I decided to start a thread to document things as I move allong in cracking the ECU of a '99 Saturn. The specific type I pulled from the JY was from a 1.9L DOHC Auto tranny. I have it sitting on a computer with a script running to try to find the seed/key pair. There are 2 halves to this board, one primarily for the engine side and one for the tranny. I have socketed the 28F010 flash ROM's and pulled both the straight image and one that has the address lines straightened out (the A1-A13, 2-A12, A3-A11, and A10-A14 lines were swapped). With the decoded image I have mostly made it through the decoding of the tranny side chip, it is fairly straightforward with only the lower 64K of the chip used. The engine side chip is a little more tricky. It has several routines near $5666 for bank switching the between the lower and upper halves of the flash (128K flash) using port G on the 68HC11F1 processor.

The only other thing at the moment that I have is that there is a 40 header on the board which accesses many of the address and data lines :
A4 1 2
A3 3 4 Ground
A2 5 6
7 8 A1
9 10 A0
11 12 A5
A6 13 14 A8
A7 15 16 A9
A10 17 18 D7
A11 19 20 D6
A12 21 22 D5
A13 23 24 D4
A14 24 25 D0
A15 27 28 D1
R/W 29 30 D2
Reset 31 32 D3
Moda/LIR 33 34 PD1/TXTD
PD1/RXD 35 36 E
ModB/Vstby 37 38 ground
XIRQ 39 40

I did the ohm checks on the tranny side and I have not yet compared them the the engine side, I would suspect at least adding support for A16.

Well that is where I am for now. I will keep things up to date when I learn them.