Tuesday, 19 January 2016

Of ROMs, PROMs and EPROMs....

So…. Looking at the resulting MINOL ROM, it’s about 2,500 bytes or thereabouts in total.

Thinking in hardware terms, how you would actually put this on a machine - this isn’t good. I’m wasting 1/2k of ROM here – now, that isn’t much in 2016 terms, but in 1975 terms it is a heck of a lot.

So I have decided on some reorganisation.

Out from the monitor ROM go the Mathematics routines – which now aren’t used by anything – and into the monitor ROM goes the scrolling screen handler.

I’ve tweaked it a bit so there are vectors at 3 (Print Char/String) 5 (Get Char) 7 (Get String) and obviously changed MINOL so it calls these routines directly. This is also handy in that I can call these routines from MINOL if required.

The monitor ROM now has two sets of screen drivers – the roll to the top and the scrolling version. It also has two keyboard input routines, but this isn’t really worth fixing.

There’s one problem ; I fixed up the integer printer so when it is left it sets P3 to the Print routine, so this can’t happen any more – the Print routine is elsewhere – so this is set up manually after the three calls to this routine.

It works quite nicely, and each ROM image (both 2 x 1k ROMs) has about 128 bytes of free program space, which is handy if there are any messy bugs.

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