Extended Algol and ESPOL compiler sources and ported them to the Algol variant used by the modern successor to the B5500, Unisys ClearPath MCP systems. That effort was successful, producing what we term the ALGOLXEM and ESPOLXEM cross-compilers, but we were going to need more source code -- lots of it. Alas, it appeared it was going to take us a couple of years' work to prepare enough source code from the scanned listings to be able to make our emulator run.

Despite not having system software in a usable condition, and little hope for having much for a long time, we nonetheless started coding for the emulator itself exactly a year ago, over the (U.S.) Memorial Day weekend at the end of May 2012.


And Then the Most Amazing Thing Happened...

Sid McHarg of Seattle, Washington contacted me in early July 2012. I knew Sid slightly from the annual Unisys user conferences. It turns out that Sid is an old B5500 hand, and over the prior year had been working on a B5500 emulator of his own, which he had recently gotten working. Our surprise at finding that someone else was crazy enough to try to emulate this old machine was matched by Sid's surprise that our project existed. That was surpassed only by our surprise to learn that not only did Sid have a working emulator, he had software!

Sid started out in the same position with respect to software (or rather, the lack thereof) that we were in, but Sid had something we didn't -- a set of 7-track Burroughs Mark XIII release tapes from 1971 that had been sitting on a shelf for 40 years. Amazingly, Sid found someone who had a 7-track drive that still worked, and even more amazingly, his tapes proved to be readable after all this time. He was in possession of a complete set of machine-readable source and object files for the B5500 system software, as they existed just past the apogee of the B5500's productive life.

Sid generously started working with Unisys on a license to share this data with others. We felt obliged to keep quiet about this while Sid was negotiating with Unisys, and since we did not know when or if that software might be available to us, we kept trudging along on our original path. We were eventually able to obtain raw images of Sid's tapes in late October 2012, and have recently acquired our own license for the Mark XIII software from Unisys.


The Road to First Halt/Load

With usable system software now in hand, the priority in the project shifted late last year from source transcription to getting the emulator working. I started working on I/O in early December, and by January of this year had initial implementations for the Input/Output Units, Head-per-Track disk, and SPO (supervisory keyboard/printer). I also built a web-based testbed to exercise individual instructions, which eventually evolved into the B5500SyllableDebugger, a basic debugging environment that could load and run whole programs.

Using our ESPOLXEM cross-compiler, I wrote a program to exercise the Character Mode instruction syllables for the system, used the SyllableDebugger to test those instructions, and found and fixed many bugs in the process. I am not very good at writing these kinds of tests, however, so started casting about for an existing program I could use to debug more of the Word Mode side of the instruction set. I settled on the KERNEL, a small bootstrap program that was typically stored on disk. The hardware load mechanism would load and initiate KERNEL, which in turn would bring in the MCP's initialization code to finish the system boot process.

Booting the MCP is termed a "halt/load," after the two buttons on the operator's console that had to be pressed in series to accomplish the act. That term is still used in the modern Unisys MCP systems, even though the physical buttons disappeared long ago.

The KERNEL proved to be a very good vehicle for testing out a significant part of the instruction set. I compiled its source with ESPOLXEM to get an object file and a listing that included the generated machine instructions. I would then load the object file into the SyllableDebugger and single-step through the code, inspecting the processor registers and memory at each step for proper operation. Working this way I was able to identify and correct several problems in the processor portion of the emulator.

As part of the implementation of disk I/O earlier, I had written a standalone utility, B5500ColdLoader, that would initialize the disk subsystem, prepare a skeleton directory structure, and load files from the binary images of the tapes we had obtained from Sid. The standard B5500 software release had a couple of standalone programs that were loaded from cards to perform these functions -- COLD to initialize the disk subsystem and create an empty directory, and TAPEDSK to copy files from Library/Maintenance tapes (an MCP tape format used to dump and restore disk files, somewhat similar to Unix tar). We did not have the card reader working yet, though, and the HTML5 IndexedDB mechanism we were using to implement persistent storage within the browser for the B5500 disk subsystem required some initialization and setup of its own, so building a custom tool to do these initialization functions seemed like the best idea.

The ColdLoader and SyllableDebugger are just HTML files with a bunch of Javascript embedded in them, and you run these utilities from within a standard web browser, just as with the emulator itself.

Thus, by the time in mid-February I was single-stepping through KERNEL with the SyllableDebugger, we already had a disk subsystem in place, and some of the system files, including the Datacom MCP, loaded into it. Over a period of several days, I was able to progress through the instructions in KERNEL, finding and fixing problems as I went, eventually arriving at the point where the program reads sector 0 from EU (disk Electronics Unit) 0 to get the MCP bootstrap address, which had been placed there (correctly, I hoped) by the ColdLoader utility. That eventually worked, so I continued stepping into the code that actually reads the MCP initialization segment.

