A Newbie Question

[David A. Cooley]

>I'm trying to learn to program some proms, I have a few hex programs, and I
>have a proprietaery program for programming 1986-87 GN proms. Thats neat,
>BUT, I'd like to find a resource on what locations mean, what purpose they
>serve, and what the common adjustments are in those locations. I'm not
asking
>for any trade secrets, I just wana get an idea of whats what. I'd like to
>innovate on my own. I downloaded a stock GN prom table (its a spreadsheet
in
>excel) for the GN's and while looking at the same chip in rom format in a
>typical hex editor, cant find where some of the info is, for example a
>certain address might be hi rpm cutoff..theres a hex value, and I assume at
>the end of the line a checksum value for that line? Like there might be:
(I'm
>not looking at anything now, this is erroneus data)
>
>$33F0   AA  AB AC CD EE FF CC DD-EE FF AB AC AD AE 99 F0
..(..!.ac........



Actually, this line is $33f0 thru $33ff.. 16 addresses.
The junk at the far right is what ASCII characters represent the HEX numbers
in the bin... Ignore them.
I guess first, Hex numbers go $0, $1, $2, $3, $4, $5, $6, $7, $8, $9, $A,
$B, $C, $D, $D, $E, $F, $10 etc...
If you are running windows 95 or 98, the calculator in Accessories, in
advanced mode, will let you enter HEX numbers, then convert to decimal and
vice-versa.

>
>I know the first number is the base memory address, and each of the 15
values
>are a parameter till the next line which would go to $3400, but the last
>pieces of info on that like..the ...(.!..ac...... stuff..what is that? And
>what signifigance does it have? Is it that lines entry for the overall
>checksum? Maybe someone can explain that to me as well...ANYWAY..in the
table
>(the spreadsheet) I have, the last entry in a line is like KMRPMHI which
I've
>gathered was a GM engineer abbreviation for hi max rpm hi. Anyway...where
can
>I find this info in the chip in a hex editor, or what could I open the rom
or
>bin file to SEE that info. My intention is to go beyond ONLY doing Buick
>proms, but unfortunatly my knowledge is still in its infancy. Hopefully
some
>of you guys/gals will be generous with your knowledge to help me learn a
>little at a time
>

There isn't any checksum for each line... The GN chips have a checksum for
the entire PROM image, except for addresses $0000 and $0001 (the checksum
itself).
Also, if you are in say the Main spark advance table... The numbers you see
(converting hex to decimal) aren't the degrees of advance... GM uses
different formulas to get the proper range they want from the 255 possible
hex values that can be in one byte.

For example:  N=E*256/90

The number in the table is N, the real world advance value is E.
so if you wanted to see 26 degrees advance at one byte, you'd use:
26*256/90=73.955555... Rounded to 74 and converted to HEX is $4A

Clear as mud yet?
It takes a lot of getting used to... I've been playing with it for 8 or 9
years and I still don't understand some of it... GM's terms can be
misleading!