Re: Software for MS-DOS 1.25

Den Thu, 12 Oct 2017 13:18:02 +0200 skrev Michał Pleban
<lists@michau.name>:
> Hello!
> 
> Baltissen, GJPAA (Ruud) wrote:
> 
> > I run MS-DOS 6.22 on my PC20-III which is PC-XT compatible.
> > Normally 3.3 would be a better choice because it uses less memory.
> > Its disadvantage is that only supports FAT12 which means the
> > maximum hard disk drive partition is 32 MB. My PC20 has an IDEXT
> > card with an 8 GB HDD. And then 32 MB is rather small.
> 
> Let's say I will be worrying about it when I find a way to attach a 8
> GB hard disk to the 710 (and make it accessible from the 8088
> side) ;-) I think we shouldn't set the bar too high at the beginning,
> so it's MS-DOS 3.3 for now.

I'd suggest starting with DOS 1.1 or whichever version that the source
code is available for. That way (I think) it would be far easier to
debug. Just outputing debug messages (from a recompiled/reassembled DOS
1.1 IO.SYS) at the various stages during boot would probaly be a useful
feature.

> > Oops, just popped up in my mind: I replaced the PAL that controls
> > the RAS/CAS signals to the DRAMs in an original IBM XT with a GAL
> > and replaced the two banks of 4164 DRAMs with one row of 41256s. It
> > now has the usual 640 KB plus 128 KB from 0D0000h to 0F0000h. A
> > special program, USE!UMBS, enables me to use UMB on this XT. And
> > thus profit from the extra memory available under the 640 KB limit
> > when using 6.22.
> 
> IIRC, a similar method is used to upgrade the memory in the 710 to 1
> MB, correct?

Such upgrade would be good for running a periodic code to copy emulated
PC display card over to the CBM display hardware.

> > My first impression: you are absolutely right, what we really need
> > could well fit in 4 KB. And chapeau for thinking of the 2364. But
> > then: do we need to alter the ROM at all? I don't know what the
> > original boot program exactly does but IMHO it loads IO.SYS and
> > tells the 8088 to take over and start it. IO.SYS then sets the INT
> > vectors. And IMHO it will overwrite the ones you set in ROM.
> 
> The problem here is that in MS-DOS, IO.SYS is loaded at a fixed
> location starting from 0070:0000. I tried modifying the boot sector
> to load it somewhere else and yet it's still relocating itself there.
> 
> So if we wanted to use the IO.SYS as is, then we would have to either
> (a) fit all the code below segment 0070 (not possible) or (b) place
> the code at the top of the RAM where it would be easily overwritten.
> 
> That's why I would like to place the code in the ROM on the card.
> There's all this 3 kB of wasted space there ;-)

I'd suggest some kind of loader that loads in top of memory and patches
the various INT vectors and emulates a PC BIOS good enough to run a DOS
version for IBM PC.

AFAIK DOS doesen't test how much RAM the machine has, it either reads
some specified memory localtion or calls some BIOS INT to figure that
out, so it should be easy to just hide the top of memory.

That way atleast in theory it would be possible to run all MS-DOS
versions for PC that works on any 100% IBM compatible PC, i.e. atleast
up to DOS 6.x.

Of course changing a ROM is far easier than any other mod, but it's
still a mod and if you go down that path you might as well add the few
bits and pieces that's required to use ISA cards. Then you have could
have both hard disk and a graphic card that's 100% IBM compatible...




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