K-Prompt

A ZX Spectrum in your browser. Load a tape and play.

K-Prompt is a ZX Spectrum 48K emulator in TypeScript: a Z80 at 3.5 MHz, the ULA drawing the screen line by line (border stripes and all), the beeper, the keyboard and the cassette. It also runs Microdigital's Brazilian clones of the Spectrum, the TK90X (1985) and the TK95 (1986), on their own ROMs.

The name comes from the flashing K cursor the Spectrum waits at, as A-Prompt, its CP/M sibling, is named after A> and Ok-Prompt, its MSX sibling, after Ok. The Z80 core comes from A-Prompt, where zexdoc and zexall pass; here it times every memory and I/O cycle on its own, so the ULA's contention delays the CPU exactly as on a real 48K (FUSE's core tests pass, every bus event at its T-state), and unattached ports read the floating bus.

What it does

• ZX81 and ZX80, on the same Model menu: the ZX81, ZX80, and Microdigital's TK85, TK82C and TK80 at 60 Hz, as sold in Brazil. The CPU draws the picture as on the real machines (the display file executed, characters turned into NOPs and pixels, a line interrupt from R), SLOW and FAST included; .p and .o programs load and SAVE through the ROM's LOAD and SAVE.

• Tapes: .tap and .tzx. Standard blocks load at once (the ROM's LD-BYTES is trapped); turbo loaders, and every tape when you turn fast loading off, play in real time through the EAR input, with the stripes in the border, and fast-forward while they play. SAVE records onto the tape in the deck, which you can download as a .tap.

• Listings: paste a BASIC program as a magazine printed it; it is tokenized (numbers get their hidden 5-byte values), put on a tape with an autostart line, and loaded by the ROM's own LOAD.

• Snapshots: .z80 (versions 1 to 3) and .sna, 48K and 128K, and .z80 for the TS2068; download the running machine as either. A snapshot switches to the machine it was made on.

• Interface 1: Sinclair's Interface 1 (ROM v2 or v1) with eight Microdrives: FORMAT "m";1;"name", SAVE *"m";1;"prog", LOAD *, CAT 1 on .mdr cartridges kept in the browser. Its RS232 is wired to the telnet modem, so BASIC can call a BBS with OPEN #4;"b", and I1TERM.tap on the shelf is TERM through the Interface 1 at 19200 baud (programs/io-if1.asm, with its own bit-timed serial routines).

• AI board: in TERM, ATDT AI calls a BBS whose sysop is DeepSeek V4.1 Flash (your own API key, kept in your browser): chat with it or with the Bastard Operator From Hell, or order a program in BASIC or Z80. It tests the program on a hidden Spectrum, sends the ROM's error reports or the assembler's errors back to the model, and sends you the tape by ZMODEM: TERM saves it to the tape in your deck. To have your own BASIC program fixed, SAVE it to tape and upload it from TERM (EXTEND, then U).

• Quick save and rewind: F2 keeps the machine in the browser and F3 brings it back; F4 steps back about five seconds at a time, up to 30.

• Keyboard: the PC keyboard types by character (" is SYMBOL SHIFT + P), and an on-screen 48K keyboard shows every keyword and symbol; tap it on a phone. CAPS and SYMBOL SHIFT latch for one-finger typing.

• Joysticks: Kempston, Sinclair or cursor, from the arrow keys or a gamepad.

• Modem: a Hayes modem that dials telnet BBSes, on I/O ports BASIC can reach (IN 447, IN 703, OUT 703,x), with the dial tone, ringing and handshake on the speaker. TERM.tap, on the shelf, is a 64-column terminal written in Z80 for it (programs/TERM.asm). Boards with their own WebSocket gateway are dialed straight from the browser; the others go through the Worker's relay, which only reaches the boards in src/phonebook.ts.

• ULA.tap, on the shelf: colour clash and multicolour side by side. Two balls drag their colours over a pattern 8×8 cells at a time, and beside them a band whose every pixel line has its own colours, rewritten 64 times a frame by code counted to the T-state (programs/ULA.asm).

• Machines: ZX Spectrum 48K, TK90X, TK95, 128K and +2: the 128's menu and editor, its eight RAM banks and second screen, and the AY sound chip (which a 48K can have too, as an add-on). Pasted listings may use 128 BASIC's PLAY and SPECTRUM on a 128K. And the American Timex Sinclair 2068: its 60 Hz frame, hi-colour and 512-pixel screen modes, EXROM, AY and joystick.

Run

npm install
npm run dev        # http://localhost:5173
npm test           # vitest; ZEX=1 npx vitest run tests/zex.test.ts for the Z80 exercisers
npm run build      # static site in dist/

ULA2: a photo without colour clash

ULA2.tap on the shelf: a beach photograph (CC0, by Anders Wideskott, via Wikimedia Commons) on a 48K. The sides are as the ULA shows any picture, two colours per 8×8 cell; down the middle, 16 columns by all 192 lines, every pixel line has its own two colours, rewritten 192 times a frame while the beam is in the border. M swaps the middle between the two ways; SPACE goes back to BASIC.

Fonts

TERM's 4-pixel font is Tom Thumb (© 1999 Brian J. Swetland and Vassilii Khachaturov, portions © Dan Marks, readability changes by Robey Pointer), under the 3-clause BSD licence: see programs/FONT-LICENSE.txt.

ROMs

The Spectrum ROM is © Amstrad, who allow it to be distributed with emulators. The TK90X, TK95 and TK85 ROMs are © Microdigital. The ZX80 ROM is © Amstrad and the ZX81's by Nine Tiles, shared with emulators. See src/roms/README.md.

Licence

MIT, © 2026 Eduardo Stern. Built with Claude Code (Opus 5.5).