The offset value

[lcontador]

I´m tried to open a file to write in it, but in every place that I look for, they say to me that I need enter with the off set value, like in the bellow example .

mov ax,3d02h
mov dx,offset cs:fname ; DX=*FileName
int 21h ; DOS/FileOpen
jc errorlbl ; Jump On Errors

mov word ptr [offset cs:fname],ax ; BX=Handle
mov bx,ax

mov ax,4200h
xor cx,cx ; Segment
mov dx,23ceh ; Offset
int 21h ; DOS/FileSeekSet
jc errorlbl ; Error !

mov ax,4000h
mov bx,word ptr [offset fname] ; BX=Handle
mov cx,6 ; Lenght
mov dx,offset patch ; Buffer
int 21h ; DOS/WriteFile
jc errorlbl

mov ax,3e00h
mov bx,word ptr [offset fname] ; BX=Handle
int 21h ; DOS/CloseFile
jc errorlbl

My answer is : How can i found the offset value of a specifie instruction ?

 

[Prattaratt]

The first that catches my eye is this line:
mov dx,offset cs:fname ; DX=*FileName ????

The first thought that comes to mind is that you are trying to write a COM program where DS = CS in which case this will work much more efficiently:
MOV DX, OFFSET fname;

My second thought is that you declared fname as a variable in the code seg, in which case you need to save DS and then set DS = CS before performing the INT, and then restore DS before returning. But the CS segment overide is still not neccessary for the MOV DX, OFFSET fname.

 

[Guest_imported]

Hi.

Put a label before your opcode (instruction), and get the offset b using - offset label.

Example:
label1:
mov bx,10
mov ax,30
mov dx,offset label1

;At this point, dx will contain the offset of the instruction: mov bx,10 (the first byte).