Port B is then configured to output and the first byte
placed onto the emulator data bus.
The WE signal is taken LOW and the CS signal HIGH to
load the first byte.
The counter reset is removed and the ADCLK line is
taken HIGH and then returned LOW to advance the address
The next byte is fetched and loaded and so on until the
full 2k bytes are transferred.
A similar sequence may be employed to read back the contents of
the emulator RAM into the host Nascom.
To use the emulator to develope hardware, short test
routines may be written using ZEAP, tested by single-stepping if
necessary and then re-assembled with origin 0 but located in the
Nascom RAM at 4000H by means of ZEAP’s P option. Executing the
loader routine will then transfer the program into the emulator.
The target hardware may then be powered up and its own processor
reset. The target hardware will then be able to access the
program as if it were in EPROM. The program may readily be
changed until the hardware has been thoroughly tested. The final
program can then be written and debugged using the same
procedure and then eventually blown into an EPROM.
The emulator as described has been in use now for some
months and has helped to develope several small Z80 based
projects. Although its minimal nature does not offer the
sophisticated facilities of more expensive devices, I have found
it to be a great time saver and now regard it as invaluable for
home projects. Better software is definitely required. This will
be a future aim.