Simple monitor program

README.md

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+# Z80 UART monitor
+
+This ROM provides a small interactive monitor on the UART/CTC card at I/O base
+`20`. Connect a terminal at **2400 baud, 8 data bits, no parity, 1 stop bit**.
+Commands are case-insensitive and all numeric arguments are hexadecimal; an
+optional `0x` prefix is accepted.
+
+| Command | Action |
+| --- | --- |
+| `D 8000` | Dump 40 bytes starting at address 8000 |
+| `D 8000 20` | Dump 20 bytes starting at address 8000 |
+| `W 8000 3E 41 D3 25 C9` | Write bytes starting at address 8000 |
+| `I 20` | Read I/O port 20 |
+| `O 20 57` | Write 57 to I/O port 20 |
+| `G 8000` | Execute code at address 8000 |
+| `HELP` | Show the command summary |
+
+`G` transfers control with a return address on the stack, so code ending in
+`RET` returns to the monitor. Code that changes the stack pointer, interrupt
+mode, or UART/CTC setup must restore it before returning.
+
+Build the ROM and programmer-format images with:
+
+```sh
+make
+```
+
+The outputs are `build/metal/monitor.bin` (32 KiB ROM image) and
+`build/metal/monitor.prog.bin` (compact programmer image).