Thread: Zx81
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Old 25th Mar 2020, 8:28 pm   #187
Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
Posts: 9,184
Default Re: Zx81

I think that on the ZX81, like its later big brother, you can switch out the onboard ROM and replace it with an external ROM just by pulling one line on the edge connector.

If you knew that the entry / execution address for your direct-injected code was going to be address nnnn, then you could clone the normal ZX81 ROM and just make a few changes to the code it runs at reset, so that it would jump straight to your code instead of initialising the RAM, which of course is the last thing you would want it to do.

This would also leave the majority of the original ROM content as found so that your code could make use of the OS subroutines, character set, etc, if required.
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