video address/script issues, PLEASE HELP

[Wannabe Developer]

Below I have a script that's supposed to place a certain colored pixel at a given position (address)...but I get the error message "error: beroset-p-637-invalid effective address" at line 28. Any help on this would be nice, many thanks!

color db 0xff0000         ; 27 - pixel color
position db 0xa000, 0000      ; 28 - pixel position
label1: mov ebp, [color + position]   ; 29 - write (color, position->ebp)

%macro clearreg 1-*         ; 31 - clear register
   %rep %0            ; 32
      push %1         ; 33
   %rotate 1         ; 34
   %endrep            ; 35
%endmacro            ; 36

offset db 0x13            ; 38 - video memory offset
label2: mov 0xa000, [offset + ebp]   ; 39 - write (ebp->video memory)

%macro clearvid 1-*         ; 41 - clear video memory
   %rep %0            ; 42
      push %1         ; 43
   %rotate 1         ; 44
   %endrep            ; 45
%endmacro            ; 46

[Keith Kanios]

I think that you are off by one line in your source code counting, and I believe that this is the line in question:

Code: [Select]

label1: mov ebp, [color + position]   ; 29 - write (color, position->ebp)

As a rule, don't use two potentially relocatable addresses (e.g. any label/address that can be move around by optimization) as a combined memory reference.

Within the context of your code, you are probably trying to achieve the following:

Code: [Select]

color dd 0xff0000         ; 27 - pixel color
position dd 0xa0000000      ; 28 - pixel position
label1: mov ebp, DWORD[color]   ; 29 - write (color->ebp)
add ebp, DWORD[position]   ; 30 - add (position->ebp)

%macro clearreg 1-*         ; 32 - clear register
   %rep %0            ; 33
      push %1         ; 34
   %rotate 1         ; 35
   %endrep            ; 36
%endmacro            ; 37

offset dd 0x13            ; 39 - video memory offset
label2: add ebp,DWORD[offset] ; 40 add (offset->ebp)
mov DWORD[0xa000],ebp ; 41 - write (ebp->video memory)

%macro clearvid 1-*         ; 42 - clear video memory
   %rep %0            ; 43
      push %1         ; 44
   %rotate 1         ; 45
   %endrep            ; 46
%endmacro            ; 47

Note that there are quite a few more mistakes in your code besides the ones I implicitly corrected via the above example.

I would suggest (re)reading the NASM Documentation in regard to memory references and the like.

[Wannabe Developer]

I think that you are off by one line in your source code counting, and I believe that this is the line in question:

Code: [Select]

label1: mov ebp, [color + position]   ; 29 - write (color, position->ebp)

As a rule, don't use two potentially relocatable addresses (e.g. any label/address that can be move around by optimization) as a combined memory reference.

Within the context of your code, you are probably trying to achieve the following:

Code: [Select]

color dd 0xff0000         ; 27 - pixel color
position dd 0xa0000000      ; 28 - pixel position
label1: mov ebp, DWORD[color]   ; 29 - write (color->ebp)
add ebp, DWORD[position]   ; 30 - add (position->ebp)

%macro clearreg 1-*         ; 32 - clear register
   %rep %0            ; 33
      push %1         ; 34
   %rotate 1         ; 35
   %endrep            ; 36
%endmacro            ; 37

offset dd 0x13            ; 39 - video memory offset
label2: add ebp,DWORD[offset] ; 40 add (offset->ebp)
mov DWORD[0xa000],ebp ; 41 - write (ebp->video memory)

%macro clearvid 1-*         ; 42 - clear video memory
   %rep %0            ; 43
      push %1         ; 44
   %rotate 1         ; 45
   %endrep            ; 46
%endmacro            ; 47

Note that there are quite a few more mistakes in your code besides the ones I implicitly corrected via the above example.

I would suggest (re)reading the NASM Documentation in regard to memory references and the like.

Thanks, I'll try it out!