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Date: Mon, 23 Mar 2009 22:02:53 -0300
Message-ID: <5d6222a80903231802r648aa025ieef9f61008fd1a37@mail.gmail.com>
Subject: Re: [Qemu-devel] [PATCH] compute checksum for roms bigger than a
	segment
From: Glauber Costa <glommer@gmail.com>
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To: qemu-devel@nongnu.org

On Mon, Mar 23, 2009 at 9:48 PM, malc <av1474@comtv.ru> wrote:
> On Mon, 23 Mar 2009, Glauber Costa wrote:
>
>> =C2=A0 =C2=A0xor =C2=A0ax, ax
>> =C2=A0 =C2=A0xor =C2=A0bx, bx
>> =C2=A0 =C2=A0xor =C2=A0cx, cx
>> + =C2=A0xor =C2=A0dx, dx
>> +
>> =C2=A0 =C2=A0mov =C2=A0ch, [2]
>> =C2=A0 =C2=A0shl =C2=A0cx, #1
>> +
>> + =C2=A0jnc checksum_loop
>> + =C2=A0mov =C2=A0dx, cx
>> + =C2=A0mov =C2=A0cx, #0xffff
>
> =C2=A0 =C2=A0 xchg dx, cx
> =C2=A0 =C2=A0 dec =C2=A0cx
>
>> +
>> =C2=A0checksum_loop:
>> =C2=A0 =C2=A0add =C2=A0al, [bx]
>> =C2=A0 =C2=A0inc =C2=A0bx
>> =C2=A0 =C2=A0loop checksum_loop
>> +
>> + =C2=A0cmp =C2=A0dx, #0
>
> =C2=A0 =C2=A0 test dx, dx

ok.

>
>> + =C2=A0je checksum_out
>> +
>> + =C2=A0add =C2=A0al, [bx]
>
> =C2=A0 =C2=A0 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.

>
>> + =C2=A0mov =C2=A0cx, dx
>> + =C2=A0mov =C2=A0dx, ds
>> + =C2=A0add =C2=A0dx, #0x1000
>
> =C2=A0 =C2=A0 add =C2=A0dh, #0x10

Why is it preferred?

--=20
Glauber  Costa.
"Free as in Freedom"
http://glommer.net

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