Hardware load operations finish by branching to memory location @20 (@ indicating an octal literal in ESPOL), so the ESPOL compiler generates object code with the assumption that their execution will start at that address. The MCP cannot be read directly into that address, however, since KERNEL (which is about 250 words in size) was loaded there and is still running in that area of memory. Therefore, KERNEL reads the MCP code into the next available 4KW memory module, generally at address @10000. Disk reads are limited to 63 sectors (1890 words) each, so the MCP initialization segment is read in three chunks into addresses @x4236, @x0474, and @x0020, in that order. Disk I/Os take their disk address from the first word of the memory buffer, thus the backwards sequence allows each read to overlay the disk address word of the prior one.

After reading the MCP initialization code into the higher memory addresses, KERNEL deposits a small in-line Character Mode routine, in raw machine code, into addresses @15-17, and branches to it. That routine simply slides 4042 words starting at address @x0040 down to address @20, overwriting the memory used by the rest of KERNEL. The routine exits back to Word Mode while executing the last syllable at address @17, thus cleverly falling into the first syllable of the MCP initialization code that now resides at address @20.

So there I was, having started out intending just to debug some instructions, but now sitting there with the first word of actual Mark XIII MCP code loaded into the emulator's processor registers, poised to execute a LITC @704 syllable that had been generated in 1971. I thought, why not? and kept right on stepping into the MCP. The date was Saturday, 2 March 2013.

Much of my spare time over the next couple of weeks was taken up with continuing to step through the MCP initialization segment, which is quite large. That consists of the INITIALIZE procedure itself, at about 570 words, plus the kernel MCP routines for allocating memory and doing disk I/O, plus DIRECTORYBUILDER, which implements the complex process of "complementing" the disk directory to determine which areas of disk are not allocated and to build the tables that manage available disk space. Not surprisingly, there were quite a few more emulator bugs resolved during this process.

On 13 March, I saw the first halt/load message to come out of the emulator. This screen shot captured the moment, with the SyllableDebugger and emulated SPO shown running in Mozilla Firefox:
retro-B5500 first halt/load message -- click to enlarge
This message comes out at the point after most memory tables have been initialized and just before initialization of the disk tables begins. The text may be difficult to make out clearly. It reads:
The underlined characters are junk, and should not be there. They were due to a bug in Character Mode that did not reset the BROF flip-flop when a memory transfer ended on a word boundary. That bug was easily found and fixed. There were several more emulator bugs as I got further into the disk initialization code, but by the 18th, it appeared I had gotten all the way through it, and the SPO now produced this:
DKA EU0 SU 1,2,3,4,5, EU1 SU 1,2,3,4,5 WENT READY
 TIME IS 0000
This was very encouraging. The date was not what it was supposed to be, but alas, the SPO would not respond to any input requests so I could change it. It turned out that the MCP was looping endlessly in its NOTHINGTODO loop, and the reason for that eventually turned out to be a bug in handling the F register during accidental entries (thunks). That was easily fixed, and on the 19th I was finally able to see a reasonable halt/load completion with the MCP responding to SPO commands. The underlined text represents my inputs to the system:

 TIME IS 0026
DT 3/19/83
INV KBD DT 3/19/83
DT 03/19/83
INV KBD DT 03/19/83
DT 3-19-83
INV KBD DT 3-19-83
DT 031983
INV KBD DT 031983
DT 3/19/73
INV KBD DT 3/19/73
TR 2128
 TIME IS 2128
DT 3/19/83
INV KBD DT 3/19/83
PD =/=

0:MCP/DISK= 0:SAVE=13485 OLAY=3649

The problem with the DT command (which sets the system date) turned out to be another Character Mode bug, which was fixed the next day. The PD command lists the files in the disk directory (these were all put there by the ColdLoader utility). The MX command lists the tasks in the "mix" -- the set of currently-running jobs. CU prints the memory in use by all tasks in the mix. OL reports on the status of a specified peripheral unit -- DK for a disk controller, SPO for the SPO, and LP for a line printer.

I had long wanted Nigel to share in the experience of the emulator's first successful halt/load, but was feeling a little guilty about that at this point, because I had been slowly, manually, doing the first halt/load all by myself inside the SyllableDebugger. I had been keeping him posted on my progress by email, but there wasn't much else to be done about it, as Nigel is in Hobart, Tasmania, while I am in San Diego, California -- 12,840 km (almost 8,000 miles) away. At that time of the year we are 18 time zones apart, so most days he is going to bed just as I am getting up. This does not make for close collaboration.

