Z80 UART monitor

This ROM provides a small interactive monitor and a bundled Z80 Tiny BASIC interpreter on the UART/CTC card at I/O base 20. At reset, a small ROM bootstrap copies both programs into the top 16 KiB of RAM and starts the monitor. Only occupied code/vector bytes are stored and copied; uninitialized data is cleared directly in high RAM. The low ROM can subsequently be disabled without taking away the monitor, Tiny BASIC, monitor data, or interrupt table. No runtime RAM below C000 is used.

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
`G D000`	Start the bundled Z80 Tiny BASIC interpreter
`B` or `BASIC`	Shortcut for `G D000`
`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:

make

The outputs are:

build/metal/monitor.bin: complete 32 KiB ROM image
build/metal/monitor.prog.bin: compact image ending at the last occupied byte
build/tinybasic/tinybasic.bin: standalone relocated Tiny BASIC payload

The current compact image is 6,157 bytes. Its ROM layout is:

0000-007F: reset/copy bootstrap
0080-0F6E: monitor image copied to RAM at C000-CEEE
0F6F-0FFF: unused space reserved for monitor growth
1000-180C: Tiny BASIC image copied to RAM at D000-D80C
180D-7FFF: unused space available for additional ROM content

The monitor executable starts at C080; its IM 2 vector table is at C040. The bootstrap obtains both copy lengths from generated/linker symbols, so the copied sizes track changes to the programs automatically.

The resulting high-RAM layout is:

C000-CEEE: monitor vectors, services, and code
CEEF-CFFF: free
D000-D80C: Tiny BASIC code
D80D-DFFF: free
E000-E7FF: Tiny BASIC workspace and BASIC program text
E800-E877: Tiny BASIC variables and input buffer
E878-EFFF: Tiny BASIC stack space
F000-FFFF: monitor globals and monitor stack

Tiny BASIC

The original GNU-z80asm listing is retained as tinybasic/tinybasic.asm. Its sdasz80 port is tinybasic/tinybasic_sdas.asm and is built automatically. The port replaces fixed low-page RST calls with relocatable calls and uses a marker-based command table so addresses in the D000 page work correctly.

At reset the bootstrap copies Tiny BASIC from ROM offset 1000 to D000. Start it from the monitor with:

BASIC and G D000 are equivalent alternatives.

Tiny BASIC calls the monitor ABI at C050 for output and C053 for non-blocking input. The assembly ABI wrappers preserve the working registers, and Tiny BASIC keeps interrupts enabled so it can share the monitor's configured interrupt-driven UART.

Line editing accepts either BS (08) or DEL (7F) for backspace and emits BS SPACE BS so ANSI-style terminals erase the character visually. Ctrl-U clears the complete input line.

During execution, Ctrl-O toggles output suppression without stopping the BASIC program. Press Ctrl-O again to resume output. Ctrl-C stops execution and returns to the Tiny BASIC prompt.

Tiny BASIC resets the stack pointer when it starts, so it does not currently return through the monitor's G call. Reset the machine to return to the monitor. The first currently unused ROM byte is 180D; 1900 is a convenient aligned starting point for another ROM payload.