Re: Calculating checksums...

Alex Buell  <spamtrap@[EMAIL PROTECTED]
> wrote:

>I have a snippet of FASM code:
>
>; The last byte (512th) will be the patch_byte for the checksum
>; patch_byte is calculated and automagically inserted below
>PREV_CHKSUM = 0
>repeat $
>   load CHKSUM byte from %-1
>   CHKSUM = (PREV_CHKSUM + CHKSUM) mod 0x100
>   PREV_CHKSUM = CHKSUM
>end repeat
>store byte (0x100 - CHKSUM) at ($-1)  ; store the patch_byte
>
>How would you suggest I could rewrite this in NASM?

This is one of the easier algorithms to code in assembly.

    mov  cx, 511
    xor  dl,dl
again:
    lodsb
    add  dl,al
    dec  cx
    jnz  again
    neg  al
    mov  ds:[si],al
-- 
Tim Roberts, timr@[EMAIL PROTECTED]
 & Boekelheide, Inc.