Quote:
[...] using another chip for isolation [...]
Just a quick note to mention that propagation delay needn't prevent adoption of this approach. FET-based bus switch ICs are available which can, for a project like this, replace the '244 tristate buffers which superficially seem to be required. FET bus switches boast near-zero propagation delay. IOW a ROM emulator could use such ICs to connect or disconnect the target system with an ordinary RAM chip, and that chip needn't be any/much faster than the ROM being emulated.
Attached is info for one such IC -- but the overall selection is startlingly extensive. Some of the FET bus switches also offer level-shifting
-- useful when connecting devices operating at different supply voltages.
--Jeff
ETA: The FET switches aren't quite a
perfect solution, however, as they occupy space on the board and in the budget. And there'd be an increase -- probably tolerable -- in capacitive loading on the target sysytem. So, the dual-port ram still bears consideration.
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In 1988 my 65C02 got six new registers and 44 new full-speed instructions!
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