Z80 Monitor From Scratch

This repo documents the evolution of a simple Z80 monitor for single board computer like Z80-MBC2.

Z80-MBC2

This little computer already uses the Z80 version of WozMon, the tiny Apple ][ ROM monitor. Due to the reduced commands I decided to start a new monitor with extended command set.

The Monitor

Setup

80s-like this monitor is written in assembler with the SLR z80asm.com format. Development happens on my Linux machine using zxcc or alternatively tnylpo as CP/M emulator. During development the monitor is an CP/M program to simplify the testing - it can easily switched to a standalone program in ROM. The only two HW-related functions are

• conin (wait for char and return it in register A)
• conout (output char in register A)

The typical use case for Z80 SBCs is the combination with a terminal program running on the PC that connects to the serial port of the SBC. So the simplest way of program storage is to copy it from or to the terminal screen and store it on the PC. Therefore I decided to support the WozMon format for simple data storage.

Commands

The monitor program offers these functions with the following syntax:

<CMD><ADDRESS>[ <ARG> <ARG> ...]<RETURN>

• VIEW memory content

  • VAAAA - View 16 bytes at AAAA as hex and ASCII (AAAA DD DD ... CC ..).
  • V (or CR alone) - View the next 16 bytes after previous dump.
  • WAAAA - Dump 16 bytes at AAAA in WozMon fmt (AAAA: DD DD ...).
  • W (or CR alone) - Dump the next 16 bytes after previous dump in WozMon fmt.

• MODIFY memory content

  • MAAAA DD DD ... - Modify mem at AAAA, no space between 'M' and the address!.
  • M DD DD ... - Modify next mem bytes.
  • AAAA: DD DD ... - Modify mem at AAAA using WozMon fmt (see SHOW).

• GOTO program location

  • GAAAA - Goto program at AAAA.
  • G - Goto program counter of user context, e.g. resume from last breakpoint.
  • G0 - Exit to CP/M or restart the Z80-MBC2 monitor.

• BREAKPOINT set or clear

  • K - clear breaKpoint.
  • K0 - clear breaKpoint.
  • KAAAA - set breaKpoint at AAAA.
  • KAAAA BBBB - set breaKpoint at AAAA and reactivate it at BBBB. Breakpoints and reactivation points are implemented by replacing the opcode at AAAA and BBBB with RST instructions.
    This only works if the addresses AAAA and BBBB point to the first byte of the opcode (M1).

• REGISTER display

  • R - show the user register including the 2nd register set AF', BC', DE', HL' and, if applicable, the address of the breakpoint and its recovery.

           A  F  BC   DE   HL  A' F'  BC'  DE'  HL'  IX   IY   I  R  PC   SP
          DE 8B 0600 0003 C504 55 00 0000 0000 0000 0000 0000 00 1B 100C 0740
          Sz...p.C             sz...p.c                      BrkPt: 100C/100D

• SET REGISTER prepare user context for the following G command
  • A dd - set register A
  • B dd - set register B
  • C dd - set register C
  • D dd - set register D
  • E dd - set register E
  • H dd - set register H
  • L dd - set register L
  • F dd - set flags
  • I dd - set interrupt vector register
  • P dddd - set program counter
  • S dddd - set stack pointer
  • X dddd - set register IX
  • Y dddd - set register IY

The command line input uses BS or RUB to delete to the left. ^C returns to CP/M.