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Re: [Qemu-devel] [PATCH] compute checksum for roms bigger than a segmen
From: |
Glauber Costa |
Subject: |
Re: [Qemu-devel] [PATCH] compute checksum for roms bigger than a segment |
Date: |
Mon, 23 Mar 2009 22:02:53 -0300 |
On Mon, Mar 23, 2009 at 9:48 PM, malc <address@hidden> 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."
[Qemu-devel] [PATCH] compute checksum for roms bigger than a segment, Glauber Costa, 2009/03/25