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boot32.asm: fix, reset full ax after int 10h function 0Eh

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We discovered [1] that some ROM-BIOS implementations apparently
corrupt the ah register contents upon running int 10h function
0Eh so that writing only al afterwards isn't certain to set up
the registers for another function 0Eh call.

In the talk about this I referenced an earlier problem [2] in
which it was the al register, not ah, which got corrupted. So
all in all the int 10h function 0Eh call should be expected to
clobber the entirety of ax, alongside bp. (The bp corruption is
stated in the Interrupt List [3].)

[1]: https://github.com/SvarDOS/bugz/issues/174#issuecomment-3368497819
[2]: 977023f85c
[3]: https://fd.lod.bz/rbil/interrup/video/100e.html
This commit is contained in:
E. C. Masloch 2025-10-09 17:28:50 +02:00 committed by Kenneth J Davis
parent 56a86d2ad4
commit e3bfd6a6f0
1 changed files with 1 additions and 1 deletions
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boot/boot32.asm
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@ -223,7 +223,7 @@ c6:
boot_error: boot_error:
mov ax, 0E00h | '!' mov ax, 0E00h | '!'
int 10h ; display the error sign int 10h ; display the error sign
mov al, 07h mov ax, 0E07h
int 10h ; beep int 10h ; beep
xor ah,ah xor ah,ah
int 0x16 ; wait for a key int 0x16 ; wait for a key