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Article #621688Re: IMSAI 8080 and War Games
From: Gareths Downstairs Computer
Date: Sat, 31 Mar 2018 22:25
Date: Sat, 31 Mar 2018 22:25
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On 31/03/2018 22:36, Charlie Gibbs wrote: > > The IMSAI took some shortcuts when displaying and alterming memory > through the front panel. IIRC it tricked the 8080 chip into fetching > a byte as if it were an instruction, then somehow froze things and > displayed the byte on the lights. A similar technique was used by John Miller-Kirkpatrick (RIP) in his SCAMPI design based on the SC/MP microprocessor. The "somehow freezing" is using the delaying input on memory accessing to allow memories which are slower than the micros bus timing to be used and holding the signal indefinitely until the next byte. This is only good on static logic designs such as the Intel chips and not possible on dynamic logic chips such as the Motorola chips. What happens is this, the micro resets and starts to fetch instructions from byte zero onwards. However, the response to the fetch is to assert the delay pin to extend the memory access. Then, using the front panel switches the byte to be written to address zero is set up, the read line changed to a write, and then a NOP is electronically placed on the bus, the delay released and the micro then tries to fetch from address one, and so on and so forth. Not sure that's a sufficiency, but it's only off-the-cuff!
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