I looked at int 15h lately, int 15h is not supported on older BIOS versions
but every BIOS has it in the time called now. :)
Error code 86h means "function not supported" and if you get 80h it means
"invalid command"
Op woensdag 20 juli 2016 heeft Eric Auer <e.a...@jpberlin.de> het volgende
geschreven:
>
> Hi experts,
>
> some interesting observations about an old 486 PC in the thread
> with Dimitris about "Which freedos on 486" on freedos-user...
>
> His BIOS answers with AH=86h, carry=UNCHANGED to int 15.e801,
> also leaving AL unchanged. So it seems to be a good idea to
> STC before INT 15 when doing the 15.e801 call :-) Note that
> AH=86 is a common failure code for int 15 functions. I would
> also suggest to, in addition, treat AH=UNCHANGED as failure.
>
> Another issue is that RBIL says for this call that if AX=BX=0,
> one should use CX and DX as memory size info instead. However,
> DOSEMU only sets AX and BX and returns CX=DX=0 while FreeDOS
> HIMEMX and JEMMEX both look ONLY at CX and DX, without even
> checking for AX and BX! Luckily DOSEMU also supports the two
> other int 15 calls for memory info afair, but it seems tricky
> that DOSEMU and FreeDOS disagree about 15.e801, no?
>
> Also, it is interesting that the FreeDOS drivers say that if
> CX is not 3c00 (the max value) then DX must be treated as 0.
>
> This sounds like "memory hole below 16 MB means end of RAM"?
>
> Finally, JEMMEX and HIMEMX both say "if CX plus scaled DX < 64
> then fall back to 15.88", which is a somewhat weird way to say
> "if CX and DX are both 0".
>
> RBIL writes about int 15.88 versus the carry flag:
>
>> the standard BIOS only returns memory between 1MB and 16MB; use AH=C7h
>> for memory beyond 16MB
>
>> not all BIOSes correctly return the carry flag, making this call
>> unreliable unless one first checks whether it is supported through
>> a mechanism other than calling the function and testing CF
>
> Interestingly, JEMMEX and HIMEMX decide to trust int 15.88
> even if it leaves carry unchanged: They CLC before int 15.
>
> I think it would be a good idea to check if AH=80, 86 or 88
> after the int 15 call even if carry is not set, as all those
> values would be possible return values if int 15.88 failed.
>
> What is confusing: JEMMEX works on the mentioned 486, while
> HIMEMX does not work, not even with /NOABOVE16 and /X, why?
>
> Regards, Eric
>
> PS: In theory, int 15.c7 would also be a possible call to get
> info about memory beyond the first 16 MB without 15.e??? BIOS
> support, but that would only help for a very specific group
> of PS/2 machines identified by int 15.c0 - not very useful.
>
> PPS: The same PC also fails to boot from FAT32 with any SYS
> newer than 2.6, so SYS apparently sees LBA support while it
> does not really boot later? See freedos-user, still ongoing.
> Unclear regarding SYS 3.6e /FORCE:LBA and /FORCE:CHS options.
>
>
>
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