Re: Calculating checksums...

Tim Roberts <spamtrap@[EMAIL PROTECTED]
> wrote in part:
> 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

Small bug?  Shouldn't those last instructions use 
dl as source, not al?  And isn't the ds: redundant?
I presume DF is correctly cleared [upward].

-- Robert