Re: [Qemu-devel] [PATCH] compute checksum for roms bigger than a segment

[Glauber Costa <glommer@gmail.com>]

On Mon, Mar 23, 2009 at 9:48 PM, malc <av1474@comtv.ru> wrote:
> On Mon, 23 Mar 2009, Glauber Costa wrote:
>
>>    xor  ax, ax
>>    xor  bx, bx
>>    xor  cx, cx
>> +  xor  dx, dx
>> +
>>    mov  ch, [2]
>>    shl  cx, #1
>> +
>> +  jnc checksum_loop
>> +  mov  dx, cx
>> +  mov  cx, #0xffff
>
>     xchg dx, cx
>     dec  cx
>
>> +
>>  checksum_loop:
>>    add  al, [bx]
>>    inc  bx
>>    loop checksum_loop
>> +
>> +  cmp  dx, #0
>
>     test dx, dx

ok.

>
>> +  je checksum_out
>> +
>> +  add  al, [bx]
>
>     DS:[0] is added twice, intended?
it's not DS:[0]. It's the last byte from the previous segment that is
not caught by the loop
Note it happens before cleaning bx and increasing cs. It works with
it, does not without it.

>
>> +  mov  cx, dx
>> +  mov  dx, ds
>> +  add  dx, #0x1000
>
>     add  dh, #0x10

Why is it preferred?

-- 
Glauber  Costa.
"Free as in Freedom"
http://glommer.net

"The less confident you are, the more serious you have to act."