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:
>
>> Some option roms (e1000 provided by gpxe project as an example)
>> are bigger than a segment. The current algorithm to compute the
>> checksum fails in such case. To proper compute the checksum, this
>> patch deals with the possibility of the rom's size crossing a
>> segment border.
>>
>> We don't need to worry about it crossing more than one segment
>> border, since the option roms format only save one byte to store
>> the image size (thus, maximum size = 0xff = 128k = 2 segments)
>>
>> Signed-off-by: Glauber Costa <glommer@redhat.com>
>> ---
>>  bios/rombios.c |   33 +++++++++++++++++++++++++++------
>>  1 files changed, 27 insertions(+), 6 deletions(-)
>>
>> diff --git a/bios/rombios.c b/bios/rombios.c
>> index bc43251..6fede17 100644
>> --- a/bios/rombios.c
>> +++ b/bios/rombios.c
>> @@ -10162,22 +10162,43 @@ no_serial:
>>    ret
>>
>>  rom_checksum:
>> -  push ax
>> -  push bx
>> -  push cx
>> +  pusha
>> +  push ds
>> +
>>    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
I actually skipped this one.

Ok for xchg, but cx at this time holds the remaining bytes, so dec cx won't
have the effect you think it will.


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

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