Without adding any
extra chips, is it legal to increase the capacity of an
existing chip? Maybe that's bending the rules too much, but I'll share this idea anyway...
BigEd wrote:
Use NMI as the input
Another approach is to double the size of the EPROM and use the new address line as an input; ie, don't drive it as usual from the CPU. Instead, drive it from whatever external signal you want to read.
The EPROM contains two entire copies of the firmware, and the copies are largely but not 100% identical. At any point where the "input" needs to be tested, one copy of the firmware contains a BRA or JMP and the other copy simply contains NOP's (or a JMP/BRA to a different destination).
( Back in the 20th century I used this trick to get out of a tight spot -- see the "Slightly OT" section of
this post. And the goal was real, not just a forum brain teaser. I wanted to upgrade an existing system, but what I was attempting seemed impossible in light of the speed that was required. )
-- Jeff
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In 1988 my 65C02 got six new registers and 44 new full-speed instructions!
https://laughtonelectronics.com/Arcana/ ... mmary.html