Re: Software for MS-DOS 1.25

From: Michał Pleban <*lists_at_michau.name*> Date: Thu, 12 Oct 2017 22:05:45 +0200 Message-ID: <59DFCB19.4050108@michau.name> · lists_at_michau.name

Michał Pleban wrote:

> So I am pretty sure it DOES hook into original INT 13h and not overwrite
> it. In the evening I will run PCem to verify this.

I just debugged MS-DOS on PCem and it seems I was right.

The patched INT 13h checks if a function 08 or 15 is called, if yes,
converts them to function 01. Otherwise, it calls the original INT 13h
handler.

After that it checks if the original BIOS returned "DMA boundary
exceeded", if yes, tries to work it around. If not, it just returns.

So basically, since we are not concerned with DMA, the DOS just calls
the original INT 13h and that's it.

Regards,
Michau.

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