Pentagon-128 vs Pentagon-512

Immediately press RESET, All!

You can attach a decoder to the data bus and if in the COP reading cycle there was an
code out (xx), a (like 0xdf) then in the next one (if there was no another
COP)
output cycle, block 6 bits of the data bus to 0, for Pentagon-1024 you can block 5
bits. Thus, when accessing a port using a shortened address
only the memory of the standard Spectrum-128 will be used, and when accessing
by a full
address, you can use all the memory.

* Originally in ZX.SPECTRUM
* Crossposted in REAL.SPECCY