z80_monitor/README.md

Z80 UART monitor

This project provides a resident BIOS, an interactive monitor, and ROM-resident applications for a Z80 system with a UART/CTC card at I/O base 20.

At reset, a small ROM bootstrap copies only the BIOS and monitor into the top 16 KiB at C000. Before copying, it writes 06 to banking port 00: ROM remains enabled, banking is enabled, and C000-FFFF maps to the upper physical 16 KiB. The BIOS then initializes interrupt mode 2 and the UART and starts the monitor. Applications remain in ROM and execute in place while calling the stable BIOS jump table in high RAM. Tiny BASIC is the first such application, linked at ROM address 1000.

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
C 8000 BFFF	Compute the 16-bit additive checksum of an inclusive range
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 1000	Start the ROM-resident Z80 Tiny BASIC interpreter
B or BASIC	Shortcut for G 1000
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.

C adds every byte from the start through the end address, inclusive, and prints the low 16 bits of the sum as four hexadecimal digits. The end address must be greater than or equal to the start address.

Build the ROM and programmer-format images with:

make

Uploading Intel HEX files

program_ihex.py can write an Intel HEX file to memory through the running monitor. It sends data in small blocks with W, checks every block with C, and retries a failed write or checksum before giving up. It requires Python 3 and pySerial.

python3 -m pip install pyserial
python3 program_ihex.py /dev/ttyUSB0 program.hex

The default connection is 2400 baud, the default block size is 16 bytes, and three retries are allowed after the initial attempt. For example:

python3 program_ihex.py /dev/ttyUSB0 program.hex --retries 5 --block-size 24

Use a Windows port such as COM3 in place of /dev/ttyUSB0. Run python3 program_ihex.py --help for all options. The monitor must be sitting at its > prompt before starting the upload. Only data records present in the HEX file are written; gaps are left unchanged. The HEX addresses must refer to writable memory and must not overwrite the running monitor at C000-FFFF.

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
• 0080-0FEA: packed BIOS+monitor image copied to RAM at C000-CF6A
• 0FEB-0FFF: unused space reserved for the resident image to grow
• 1000-180C: Tiny BASIC, executed directly from ROM
• 180D-7FFF: unused space available for additional ROM content

The resident image starts at C000, its IM 2 vector table is at C040, and linked code starts at C080. The bootstrap obtains the resident copy length from a linker symbol, so it tracks BIOS and monitor changes automatically.

The resulting RAM layout is:

• 8000-AFFF: Tiny BASIC workspace and BASIC program text
• B000-B077: Tiny BASIC variables and input buffer
• B078-BFFF: Tiny BASIC stack space
• C000-CF6A: resident BIOS, vectors, services, and monitor
• CF6B-EFFF: free space for resident-image growth
• F000-FFFF: BIOS globals, UART buffers, and monitor stack

The BIOS and monitor continue to work if low ROM is disabled. ROM-resident applications naturally require ROM to remain mapped while they execute.

The banking latch bits used by the bootstrap are bit 0 ROM_DISABLE, bit 1 BANK_SELECT, and bit 2 BANK_ENABLE. Its initial value 06 leaves ROM on and selects the lower physical 48 KiB plus the upper physical 16 KiB. Writing 07 later disables ROM without changing that split mapping. Writing 05 selects the complete upper physical 64 KiB; the monitor remains available because its C000-FFFF image was placed in that upper bank during boot.

BIOS interface

Assembly applications include include/bios.inc and call these fixed entries:

Address	Service	Interface
C050	BIOS_PUTC	Write character in A
C053	BIOS_TRY_GETC	Return character in A, or zero
C056	BIOS_GETC	Wait for and return a character in A
C059	BIOS_IO_OUT	Write L to port A
C05C	BIOS_IO_IN	Read port A, returning the value in A
C05F	BIOS_DELAY_MS	Delay approximately HL milliseconds
C062	BIOS_MONITOR	Warm-start the monitor; does not return
C065	BIOS_BANK	A=0: split lower 48K/upper 16K; otherwise select upper 64K
C068	BIOS_ROM_DISABLE	Disable ROM while retaining the selected RAM mapping

The service wrappers preserve working registers except for documented return values. The monitor uses the corresponding C API declared in bios.h.

The banking latch at port 00 is write-only, so the BIOS keeps a persistent shadow byte initialized to 06 by the copied resident image. BIOS_BANK always leaves banking enabled and changes only the bank selection while retaining the shadowed ROM-disable bit and auxiliary-output bits. BIOS_ROM_DISABLE sets the ROM-disable bit and is idempotent. Code should not write port 00 through BIOS_IO_OUT or the monitor's O command after using these services, because such writes bypass the BIOS shadow state.

The equivalent C API is bios_bank(uint8_t bank) and bios_rom_disable(void). A ROM-resident caller cannot return normally after selecting the complete upper 64 KiB or disabling ROM, because its return address is no longer mapped; it must arrange its continuation in RAM.

Project organization:

• boot.asm: ROM bootstrap, high vectors, BIOS jump table, and assembly glue
• bios.c / bios.h: UART, buffers, interrupts, and resident BIOS API
• monitor.c: command interpreter implemented on the BIOS API
• apps/tinybasic.asm: first ROM-resident application
• apps/README.md: ROM application memory and integration contract
• include/bios.inc: stable assembly interface for ROM applications

Tiny BASIC

Tiny BASIC is built from apps/tinybasic.asm. The port replaces fixed low-page RST calls with relocatable calls and uses a marker-based command table so it can be linked at a nonzero ROM address.

Tiny BASIC is linked at and runs directly from ROM address 1000. Start it from the monitor with:

B

BASIC and G 1000 are equivalent alternatives.

Tiny BASIC calls the BIOS at C050 for output and C053 for non-blocking input. Tiny BASIC keeps interrupts enabled so it can share the BIOS-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.