ROMBAT

ROMBAT is a 256 byte or less mini-monitor for the Z80 processor. It provides
functionality to read and write memory and I/O as well as to execute code.
ROMBAT can function without working RAM and is primarily intended to be used
as a board bring up tool or as a fallback if boot code finds hardware
problems.

Commands

AAAA R		Display the contents of address AAAA and increment
R		Display the contents of the working address and increment
AAAA DD W	Write DD to AAAA, increment AAAA
DD W		Write DD to the current working address and increment
AAAA X		Execute
X		Execute from working address (not usually useful)
AAAA I		Display the contents of I/O port AAAA
I		Display the contents of the current I/O port
AAAA DD O	Output DD to the I/O port AAAA
DD O		Output DD to the current I/O port

Space simply echoes a space for neatness, newline produces a newline and prompt
likewise. Delete deletes back through the last address typed.

How It Works

Hex digits are stored in a 24bit value in EHL. Each digit shifts the value
left four bits and adds it on. W and O move EH to HL and output the old value
of L to it, W also increments HL. On read the same theory applies R reads a byte
from HL and increments, O moves HL to BC and does in (c).

The code execution uses the stack as a threadcode engine. Each ret is
effectively a move to the next operation word.