Calculating the Checksum
Don.F.Broadus at ucm.com
Don.F.Broadus at ucm.com
Fri Sep 11 19:24:14 GMT 1998
Could you please explain checksum for me. I thought checksum was a
hex value
That was used to verify if an EPROM had the correct program data.
Thanks Don
-----Original Message-----
From: Roger Heflin [SMTP:rah at horizon.hit.net]
Sent: Friday, September 11, 1998 12:55 PM
To: diy_efi at efi332.eng.ohio-state.edu
Subject: Re: Calculating the Checksum
A couple of questions. I have a 93 Z28 (32kb) eprom. I am working
on the
disassembly. I have put the eprom in memory at several locations,
the
location that seems to work is $8000, with it there, the code seems
to
be writing to things listed as ram in my databooks, any only reading
from
things that are known to be rom. Is this a resonable method to
use?
Another thing, I assume the checksum code is in the build in rom,
and not
in the rom I have, since you really can't validate a checksum with
code
you don't even know if works.
Mostly right now I am just decoding what looks like initialization
stuff.
I assume that probably the block learns/integrators would be
initialized to
128. I also would guess that alot of the other things would be
initialized to
0. So far things look pretty simple, but I am only into the simple
stuff.
I have a 68hc11 disassembler program, a detailed 6809 instruction
book and
decoder card, and a 1988 vintage motorola single chip microcomputer
data
book that lists quite a few variants that all use pretty much the
same
instruction set with a few additions, and extra ram/rom/portio/ad
varing
depending on what was specified. I would guess that GM is using a
custom one of these chips, probably with a burned into the chip rom,
for
default operation.
Roger
On Fri, 11 Sep 1998, Georg Faustmann wrote:
> If the system is a Delco unit, then the Checksum on/off byte may
be at
> $0008, set this to $AA, the checksum (stored at $0006,$0007)
should be the
> total of all the bytes from $0008 to then end of the chip.
>
> In some cases the address of the checksum switch could be offset,
eg $3008,
> $8008.
>
> Notes on how to find checksums:-
>
> If you had access to an emulator with a Romwatch (real time trace)
type
> function where the actual bytes being read at that moment in time
are
> highlighted, then you can sit there and watch where the bytes
start to be
> sequentially accessed by the CPU (usually every 10 seconds or so),
you can
> then scroll down until you find out where they stop being
accessed, and
> will give you the start and end point of the checksum. Then add
these range
> of numbers up, and look for a 16 bit value that is the last 16
bits of this
> sum, and search for this number outside of the checksum range,
because it
> can't be in the checksum area.
>
> Georg
>
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