NorthStar Horizon Z80 ROM monitor/diagnostic tool

The NorthStar Horizon is a S-100 bus Z80 computer from 1977. This ROM monitor
was created as part of a restoration project. The initial need was a diagnostic
to isolate and troubleshoot basic hardware problems. The minimum hardware
required for this ROM was only the Z80 CPU and console serial port. Basic
function without memory was a design goal to assist with memory troubleshooting.
Memory from this era was a common source of failure for many retro computers.

Design requirements:

  1. No RAM, which includes no stack.
  2. Relative code, no fixed addressing.
  3. Modified Z80 CPU card uses ROM 2716, 2048 bytes max size


  1. Map Memory
  2. Test Memory
  3. Display Memory
  4. Edit Memory
  5. Go to address
  6. Boot - E800
  7. Fill Memory
  8. Loop address - for testing
  9. Intel-HEX load


Develment environment was Ubuntu using z80asm. No preprocesser in this version which
should make it easier to port to other z80 assemblers.

For example:

$ z80asm -onsz80rom.bin -lnsz80rom.lst nsz80rom.mac

This ROM requires 2K and works with a 2716 using this on the CPU card:


Welcome! A RAM-free test ROM is a nice project - the Z80’s plentiful registers must be a bit of a help here.