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Old 16th Mar 2021, 6:17 pm   #1061
SiriusHardware
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Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
Posts: 7,318
Default Re: Non-working Commodore PET 3016

Quote:
I might be wrong but I think that page 7 is referring to the system RAMs
Absolutely so, but that is the area where I think the next fault is. The screen shows random stuff if we prevent the CPU from clearing it by taking out the PROM which has the clear-screen routine in it. It gets cleared if we put the PROM back in, but then it doesn't make it all the way through initialisation.

The chief suspect now is possibly the system RAM because the return-from-subroutine mechanism, widely used in the OS PROMs, can not work unless you have working system RAM.

It so happens that the clearing of the screen RAM and initialisation of the peripheral ICs takes place in the very first subroutine, so those tasks appear to be being carried out successfully, but the hypothesis is that when the system then tries to return-from-subroutine it fails because the stack, which is held in system RAM, is not working.
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