new Z80 monitor ROM works with no RAM

[Eric Smith]

I wrote a new monitor ROM for the Z80, called umonz, which can provide some
basic monitor functionality even if there is no working RAM. In other
words, it doesn't use a stack or store any variables in RAM. It supports
reading and writing memory, I/O ports, and loading and dumping Intel hex
format.

    https://github.com/brouhaha/umonz

It's tricky to write any non-trivial Z80 code without a stack. I used IX
and IY as subroutine return address registers, allowing two levels of
subroutines, though a subroutine that uses IX can't call another subroutine
using IX, and similarly for IY.

The background is that John Doran built a custom Z80 (equivalent) computer
system which he calls the Z80/M. It doesn't use an actual Z80 CPU; there is
an Altera (now Intel) MAX 10 FPGA on the control panel board, and a
Z80-equivalent core, along with the front panel logic, is in the FPGA.
There is a 16-slot expansion bus using 96-pin DIN 41612 connectors. The
cards are Eurocard 3U x 220mm.


https://www.flickr.com/photos/_brouhaha_/52032832163/in/album-72177720298429134/

There's an expansion card that provides 512K of flash, 512K of SRAM, page
mapping, a Z80 SIO for two serial ports, and a CF card, all compatible with
the equivalent RC2014 cards.

He expected that ROMWBW and CP/M or ZSystem would "just work". However we
had to track down several minor hardware design issues to get it to work.
After writing a few simple one-off test programs, I got tired of the
assemble, sneakernet, program flash chip, test in target loop, so I decided
to write a monitor. That definitely sped up the debug process.