Thread: Old exam paper
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Old 21st Nov 2020, 10:27 pm   #6
JohnBHanson
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Join Date: Aug 2009
Location: Worthing, Sussex, UK.
Posts: 661
Default Re: Old exam paper

I agree - very hard to give definitive answers. Eg
There are different ways of calculating relative addressing depending when
in the instruction the PC is incremented.

The partial instruction set matches the 6502
The partial circuit has rom at address 0 - therefore not 6502 (Stack in rom is silly)

Floppy disk diagram item D could be
Density Select sensor
Speed Select sensor
Disk Present sensor
Write protect/Write enable sensor

Divide routine - I would preset Q to 1 in step A and look for carry
after shifting Q left. But depends on how flags behave.

I would hate to mark this exam - so many different answers that could be argued to be correct.


PS - ZX80/ZX81 used the cpu as a crt controller to time character generation on a vdu! I bet it was not designed for that!
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