by Brutman » Wed Oct 17, 2012 6:19 am
The DOS 5 patch (and DOS 6.x, ...) tells the machine to lie about how much contiguous memory is available for DOS. It does it by setting the one BIOS memory area that holds the amount of contiguous memory is available for DOS to the same value as the total expansion memory.
Making the BIOS do that would let you boot an unmodified DOS 5 on the machine, assuming you still had the stacks 0,0 line in config sys. But you still have that hole where the video buffer is at 112K, so the lying part makes me uncomfortable. You still need a bootable device driver like jrConfig to move the video memory, and that makes the diskette not bootable on a standard machine too.
The patch really belongs in the boot sector of any diskette or media you are going to boot ... The Racore expansion units were able to do it in hardware, but they also had the code to move (and neuter) the video memory. If the BIOS were to move the video memory then it would be acceptable to do both in hardware, but moving the video memory also potentially limits the machine unless we just make the video buffer a fixed 32K. (It could be grown much bigger than that, but I doubt anything ever set more than 64K.)