Star Wars mathbox math

Working on two SW main boards that complain about bad RAM at 5F and 5H even when replaced with known good chips. I think it's elsewhere as they also fail four of the six matrix tests (15, 16, 18 and 20), all the divider tests, and tests A, B, C and D. I would love to know how the math works as shown in the SP-225 Troubleshooting Guide for example:

test 17 (passes) performs (5555-0000) x 4000 = 5555

test 18 (fails) performs (0000-5555) x 0000 = 5555 if you go by the test or (0000-5555) x C000 if you go by the first diagram on page 12

test 19 (passes) performs (2AAA-0000) x 4000 = 2AAA

test 20 (fails) performs (0000-2AAA) x C000 = 2AAA

test C performs ACC = (1B2C-0000) x 4000 + (196A-0000) x 4000 whose result should be 3496

test D performs ACC = (2696-1B2C) x 4000 whose result should be 0B6A

I understand binary and hex but neither is helping me understand how any of the above operations could calculate the way the guide says they do.

Can anyone explain this?

-Douglas

PS. seems my Accumulator seems to hold onto the first value it gets, which is why 17 and 19 pass but 18 and 20 fail with 0000, A with 0001, C with 196A and D with 2696 which is interesting. Test 15 and 16 fail with 1B2C which I can't explain and B ends with 0000.

PPS. Setting bit 8 of the instruction PROMs loads the accumulator, but what does it mean to set bit 9 and 'Road' the accumulator?
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Received on Thu Sep 26 01:40:44 2013