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XT-IDE BIOS ponderings (Users)

posted by mceric, Germany, 04.02.2024, 20:52

Given that it has been mentioned that the Book8088 XT-IDE BIOS is involved in a crash triggered by the FreeDOS kernel and/or FDISK when they attempt to ask int 13.41 more than once whether EBIOS LBA support exists, which means that you can probably use SYS CONFIG to disable LBA checks and attempts in the FreeDOS kernel as a workaround until something in one of the BIOS handlers gets fixed:

> I changed the CPU for 86Box from V20 to 8088. Same effect, system hangs...
I have been digging around in the XT BIOS sources a bit. They basically assume that V20 supports all 186 opcodes such as pusha, popa, immediate push, immediate shifts etc. but no 386 opcodes such as movzx.

One odd detail here:

https://www.xtideuniversalbios.org/browser/xtideun...IDE_Universal_BIOS/Src/Handlers/Int13h.asm#L122

You do not have to use a convoluted macro for an immediate shift by ONE bit because all CPU support that. In addition, the macro has the strange property that it depends on CHECK_FOR_UNUSED_ENTRYPOINTS, why that?

https://www.xtideuniversalbios.org/browser/xtideun...ios/trunk/Assembly_Library/Inc/Emulate.inc#L660

However, if that were broken, nothing would work, so that is NOT why int 13.41 causes instability if MODULE_EBIOS for LBA support is not compiled in.

I do not know whether the XT-IDE in question is compiled with local stack in BDA. Probably not, because that would be 286+ according to code comments.

So you basically get an optional CLD, lots of registers and some local stack frame get pushed and allocated, a BIOS DS and DPT pointer get loaded (NUL for unknown DL), bx=2*ah, ah gets compared to 25 and either a jump table is processed or you go to unsupported function, which uses a stack trampoline to call whatever the int13 handler outside XT-IDE was. Then most registers are copied to the stack of XT-IDE, DS is set, IF and CF are processed on stack, the local frame is released, lots of registers get restored from stack and you end with an IRET. Seems all sane at first glance. There also are checks for DL, only 13.08 is always hooked, the rest depends on whether the disk is XT-IDE handled.

Makes me wonder what the original BIOS int13 handler WITHOUT XT-IDE will do for int13.41 LBA EBIOS support check calls?

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FreeDOS / DOSEMU2 / ...

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