80-Bus News


May–June 1984, Volume 3, Issue 3

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sets that can be selected by means of an on-board DIL switch. (The character generator is actually intialised with the English character set, and then, depending on the DIL switch settings, a small sub-set of characters may be overwritten by their foreign language versions.)

The two standard alpha display modes are now 80 x 25, (80 characters by 25 lines) and 40 x 25. With the increase in the size of the alpha screen memory from 2k to 4k, (remember the other 4k is used as a character generator), it was decided to add the concept of multiple ‘screens’ I felt that this offered certain advantages over treating the 4k of memory as a single screen, although the latter would offer the possibility of being able to scroll back through the last 50 lines (or 100 lines in 40-wide format) that had been displayed. With the ‘multiple screen’ concept you can select (via an escape sequence) which of the available screens is to be displayed, and which is to be updated by the incoming data/​commands from the Host. My reasons for doing this I list below, but I must confess that reason (1) was perhaps a little selfish!

  1. It required a very small change to the existing software to support it.
  2. This feature had zero impact on the current performance of the SVC. (The alternative approach would have meant scrolling 4k of data, and also added the problem of handling any ‘locked’ screen lines.)
  3. It allows programs to set up hidden ‘help’ screens that can be switched to instantly.
  4. It simplifies (possibly) the implementation of concurrent tasks. (If you want to have two tasks running concurrently with independent screens on a system with an IVC there is the problem of how the undisplayed screen is handled. There are two options open: a) Queue all console output for the non-displayed task, wait until its screen image is swapped into the IVC, and then send it. Or b) Duplicate all the necessary parts of the IVC monitor in the system software so that the screen image can be updated immediately.)

When set into the graphics mode the SVC uses the entire 8kx8 screen memory as a 256 x 256 pixel display. (No colour, it’s black-and-white only I’m afraid!) As a result the RAM based character set is lost, although the default character set is still present within the monitor EPROM. The SVC will still display text in this mode, but the format is now 32 characters/​line and you are restricted to the English character set. (Each incoming ASCII character is used to index into the EPROM-based character generator table from whence the bit patterns are copied out into the screen RAM. I have not as yet included any check for special sub-set characters, nor the subsequent check of the DIL switch settings to see if these should be modified.) Any enterprising Dane or German who is irritated by this could swop the relavent dot patterns of the subset characters in the default table with those in their language table, not forgetting to exchange the words ‘English’ and ‘Danish’ (or whatever) where necessary in the manual.


The IVC only offered one attribute – the selection of the alternate character generator. This is normally loaded with the complement of the main character set to give video-inverse characters.

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