BBC Micro: emulator, BBC BASIC, assembler and debugger

The BBC Micro needs Acorn’s operating system (MOS 1.20), and BBC BASIC II to do anything in BASIC; Acorn DFS 1.20 adds the disc filing system. These ROMs are still copyright: other emulators that ship them do so without a licence — jsbeeb’s ROM folder says "The ROMs here are NOT GPL. They are copyrighted still, but I think it’s fair use provided you actually own these ROMs" (jsbeeb roms/README), and the Acorn community has found no rights holder who grants permission (RISC OS Open forum: copyright status of Acorn ROMs). So this page does not include them.

Load ROM files… on the Machine tab takes the ROM images you have. Each one is recognised by its checksum (MOS 1.20 3c14fc70, BASIC II 79434781, DFS 1.20 8ccd2157, CRC-32) or by its sideways-ROM header, put in the right socket (the OS at &C000, languages from slot 15 down, service ROMs such as the DFS from slot 14 down), and kept in IndexedDB in this browser only.

Without ROMs the machine runs Tiny OS, this site’s open stand-in for the MOS: MODE 7 text output (OSWRCH, OSASCI, OSNEWL, VDU 31), OSRDCH from the keyboard, OSWORD 0 (read a line), OSBYTE &81 (INKEY) and BRK error messages. Machine code from the Assembly tab runs on it; BASIC does not. Its source is in bbc-micro-data.js.

Type at the > prompt, or use the BASIC tab: pick an example or paste a listing and press NEW, type & RUN. The text goes into the MOS keyboard buffer a line at a time, exactly as if typed. Examples: colours and double height in MODE 7, the inline assembler ([OPT pass% …] in a two-pass FOR loop, then CALL), BASIC against machine code timed with TIME, MODE 1 line art, SOUND and ENVELOPE, and a recursive tree with PROC. List it detokenises the program between PAGE and TOP; it can be saved as text or as the tokenised bytes.

The Assembly tab uses the site’s 6502 assembler: labels in the first column, NAME = value, .byte .word .text, < and > for low and high bytes; hex as &FF (BBC style) or $FF. Assemble & run (Ctrl+Enter) resets the machine, waits for the > prompt, puts the code in memory and types CALL &2000 — or, under Tiny OS, jumps there; RTS comes back. Examples: hello world with OSWRCH, MODE 7 teletext codes, MODE 2 set up by programming the 6845 and video ULA directly, reading keys with OSRDCH, a scale played on the SN76489 through the System VIA, and rainbow raster bars from a vsync interrupt and User VIA timer 1 (this one needs the MOS). Build .UEF tape and Build .SSD disc (a bootable !BOOT) make files for other emulators or a real BBC; Load as tape and Boot as disc use them here.

The Assembly tab is a split view: the editor, and beside it (below on a phone) the screen with the registers, flags, stack, code around PC and watches.

The Debugger tab works on anything that runs (BASIC, the MOS, a game). The disassembly names the OS entry points (OSWRCH &FFEE, OSBYTE &FFF4, OSWORD &FFF1, OSRDCH &FFE0 …), the vectors at &0200 (WRCHV, IRQ1V …) and every SHEILA register (CRTC_ADDR, SYSVIA_IFR, ULA_PALETTE …). Views: CRTC and ULA (all 18 registers decoded, screen start, beam position, the ULA control register and the 16-entry palette), VIAs (ports, both timers, IFR/IER, the addressable latch, keys held), Sound (each SN76489 channel’s period, frequency and attenuation), Tape and disc (the ACIA, serial ULA and 8271 state) and OS calls — a log of OSBYTE, OSWORD and OSWRCH calls with what each does.

Drop files on the screen or the Tapes and discs box:

The Software panel lists freely distributable programs this site can share, and opens files from a URL or an archive.org item.

The BBC starts with CAPS LOCK on: letters type as capitals, Shift+letter as lower case. Esc is ESCAPE, Backspace DELETE, End COPY, the arrows are the cursor-editing keys, F1–F9 are f1–f9 and F10 is f0. BREAK is a button (the Pause key works too); Ctrl+BREAK resets fully. On a phone, Keyboard shows a BBC keyboard.

Full screen fills the screen with the BBC display; ⛶ Present (or F) shows only the screen for a talk. Keys keep working; Esc closes.

The 6502 is emulated instruction by instruction with exact cycle counts, NMOS decimal mode and the stable undocumented opcodes; it passes Klaus Dormann’s 6502 functional test. Accesses to the 1 MHz devices stretch the cycle. The 6845 is stepped a scan line at a time and the video ULA decodes each line from memory (the ULA’s 4-bit palette lookup, the screen wrap-around set by the addressable latch, the hardware cursor), so palette changes between lines (raster bars) show. MODE 7 is drawn by an SAA5050 model — control codes, set-at and set-after, double height, mosaics contiguous and separated, hold graphics, flashing — using Bedstead's glyphs (CC0) with the chip’s character rounding. The VIAs run their timers at 1 MHz with the 6522’s latching and interrupt flags; the keyboard is the real 10 × 8 matrix with autoscan and CA2 interrupts. Sound is the SN76489’s three square waves and noise, replayed from the register writes and mixed at 44.1 kHz.

Share link copies the tab and the code or listing. Links can be written by hand:

Presets: hello world in machine code, MODE 7 teletext, MODE 2 by hand, BBC BASIC’s inline assembler, MODE 1 line art, the CRTC and ULA view.

Related: 6502 assembler and opcodes, Commodore 64, Atari 8-bit, ZX Spectrum, retro font editor.