M4TERM — a VT100 terminal for the Amstrad CPC

2026 update: An optimised 80×24 VT100 subset, tested with BSD vi.

An ANSI/VT100 Telnet client for the Amstrad CPC with M4 Board

Built for time travel to 1985.

Based on Ewenterm (https://ewen.mcneill.gen.nz/programs/cpc/ewenterm/) 1991 and Duke's M4 Telnet example (https://github.com/M4Duke/M4examples/blob/master/telnet.s) 2018

• Assembles with RASM (www.roudoudou.com/rasm)
• Tested with CPCEMU (https://www.cpc-emu.org/).
• Also tested by the good people on CPCWiki.
• M4 board information here: (https://www.spinpoint.org/2019/11/19/m4-board-guides/)
• CPCWiki thread here: (https://www.CPCWiki.eu/forum/amstrad-cpc-hardware/ansi-telnet-for-the-m4-board/)

A version for the USIFAC 2 is also available on ikonsgr's Dropbox (Connect to BBS.zip) https://www.dropbox.com/sh/ezzga2dppm6jlm7/AACwFC_rv2QatWh_ndKc9fhma?dl=0

Usage

For the combined CONNSTAR disk, run RUN"CONNSTAR" and select 2 for M4TERM.

For standalone use, copy src/EWEN.BAS, bin/EWENM4.BIN and bin/CHARSET.BIN to the same directory on the M4 board's SD card. On the CPC:

RUN"EWEN"

Select a preset or M for a manual address. Enter hostname:port or ip:port; the default port is 23. DEL or Backspace removes the previous character. Manual input is limited to 127 characters.

While the TCP connection is pending, press Escape (or Shift-Escape) to close the pending socket and return to the destination menu. The connecting message shows (ESC cancels). This applies to the connection wait after DNS resolution, not to the M4 DNS lookup itself.

During a session, Tab pauses/resumes reception and Shift-Escape disconnects. Other keys are sent to the remote host; ordinary Escape remains available to applications such as vi.

This is naturally a very restricted telnet client, but that is part of its appeal.

Example destinations (availability may change):

• telehack.com
  • Commands to try: cat vttest.vt, phoon, rain, starwars, clock.
• amstrad.simulant.uk:464
• ciaamigabbs.dynu.net:6400
• godwars.net:2250
• horizons.jpl.nasa.gov:6775
• sdf.org

Ideas for future work

• Telnet negotiation could be expanded. A SHOW OPTIONS function, which prints telnet commands as they arrive, is in the code but has been commented out as it can cause some display issues.
• Allow the |TERM RSX to accept a domain/IP. Then build a BASIC menu of known working servers. There is commented out code to display a default URL. Not stable yet.
  • Pull a webpage with up-to-date server lists?
• Add a few more ANSI Control codes. Could do animation?
• Maybe do something to show colour? I think it's possible to use mode 1 for more colours and half the character width.

If this is useful to you, please consider buying me a coffee. Coffee motivates :)

F Leen November 2023

VT100 support (2026)

The terminal now uses an 80-column, 24-row display with a streaming escape parser. The parser, full-screen editing, character-set, and Telnet milestones are implemented. This remains a practical 80x24 VT100 subset rather than complete hardware emulation.

Implemented:

• Bounded CSI parameters, including omitted/zero defaults and clamped cursor moves.

• Cursor movement (A/B/C/D, H/f) and display/line erasure (J/K). Erasure includes the cursor cell, preserves its position, and uses the normal background.

• Delayed right-margin wrapping and DEC autowrap mode (CSI ? 7 h/l).

• Separate CR and LF behaviour, backspace, fixed eight-column tabs, and ignored DEL.

• Full-screen scrolling across 24 rows; the CPC's extra physical row stays blank.

• Safe consumption of unsupported ESC/CSI commands and control strings, cancellation with CAN/SUB, and restart on ESC.

• Existing text attributes and ANSI CSI s/u position save/restore remain available.