It was time to try halt/loading the emulator on its own, though, so Nigel and I arranged to meet on Skype the following weekend. I wired up the Load button on the console UI to the load function in the emulator's CentralControl module, and on 22 March we watched together as our emulator booted the Mark XIII MCP all by itself for the first time.

Life After Halt/Load

This project has been an exercise in deferred gratification, and that first completely autonomous halt/load was certainly both deeply gratifying and a significant milestone for the project, but things were far from perfect. Some SPO commands (e.g., CD) crashed the system, and our initial attempts to run programs under the MCP were completely unsuccessful.

I was busy with work and travel during April, so spent far less time on the project that month than in the ones previously. I managed to chip away at several problems, however, and by the end of the month was able to see one program (XREF/JONES) run to a successful completion. There were more bugs in the emulator, including a very serious one where the Program Release (PRL) operator was looking at the wrong bit in the I/O Descriptor word to determine whether to cause a Program Release or a Continuity Bit interrupt. The consequence of that was that I/Os would not get initiated against empty buffers, which in turn caused programs to wait for I/O completion interrupts that would never occur. There were a couple of problems with the way that the ColdLoader utility was initializing the disk, one of which was the cause of the problem with the CD command. Then there were two problems for which the root cause was determined to be that Paul was no longer a competent B5500 operator. The solution for that was to RTFM, only more carefully this time.

XREF/JONES was an interesting program -- sort of a Swiss Army knife for B5500 source code -- and I had been very happy to see it included in the files on Sid's tape images. As its name implies, it could generate an identifier cross-reference, but could also do flowcharting and generate a block/procedure structure outline. It was a basic text formatter, along the lines of nroff, and could even extract and format documentary comments from source programs, making it a very early form of Javadoc processor.

XREF/JONES was controlled by "$" pragmas embedded in the source code, but since we did not yet have any way to input data into the system (the SPO is an operations console, not a user terminal), the program was running in its default mode of simply listing its input file, which in our initial tests was its own 14K-line source.

Once we got past the initial post-halt/load problems noted above (especially the one concerning the incompetent operator), XREF/JONES started to show very bizarre behavior. Sometimes it would finish successfully. Sometimes it would abort with a disk addressing error. Other times it would abort with an Integer Overflow or Invalid Address interrupt. The remaining times the system simply crashed, sometimes with an Invalid Address message as its last gasp, and sometimes with no indication at all.

Resolving this took three long, agonizing weeks in May. There were a number of problems, including some missing or misplaced break statements and missing "this." prefixes on method calls -- all really bad news when programming in Javascript. I also reworked the implementation for IP1 (Initiate Processor 1), which is what does all of the register setup to assign a task to a processor, as I had never been happy with the way I had coded it originally.

The biggest fly in the ointment, though, which I noticed entirely by accident while looking for something else, was an error in the way the processor traced MSCWs (Mark Stack Control Words) during procedure exit. Those who are familiar with the B5500 will recall that the MSCW stored the state of the F register, MSFF (Mark Stack Flip-Flop) and SALF (Subprogram-Level Flip-flop). During procedure exit, the processor had to follow the F-register links in the MSCWs backwards in the stack until it found the first one where MSFF was not set, in order to restore that word at address R+7 in memory.

Alas, I had written this mechanism to stop when either the MSFF or SALF bits in the word were not set, which sometimes resulted in the wrong word being restored (especially when a procedure was called from MCP global code in Control State), and consequently messing up the (R+7) relative addressing mode within the processor. That relative addressing mode is used infrequently, but the results were catastrophic and extremely difficult to trace back to the source. This wasn't actually a bug, but a misreading of how procedure exit was supposed to work, not that it made me feel any better about it once I discovered what the problem was.

With that problem resolved, the emulator suddenly started running much more reliably. Not only was I able to run XREF/JONES to a successful completion, but last weekend all of the following worked:
  • Compile a smaller Algol source file, SYMBOL/TAPCOPY, used to copy tapes. The compiled code is bit-identical to the version on Sid's tape image.
  • Compile XREF/JONES with the Algol compiler. The 14K-line source compiled in about 13 minutes elapsed time with default compile options, including generation of a listing.
  • Run the newly-compiled XREF/JONES against another small source file, SYMBOL/DSKDUMP.
  • Compile the Extended Algol compiler with itself. This source is 11K lines and compiled in about nine minutes elapsed time.
  • Compile SYMBOL/TAPCOPY with the new version of the compiler. The output of that is also bit-identical to the original object file.
This is hardly a scientific test, but it demonstrated an order of magnitude more capability and reliability than we had seen before. We are now ready to submit the emulator to more demanding tests, but in order to do that we need to implement more I/O capability.


