99 lines
3.9 KiB
Markdown
99 lines
3.9 KiB
Markdown
# Z80 UART monitor
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This ROM provides a small interactive monitor and a bundled Z80 Tiny BASIC
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interpreter on the UART/CTC card at I/O base `20`. At reset, a small ROM
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bootstrap copies both programs into the top 16 KiB of RAM and starts the
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monitor. Only occupied code/vector bytes are stored and copied; uninitialized
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data is cleared directly in high RAM. The low ROM can subsequently be disabled
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without taking away the monitor, Tiny BASIC, monitor data, or interrupt table.
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No runtime RAM below `C000` is used.
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Connect a terminal at **2400 baud, 8 data bits, no parity, 1 stop bit**.
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Commands are case-insensitive and all numeric arguments are hexadecimal; an
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optional `0x` prefix is accepted.
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| Command | Action |
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| --- | --- |
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| `D 8000` | Dump 40 bytes starting at address 8000 |
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| `D 8000 20` | Dump 20 bytes starting at address 8000 |
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| `W 8000 3E 41 D3 25 C9` | Write bytes starting at address 8000 |
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| `I 20` | Read I/O port 20 |
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| `O 20 57` | Write 57 to I/O port 20 |
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| `G 8000` | Execute code at address 8000 |
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| `G D000` | Start the bundled Z80 Tiny BASIC interpreter |
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| `B` or `BASIC` | Shortcut for `G D000` |
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| `HELP` | Show the command summary |
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`G` transfers control with a return address on the stack, so code ending in
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`RET` returns to the monitor. Code that changes the stack pointer, interrupt
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mode, or UART/CTC setup must restore it before returning.
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Build the ROM and programmer-format images with:
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```sh
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make
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```
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The outputs are:
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- `build/metal/monitor.bin`: complete 32 KiB ROM image
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- `build/metal/monitor.prog.bin`: compact image ending at the last occupied byte
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- `build/tinybasic/tinybasic.bin`: standalone relocated Tiny BASIC payload
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The current compact image is 6,157 bytes. Its ROM layout is:
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- `0000-007F`: reset/copy bootstrap
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- `0080-0F6E`: monitor image copied to RAM at `C000-CEEE`
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- `0F6F-0FFF`: unused space reserved for monitor growth
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- `1000-180C`: Tiny BASIC image copied to RAM at `D000-D80C`
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- `180D-7FFF`: unused space available for additional ROM content
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The monitor executable starts at `C080`; its IM 2 vector table is at `C040`.
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The bootstrap obtains both copy lengths from generated/linker symbols, so the
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copied sizes track changes to the programs automatically.
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The resulting high-RAM layout is:
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- `C000-CEEE`: monitor vectors, services, and code
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- `CEEF-CFFF`: free
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- `D000-D80C`: Tiny BASIC code
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- `D80D-DFFF`: free
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- `E000-E7FF`: Tiny BASIC workspace and BASIC program text
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- `E800-E877`: Tiny BASIC variables and input buffer
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- `E878-EFFF`: Tiny BASIC stack space
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- `F000-FFFF`: monitor globals and monitor stack
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## Tiny BASIC
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The original GNU-z80asm listing is retained as `tinybasic/tinybasic.asm`. Its
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`sdasz80` port is `tinybasic/tinybasic_sdas.asm` and is built automatically.
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The port replaces fixed low-page `RST` calls with relocatable calls and uses a
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marker-based command table so addresses in the `D000` page work correctly.
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At reset the bootstrap copies Tiny BASIC from ROM offset `1000` to `D000`.
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Start it from the monitor with:
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```text
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B
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```
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`BASIC` and `G D000` are equivalent alternatives.
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Tiny BASIC calls the monitor ABI at `C050` for output and `C053` for
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non-blocking input. The assembly ABI wrappers preserve the working registers,
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and Tiny BASIC keeps interrupts enabled so it can share the monitor's configured
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interrupt-driven UART.
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Line editing accepts either `BS` (`08`) or `DEL` (`7F`) for backspace and emits
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`BS SPACE BS` so ANSI-style terminals erase the character visually. Ctrl-U
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clears the complete input line.
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During execution, Ctrl-O toggles output suppression without stopping the BASIC
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program. Press Ctrl-O again to resume output. Ctrl-C stops execution and
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returns to the Tiny BASIC prompt.
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Tiny BASIC resets the stack pointer when it starts, so it does not currently
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return through the monitor's `G` call. Reset the machine to return to the
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monitor. The first currently unused ROM byte is `180D`; `1900` is a convenient
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aligned starting point for another ROM payload.
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