EASM: a Z80 assembler for the VTech Genio 2000

By Santiago Crespo, 2026. The instruction encoder inside it is R.T. Russell's, from his BBCZ80.

EASM is a 32 KiB cartridge for the Spanish VTech Genio 2000 that turns it into a small Z80 development system. Everything runs on the Genio's own Z80: no BASIC and no PC are needed.

• a source editor for the 4x20 LCD, with sideways scrolling, line numbers, and mnemonics on the activity keys;
• a two-pass assembler: all official Z80 instructions, labels, forward references, expressions, ORG EQU DB DW DS END, errors with their line number;
• a monitor: line-by-line assembler, disassembler, memory viewer and editor, fill, compare, run, registers after the run;
• a guide on the HELP key: one page with an example for every mnemonic and directive, and pages on the editor, commands, syntax, registers, flags, the API and the memory map;
• an API at fixed addresses (PRINTSTR, GETKEY, ...) that programs call, with built-in names.

MAME snapshots, 4x enlarged: start, the demonstration program in the editor, ASM, RUN, the disassembler and a page of the guide (HELP). tests/sheet.sh makes this picture from tests/shots.txt; the gaps between characters are the LCD's.

Machines. Written for the Spanish VTech Genio 2000, where its first runs on a real unit worked (the rest was tested in MAME). It will most likely also work on its relatives the Genius 4000, Genius LEADER 4000 Quadro, Genius LEADER 4004 Quadro L and PreComputer Power Pad, but it has not been tried on them; their keyboards carry other legends, so the key names used here and some typed characters may differ.

A first program

In the editor (ANSWER switches between the editor and the command line):

ORG USERRAM
START:
  LD HL,MESSAGE
  CALL PRINTSTR
  RET
MESSAGE:
  DB "HOLA",0

Then RUN at the command line, or the BASIC key of the activity panel. USERRAM is 0xD000 and PRINTSTR is the API entry at 0x8010; both are built-in names. HOLA is printed and the program's RET comes back to EASM. The test tests/t01_demo.txt types this program key by key in MAME and checks the bytes (21 07 D0 CD 10 80 C9 48 4F 4C 41 00), the output and the return.

Documents

• docs/MANUAL.md: the user's manual (keys, editor, syntax, commands, monitor, API, errors).
• docs/DESIGN.md: technical design: the audit of EBASIC, the choice of the assembler core, the architecture, the assembler and monitor internals, and the interfaces planned for later.
• docs/MEMORY_MAP.md: ROM pages, RAM layout, I/O ports, the API table.
• docs/COMPATIBILITY.md: what was tested and how, what was not, the risks on the real unit, and the differences from other Z80 assemblers.
• docs/STATUS.md: what is implemented and what is pending.

Building

make

Needs sdasz80 and sdldz80 (SDCC 4.5) and python3. It writes:

File	What
easm_gl2000_AT28C256_32K.bin	the cartridge image for an AT28C256 EEPROM (page 1, then page 2)
easm_gl2000_mame_64K.bin	the same twice, for MAME's -cart
out/easm.sym	every symbol of both pages (address, page, name)
out/page1.map, out/page2.map	the linker maps
out/usage.txt	ROM and RAM use (printed by make)
out/ramvars.txt	the RAM addresses, for the tests

Testing

make test

Runs every test in headless MAME (tests/): scripted key presses with checks of the screen and of memory, the whole instruction set against MAME's unidasm, and a sweep of every opcode through the disassembler. Needs MAME's genio build in ~/PROYECTOS/genio2000/mame-upstream (or MAME=...) and unidasm. See docs/COMPATIBILITY.md for the results.

Status

Tested in MAME; first steps on the real unit. A first run on a Genio 2000 (32 KiB image) worked at start-up, at the command line, with ASM and ?, and in the guide (the second cartridge page). The editor, running programs and the monitor have been tested only in MAME so far; docs/COMPATIBILITY.md lists the risks.

Licence

• 0BSD (LICENSE), Copyright (C) 2026 Santiago Crespo: EASM's own files, code and documentation, and the parts taken from EBASIC (same author and licence): the LCD and keyboard drivers, the start-up, the trampolines, the guide's viewer and compressor, the MAME driver script, and the build tools. You may use, copy, modify and distribute them for any purpose, with or without fee, without conditions. Until 2026-11-13 they were released under GPL-3.0-or-later; copies taken then may still be used under those terms.
• zlib licence: third_party/bbcz80/ holds R.T. Russell's ASMB.Z80 unchanged, with its licence.txt. src/asmb.s is an altered version of it, marked as such in its header. The name "BBC BASIC" is not used for this cartridge.