• Scroll margins (CSI top;bottom r), index (ESC D), next line (ESC E), and reverse index (ESC M). Headers/footers outside a region stay fixed.

• Origin-relative addressing (CSI ? 6 h/l), including cursor movement limits.

• DEC cursor save/restore (ESC 7/8) for position, current attributes, origin, autowrap, pending wrap, G0/G1 character-set designations, and active character set.

• Normal arrow keys (ESC [ A/B/D/C) and application arrow keys (ESC O A/B/D/C), selected by CSI ? 1 h/l for Up/Down/Left/Right. Each key is sent as a single M4 packet. Enter retains Telnet CR/LF encoding.

• DEC special graphics via ESC ( 0 / ESC ) 0, ASCII via ESC ( B / ESC ) B, and UK designation A; SI/SO select G0/G1. Graphics include connecting lines, corners, junctions, scanlines, and mathematical symbols. Control pictures use compact 8x8 glyphs. Existing CP437 BBS glyphs stay intact.

• Ready (CSI 5 n), cursor-position (CSI 6 n, respecting origin mode), and device-attributes (CSI c, CSI 0 c, ESC Z) replies. DA reports CSI ? 1 ; 0 c.

• Streaming Telnet negotiation, including fragmented commands, escaped IAC, bounded subnegotiation, and duplicate/withdrawn option handling. The client answers TERMINAL-TYPE SEND with VT100 after accepting DO TERMINAL-TYPE, and advertises 80x24 after accepting NAWS. Remote ECHO and suppress-go-ahead are supported; unsupported options are refused.

Automated assembly tests

./build.sh
python3 -m venv /tmp/m4term-tests
/tmp/m4term-tests/bin/pip install -r tests/requirements.txt
/tmp/m4term-tests/bin/python tests/test_terminal.py

The tests run the built binary in a Z80 emulator and inspect cursor state and screen RAM. CPC firmware calls are stubbed, so these do not replace testing the real display, ROM banking, cursor interrupt timing, or M4 network interface.

Repeatable CPCEMU / CPC visual test

Run this on a computer reachable from the CPC:

python3 tools/vt100_probe.py --bind 0.0.0.0 --port 2324 --fragment 1

In M4TERM, select a manual destination and enter the host's LAN IP with :2324. Each page states the expected result. Press Space for the next page, R to repeat, or Q to close the connection. Stop the server with Ctrl-C.

Pages 1–5 check cursor bounds, erasure, delayed wrapping/CR/LF/tabs, 24-row scrolling, and malformed/unsupported sequences. Pages 6–8 check scrolling regions, reverse scrolling, and origin mode plus saved cursor attributes. Pages 9–10 check normal/application arrow keys: press each arrow and verify that the server reports PASS with the correct direction. Page 11 draws two identical boxes using G0/G1. Page 12 expects three PASS reports (ready, VT100 ID, cursor 7;13). Page 13 expects PASS VT100 and PASS 80x24; it also works when repeated.

--fragment 1 sends one byte at a time to exercise sequences split between receives. Pages 1–12 send no Telnet option negotiation; page 13 explicitly exercises it.

Protocol reference: DEC VT100 User Guide, chapter 3.

Plain-text rendering optimisation

The renderer caches whether attributes or the selected character set require buffered drawing. SGR, character-set selection, reset and DEC cursor restore refresh this derived state. Plain glyphs use eight unrolled raster writes.

Compared with the preceding build, a Z80 benchmark of PRINTCHAR for A dropped from 1,588 cycles to 1,455 with caching, then to 1,333 with unrolling (about 16% fewer overall). A bold/underlined/inverse sample dropped from 2,932 to 2,852 cycles. Firmware is stubbed in these measurements; they do not measure CPC/M4 wall-clock throughput. The binary remains 9,010 bytes excluding its AMSDOS header because the additional code fits existing alignment padding.

The assembly tests compare cached rendering with forced buffered rendering across style, colour, character-set, shift-in/out, save/restore and reset changes.