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SubjectRe: [PATCH] x86-boot: don't request VBE2 information

* Peter Korsgaard <jacmet@sunsite.dk> wrote:

> The new x86 setup code (4fd06960f120) broke booting on an old
> P3/500MHz with an onboard Voodoo3 of mine. After debugging it, it
> turned out to be caused by the fact that the vesa probing now asks for
> VBE2 data.
>
> Disassembing the video BIOS shows that it overflows the
> vesa_general_info structure when VBE2 data is requested because the
> source addresses for the information strings which get strcpy'ed to
> the buffer lie outside the 32K BIOS code (and hence contain long
> sequences of 0xff's).
>
> E.G.:
>
> get_vbe_controller_info:
> 00002A9C 60 pushaw
> 00002A9D 1E push ds
> 00002A9E 0E push cs
> 00002A9F 1F pop ds
> 00002AA0 2BC9 sub cx,cx
> 00002AA2 6626813D56424532 cmp dword [es:di],0x32454256 ; "VBE2"
> 00002AAA 7501 jnz .1
> 00002AAC 41 inc cx
> .1:
> 00002AAD 51 push cx
> 00002AAE B91400 mov cx,0x14
> 00002AB1 BED47F mov si, controller_header
> 00002AB4 57 push di
> 00002AB5 F3A4 rep movsb ; copy vbe1.2 header
>
> 00002AB7 B9EC00 mov cx,0xec
> 00002ABA 2AC0 sub al,al
> 00002ABC F3AA rep stosb ; zero pad remainder
>
> 00002ABE 5F pop di
> 00002ABF E8EB0D call word get_memory
> 00002AC2 C1E002 shl ax,0x2
> 00002AC5 26894512 mov [es:di+0x12],ax ; total memory
> 00002AC9 26C745040003 mov word [es:di+0x4],0x300 ; VBE version
> 00002ACF 268C4D08 mov [es:di+0x8],cs
> 00002AD3 268C4D10 mov [es:di+0x10],cs
> 00002AD7 59 pop cx
> 00002AD8 E361 jcxz .done ; VBE2 requested?
> 00002ADA 8D9D0001 lea bx,[di+0x100]
> 00002ADE 53 push bx
> 00002ADF 87DF xchg bx,di ; di now points to 2nd half
> 00002AE1 26C747140001 mov word [es:bx+0x14],0x100 ; sw rev
>
> 00002AE7 26897F06 mov [es:bx+0x6],di ; oem string
> 00002AEB 268C4708 mov [es:bx+0x8],es
> 00002AEF BE5280 mov si,0x8052 ; oem string
> 00002AF2 E87A1B call word strcpy
>
> 00002AF5 26897F0E mov [es:bx+0xe],di ; video mode list
> 00002AF9 268C4710 mov [es:bx+0x10],es
> 00002AFD B91E00 mov cx,0x1e
> 00002B00 BEE87F mov si,vidmodes
> 00002B03 F3A5 rep movsw
>
> 00002B05 26897F16 mov [es:bx+0x16],di ; oem vendor
> 00002B09 268C4718 mov [es:bx+0x18],es
> 00002B0D BE2480 mov si,0x8024 ; oem vendor
> 00002B10 E85C1B call word strcpy
>
> 00002B13 26897F1A mov [es:bx+0x1a],di ; oem product
> 00002B17 268C471C mov [es:bx+0x1c],es
> 00002B1B BE3880 mov si,0x8038 ; oem product
> 00002B1E E84E1B call word strcpy
>
> 00002B21 26897F1E mov [es:bx+0x1e],di ; oem product rev
> 00002B25 268C4720 mov [es:bx+0x20],es
> 00002B29 BE4580 mov si,0x8045 ; oem product rev
> 00002B2C E8401B call word strcpy
>
> 00002B2F 58 pop ax
> 00002B30 B90001 mov cx,0x100
> 00002B33 2BCF sub cx,di
> 00002B35 03C8 add cx,ax
> 00002B37 2AC0 sub al,al
> 00002B39 F3AA rep stosb ; zero pad
> .done:
> 00002B3B 1F pop ds
> 00002B3C 61 popaw
> 00002B3D B84F00 mov ax,0x4f
> 00002B40 C3 ret
>
> (The full BIOS can be found at http://peter.korsgaard.com/vgabios.bin
> if interested).
>
> The old setup code didn't ask for VBE2 info, and the new code doesn't
> actually do anything with the extra information, so the fix is to
> simply not request it. Other BIOS'es might have the same problem.

thanks Peter, very nice debugging! I've applied your fix.

Ingo


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