I'm reverse-engineering some code I wrote in the middle-90s for which the source is long-lost and I'm a bit baffled by some VGA code I've encountered. I think it's probably from library or 3rd party code as I was just learning computers then and, while I did include some assembly to interact with VGA, it wasn't this informed.

If it's helpful, the app is a 16-bit DOS real-mode exe and the original source was compiled by the Turbo Pascal compiler (either version 6 or 7).

    ; function boilerplate
    push bp
    mov  bp,sp
    call 0EE2:0530  ; stack bounds check function

    ; probe vga port 03CCh
    sub  sp,0002    ; why?
    mov  dx,03CC
    in   al,dx
    and  al,0C      ; mask bits 3 & 2
    cmp  al,04      ; al == 00000100b
    mov  al,00      ; pre return value
    jne  jump_label ; return 0
    inc  ax         ; return 1
    ; store return value in [bp-01] as well, for.. reasons.
    mov  [bp-01],al
    mov  al,[bp-01]

    ; function boilerplate    
    mov  sp,bp
    pop  bp
    retf 0004 ; instance pointer?

So the question is, what is the intent here? Two parts are confusing to me:

First, bits 2 and 3 denote clock select according to the VGA docs I've read, but those docs are light on information about what that means when bit 3 is involved. For example, http://www.osdever.net/FreeVGA/vga/extreg.htm#3CCR3C2W declares the two values with the bit 3 set as undefined.

This function seems to return 0 when bit 3 is set and bit 2 isn't. But, why? What is it trying to determine about the hardware?

Second, and this is an aside, but what is the intent of mov [bp-01],al followed by mov al,[bp-01]? This seems redundant!

1 Answer 1


First: the code is only checking bit 2 (bit 0,1,2) if 25 or 28Mhz clock is set

Second: maybe its redundant but can't say without original code - could be still a problem with your disassembler

retf 0004 ; instance pointer?

is a far return with pop of 4 bytes from stack

  • Ah yes, you're right. This function returns 1 only when the clock select bits are 01 (thus 28MHz or 360/720 width). It makes sense, if the bits were 11 then that would not be true so bit 3 must be preserved. I bet it's testing for the 9-pixel-wide-char VGA mode. Thank you!
    – Mike E
    Commented Jun 2, 2020 at 14:41

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