Current State and Next Steps

The emulator consists of a number of Javascript objects:
  • CentralControl, which serves as the node through which all inter-component communication takes place. It has the memory exchange, allowing multiple processors and I/O Units (channels) to access memory. It also prioritizes interrupts, hosts the interval timer, initiates I/Os, and controls the system-load functions.
  • Memory Modules. These are actually implemented as a part of CentralControl. There can be up to eight 4K-word mods, for a total of 32K words on a system. We are currently running six mods, with a hole in the @2xxxx-@3xxxx address range to test the MCP's ability to configure around missing mods. The MCP handles that just fine.
  • Processor, which implements the instruction set. There can be two processors, but we have only tested the system with one thus far. The mechanisms to support a second processor have been implemented, but never tested.
  • I/O Unit, which is basically a DMA device that asynchronously manages input/output operations. There can be up to four of these on a system. We are currently running with two units, but need to test varying numbers. CentralControl is designed to adapt to the actual number automatically.
  • Peripheral controls, or drivers. With the exception of disk and datacom, these were implemented within the I/O cabinet and connected to the I/O Units through a peripheral exchange.
At present, we consider CentralControl and Memory to be functionally complete. Some additional work may be needed to support the maintenance display panels, but otherwise they are done.

The Processor is complete except for the double-precision multiply and divide operators. At present those have been stubbed out with their single-precision equivalents. It is likely some debugging may be necessary to get a second processor to work, but we won't know that until we try.

We currently have the SPO and Head-per-Track disk peripherals working. A small amount of coding needs to be done in the I/O Unit object to support new peripheral types, and a separate object must be created to support each peripheral type.

Nigel has taken on the task of implementing a line printer control, and I am ready to start work on one for the card reader. With those two in place, we will have a basic way to interact with the system in batch mode. That will be much nicer than typing control card images on the SPO, as we must do now, and will give us the ability to compile and run programs other than those available on Sid's tape images.

Next after that will be a control for tape drives. This will be a bit of a challenge. Getting data from the local file system into a web browser is easy, but getting data out of a browser into the local file system is, by design, extremely difficult. Tapes are input/output devices, and a reel of tape could be read in both forward and backward directions (of which the MCP sort intrinsic took advantage). We may need to import tape reels into a persistent disk mechanism as we currently have for the Head-per-Track disk, and find a way to export data out of that for output tape reels that need to be saved.

Datacom will also be a challenge. Not only is the datacom control complex (and from today's perspective, somewhat bizarre), but web browsers are by their nature client environments, and for datacom, the emulator needs to act as a server. Ideally we would like to implement something that current networking technology can connect to, e.g., Telnet NVT, but that may prove to be difficult. The Web Sockets protocol is worth looking into. At worst, we could probably implement some sort of network interface in a web server, and connect to that from the browser-hosted emulator environment using AJAX techniques.

The remaining peripheral types -- card punch, paper tape reader, and paper tape punch -- are much lower on the priority list. Unless we find a compelling reason, we may not implement paper tape devices at all.

We have the small operator console working, but would like to implement a full set of maintenance panels for the system. That is a lot of lights, and one concern is whether the performance will suffer from trying to refresh all of that state in real time.

Also on the horizon are ideas to implement the emulator in non-browser environments, such as Mozilla Rhino and Node.js. Those will probably offer richer I/O capabilities, but the user interfaces will need to be completely rethought. We have also discussed implementing the emulator in other languages. Nigel has expressed interest in Google's Dart, and no doubt to his dismay, I am still interested in Java.

It is too early to say much about the performance of the emulator. One of our goals has been to make it run at the speed of a real B5500. We have some rough indications that it is running slower than that, but within 50%. Once we can get our own programs compiled and running within the emulator, we will be in a better position to judge the actual performance, and to understand where some tuning and optimization may be required.

Parts of the foregoing may read as if we think the existing emulator components are fully debugged, but we know that is not the case. Based on the most recent problems we have seen, we suspect we are entering the stage where the easy bugs have been found, and the rest are going to be subtle, difficult to reproduce, difficult to trace, and otherwise downright nasty. The only way to flush those out is to push the system harder, and that is what we intend to do.

As I told Nigel, we have a couple of careers' worth of things that we can do yet with just this emulator, provided we aren't locked away first, muttering incoherently about MSCW F-register linkages, interrupt-driven T-register syllable injection, C-register update during Inhibit Fetch, and the like.