Re: Software for MS-DOS 1.25

Den Thu, 19 Oct 2017 12:48:56 +0200 skrev "Anders Carlsson"
<anders.carlsson@sfks.se>:
> Michau wrote:
> 
> > I'm pretty sure demos would rather use graphics mode, which we
> > cannot emulate anyway - we are emulating the MDA graphics card.
> 
> Or at least ANSI text mode with colours and other tricks, which also
> is beyond the emulation. One could possibly imagine some early text
> based demonstations, similar to the Cursor animations for the PET,
> but running on an IBM PC. Then again the IBM character set is far
> more limited graphically than the Commodore characters so MDA
> would've been an even less attractive format for demos/animations
> than the 40 and 80 column PETs (or the CBM-II natively) were.

Well, it might be a good idea to atleast be able to run text-mode CGA
programs even if most attribute data would have to be dropped.
Atribute data would even have to be dropped when emulating MDA as it
has inverse video, underline and brightness which is more than the
CBM-II can display anyway. Thus there is anyway need for software
configurable emulation of the different attributes, to make sure that
no software is hard to use because a menu selection or a curso is
invisible.

One reason for emulating CGA text mode instead of MDA text mode is
that you can get 768k instead of 704k continous RAM from $0 and upwards.

I'm not sure if it would work right out of the box to patch the BIOS to
tell PC-DOS that there is more than 640k RAM, but there surely exists
some hacks to use more memory.

Also being able to emulate both MDA and CGA text mode makes it a bit
more flexible if an ISA slot would be added at some point and someone
wants to put in an ISA display card there. That way the ISA card could
use be either CGA or MDA and the software could emulate the other kind
simultaneously. Afaik that was used by debugger software. Maybe if we
could get Xenix for 8088 to run it might use it for different virtual
consoles. (I'm 100% sure that Xenix 286 uses MDA and CGA for some
virtual consoles each, making it possible to see two virtual consoles
at the same time).

But this is mostly options that can be added at a later stage and which
doesen't require much preparation (except for however any additional
hardware turns out - but it seems almost strange to add memory that's
not in increments divisible by 64k, so any added memory would anyway be
in both MDA and CGA adress space...).


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