DOS ain't dead

finessing 16-bit (Developers)

posted by kerravon , Sydney, Free World South, 23.04.2025, 23:42

I previously proposed an extension to the MZ format for eventual use in PM16:

https://forum.osdev.org/viewtopic.php?t=57259

I'm getting closer to having CM16 working (it is currently working with kludges and limits as POC).

And I was rethinking this.

If an MSDOS program was to use compact or large (to get large data), and linked with a linker that creates nice segments, then the code relocations should be very obvious - ie multiples of 4000.

If I get a linker (ld86/slink) to pad the code segments other than the last out to 64k with NULs, I will have the segment boundaries. And the DGROUP relocation will be obvious because it is the highest value.

Microsoft C 6.0A has an "Aw" option - SS != DS and DS is not reloaded. (Au would reload DS).

So if I accept the stack that MSDOS gives me, and set DS once (or indeed, I probably don't even need that restriction), then the program will work under MSDOS, and also be something that I can run in CM16.

And don't attempt to resize the executable either - instead rely on maxalloc being set sensibly in the EXE header. Although even that request is something that could be simply ignored by the PM16/CM16 OS.

I'm not trying to get things to work in EVERY case - I'm trying to get them to work in ANY case AT ALL - or rather, a class.

Basically I need to know how - in hindsight - to properly write MSDOS programs, to future-proof them.

Huge memory model would still require some sort of change to the executable format though - and NE would be good enough for that, and try to work within the limits of Euro MSDOS 4.

Note that in my original timeline I never used huge. I was using Borland C anyway, where huge wasn't useful for my (current) purposes anyway. So getting compact and large working would be cool, now that I can see a purpose for splitting DS and SS.

Complete thread:

• finessing 16-bit - kerravon, 23.04.2025, 23:42 (Developers)
  • finessing 16-bit - kerravon, 24.04.2025, 00:49
    • finessing 16-bit - kerravon, 24.04.2025, 11:47
  • finessing 16-bit - kerravon, 24.04.2025, 16:04