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Sunday, January 19, 2014

Emulator Version 0.19 Released

Nigel and I are pleased to announce that version 0.19 of the retro-B5500 emulator was released on 10 January. 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 supports the ability to write to magnetic tape and to persist tape image data created by the emulator.

Background on Magnetic Tape Images

The emulator has had an initial magnetic tape implementation for the past few months. That initial implementation supported only reading tape images, and only from images in the ".bcd" format. This release expands on that initial one by supporting additional tape image formats and the ability to write to a tape image.

Tape images exist in the form of files in your local file system. The initial implementation allowed you to select an image file, which the emulator would load into memory and then parse into blocks in response to read commands from the I/O Unit. The ".bcd" format was originally supported because that was the one used for Sid McHarg's Mark XIII Burroughs release tapes.

Since the retro-B5500 emulator runs in a web browser, we must live with some limitations on the way that the browser is allowed to interact with your local file system. Modern browsers can read just about any file the user can select from a "file picker" form control, but at present there is not a useful way for a browser to write files under program control to the local file system. This restriction exists for very good security reasons -- without it, hackers would be able to play havoc with your local files -- but it makes getting data out of the browser environment difficult.

There are File System and FileWriter APIs that have been proposed as part of the HTML5 standards initiative, but they have two problems. First, only the Google Chrome and Opera browsers presently support them. Second, the files they create are "sand-boxed," i.e., the files exist within a virtual file system that is managed by the browser. Different browsers may format and manage this virtual file system differently. Worse, the files are generally hidden deep within the user profile directory used by the browser, are not named in obvious ways, and are difficult to find and use outside of the browser environment.

The only reasonably practical way we have found to output data from a browser is to format the data as text within a window and allow the user either to save the contents of that window to their local file system, or to copy/paste the contents into another program, such as a text editor, and save it from there. This process is neither automatic, nor very convenient for the user, but it is safe, and it works with all browsers.

Therefore, this new implementation for magnetic tape supports reading multiple tape image formats, but will output tape image data in only one format, ASCII text, as described in more detail below.

Loading Tape Images

Within the emulator, all tape image files are loaded in their entirety into a memory buffer. All tape reads and writes operate on that memory buffer. Depending on the block size used, a 2400-foot reel of 7-track tape could hold between about two and 22 million characters. It would probably be appalling to the original designers of the B5000 and B5500 that we would implement tape devices by sucking all of the data into "core" memory and operating on it exclusively there, but that turns out to be the easiest and most effective way to do it.

To support multiple tape image formats, the process of loading an image into a tape drive has changed slightly from the original implementation. The file picker control on the tape drive window has been replaced by a text box that will display the name of the image file after it is loaded. When you click the LOAD button on the tape drive window, a small dialog window now opens instead of activating the file picker as before. This window contains the file picker, a Format drop-down list to select the image format, a Write Ring check box to indicate whether the image is writable, and a Tape Length drop-down list to select the length of the tape reel:


To load an existing tape image, simply select it using the file picker. The emulator will attempt to infer the image format from the file name extension, as follows:
  • ".bcd" implies a ".bcd" tape image.
  • ".tap" is reserved for supporting the "tap" (or "taput") format in the future. See http://simh.trailing-edge.com/docs/simh_magtape.pdf for a specification of this format. It is not implemented at present.
  • All other extensions presently imply the "ASCII Odd Parity" format described below under Unloading Tape Images.
You can override the inferred tape format by making a selection from the Format drop-down list. Regardless of the format of a tape image you load into the emulator, it converts the image to the ".bcd" format and uses that format internally. The options presently available are:
  • (blank tape) -- This will load a blank tape image into the drive. Any file you previously selected with the file picker will be ignored.
  • .bcd image -- This will interpret the bytes in the selected file as a ".bcd" image and load it into the drive without conversion.
  • ASCII Even Parity -- This will interpret the bytes in the selected file as ASCII characters and convert them to an internal ".bcd" image with even-parity BCL codes.
  • ASCII Odd Parity -- This will interpret the bytes in the selected file as ASCII characters and convert them to an internal ".bcd" image with odd-parity B5500 Internal Code (BIC) codes.
Next to the Format list is a check box labeled Write Ring. If this box is checked, the internal tape image will be writable. The tape image on disk will not be affected by writing unless you later save the image over the original file when the tape is unloaded. If you merely want to read the tape, leave this box unchecked.

The last control is another drop-down list labeled Tape Length. It is relevant only when Write Ring is checked and is disabled otherwise. This specifies the length of tape on the virtual "reel" loaded into the drive. It defaults to 2400 feet, but lengths of 1200 and 600 feet are also selectable. The primary purpose of the length is to allow the drive to report it has passed the reflective EOT (end of tape) marker, which will signal the MCP that it is near the near to the end of the tape and that it should switch to a new reel. When Write Ring is not checked, the length of the tape is inferred from the size of the tape image data.

To load a blank tape into the drive, do not select a file using the file picker. Select a format of "(blank tape)," which is the default. The Write Ring box will be checked automatically and the tape length will default to 2400 feet. You may select a different tape length, if you wish. Any image file you previously selected with the file picker will be ignored.

If the Format list has a value other than "(blank tape)," you must select a file before clicking the OK button. When a file is selected using the file picker, the Format value is inferred from the file name extension, the Write Ring box is unchecked, and the Tape Length list is disabled. You may then alter the values of these controls before proceeding. The tape image is not loaded into memory until you click the OK button.

Unloading Tape Images

When a tape image is loaded with the Write Ring box unchecked, the image will be read-only, and writing to it will be inhibited. Later, when you click the UNLOAD button, that image data will simply be discarded by the drive, as it is still in the file from which it was loaded.

When the Write Ring box for an image is checked, the image will be writable, whether it was loaded as a blank tape or from an existing image file. Later, when you click the UNLOAD button, two things can happen:
  1. If the tape was not actually written to, the emulator will simply discard the image data, as for the case of a read-only image above.
  2. If the tape was written to, the emulator will ask whether you want to save the altered image data.
    • If you cancel out of this dialog, the image data will be discarded.
    • If you reply OK, however, the emulator will then open a new window and format the contents of the tape image as ASCII text. This may take several seconds for a full, 2400-foot reel of tape.
    • After the tape image text is formatted in the window, you may save it or copy/paste the data into another program. If you paste the text into another program, make sure that program will not trim trailing spaces from lines or compress multiple spaces using tab characters.
    • When you are finished with the image data, simply close the window.
 The ASCII tape image format is very simple:
  • The six-bit BCL or BIC character codes are translated to ASCII using the emulator's standard convention for the B5500 characters that do not have ASCII equivalents ("~" for left-arrow, "|" for the multiplication sign, "!" for not-equal, "{" for less-than-or-equal, and "}" for greater-than-or-equal).
  • Each tape block becomes one line of ASCII text. Lines are delimited by line-feed or carriage-return/line-feed characters, depending on the convention used by your browser.
  • An end-of-file (tape mark) is represented by a block containing a single "}" character.
  • All other blocks should be at least seven characters long, although the emulator does not enforce this.
Both even- and odd-parity tape image data converts to the same ASCII representation. The parity data is lost during this conversion.

An ASCII tape image may be loaded into a tape drive later as either "ASCII Even Parity" or "ASCII Odd Parity" format. These two formats will apply the appropriate parity to the data when it is converted to the ".bcd" format used by the drive internally.

Because the ASCII tape image format is just text, you can create tape images outside the emulator, either programmatically, or manually using an ordinary text editor.

See the Using the Magnetic Tape Drive wiki for more details on tape drive operation and the format of currently-supported tape image files.