An opensource BBC Micro emulator for Win32 and Linux
Steve Fosdick
Steve Fosdick Merge branch 'sf/vdfs4'
This merges the new version of VDFS which pushes more of the functions
into the C module vdfs.c and less in the ROM and which includes working
with MMB file and a couple of MMFS commands to do so.
Latest commit 1f3af38 Nov 7, 2018
Failed to load latest commit information.
ddnoise Merge READMEs together and form README.MD Feb 21, 2017
debian packaging: add debian files. Apr 14, 2018
discs Music 5000: seek optimisation. Oct 11, 2018
docs doc: minor corrections. Oct 27, 2018
icon Merge READMEs together and form README.MD Feb 21, 2017
roms mmb: add pre-built VDFS rom for previous commit. Nov 6, 2018
src mmb: reset selected discs on Ctrl-Break. Nov 6, 2018
tapes Add roms/discs/tapes Feb 21, 2017
utils Check for .git too Mar 5, 2017
.gitattributes squash for line-endings Feb 21, 2017
.gitignore sdf: add gtest (SDF geometry test program) to .gitignore Sep 16, 2018
COPYING squash for line-endings Feb 21, 2017
Changes.txt Add EOL normalisation Feb 21, 2017 packaging: ship a default config file to define the models. Apr 14, 2018
PKGBUILD packaging: add a PKGFILE file for Arch Linux. Apr 13, 2018 Update Linux Building. Apr 24, 2018 Clarify keeping a sync up to date Mar 1, 2017 +. Feb 26, 2017 Add for configure generation Feb 21, 2017
b-em.cfg Add Master models ROMs patched for Y2K and OSW7F Nov 2, 2018
cleanem.bat Fixes to Windows build (work in progress) Mar 1, 2017
cmos.bin Address a few niggles Feb 25, 2017
cmos350.bin Merge READMEs together and form README.MD Feb 21, 2017
cmosa.bin Merge READMEs together and form README.MD Feb 21, 2017
cmosc.bin Merge READMEs together and form README.MD Feb 21, 2017 Switch to gnu11 from gnu99 to make static_assert work, and add -lalle… May 26, 2018
hd4.hdf Add roms/discs/tapes Feb 21, 2017
hd5.hdf Add roms/discs/tapes Feb 21, 2017
makebem.bat Fix compilation on Windows. Sep 4, 2018 vdfs: add file missed from previous commit Dec 16, 2017



B-em is an emulator for various models of BBC Microcomputer as made by Acorn Computers in the 1980s along with a selection of 2nd processors. It is supported for Win32 and Linux/UNIX but may also work on other systems supported by the Allegro library.

B-em is licensed under the GPL, see COPYING for more details.


Development is active! If you want to know how to work with us, then please make sure you have a look at the file


A file exists, with ideas for future functionality for future versions of B-em.


B-em is maintained by a group of volunteers -- please enquire over on the stardot forums for more details.



You will need the following libraries:

  • Allegro 5.2 or later
  • Zlib

Linux distros which include Allegro 5.2 at the time of writing include:

  • Arch
  • Debian Stable (stretch)
  • Ubuntu 17.10 (Artful Aardvark)

Allegro 5.2 packages for Ubuntu 16.04 LTS can be had from Launchpad

Released version

Open a terminal window, navigate to the B-em directory then enter:

./configure && make

From Git Sources

./ && ./configure && make


  • B-Em looks for its config file at $XDG_CONFIGID_RI/b-em/b-em.cfg or, if $XDG_CONFIG_DIR is not defined, at ~/.config/b-em.cfg
  • If a config file cannot be found in those locations it will pick up a default config from where the package is installed or from the build directory, if being run from there. It will always be saved back to $XDG_CONFIGID_RI/b-em/b-em.cfg or ~/.config/b-em.cfg
  • This same config dir is used for CMOS RAM images and hard disc images.
  • On Linux, the debugger expects to use the terminal window from swhich you started b-em for input and output. On Windows it opens a console window for this purpose.
  • With the port to Allegro 5 all the video output modes are available on both Win32 and Linux.


You will need version of Allegro from:

Later versions have a bug which prevents the ticks in the menus from working correctly. There is a choice of compilers but these instructions only cover MingW and assume you have this correctly installed.

Unpack/clone B-Em into a folder and unpack the Allegro5 into a parallel folder, i.e. so the allegro folder and the b-em folder have the same parent and then from within the b-em folder run makebem.bat


  • On Windows B-Em looks for resource files in the same directory as the executable with the exception of b-em.cfg, the CMOS RAM image file and hard discs which are stored in a b-em.exe folder within your Windows roaming profile.


  • Emulates Models A, B, B+, Master 128, Master 512, Master Turbo and Master Compact
  • Also emulates ARM evaluation system on Master 128
  • Emulates 6502, 65816 and Z80 and 32016 tubes.
  • Cycle-accurate video emulation
  • All documented and undocumented 6502 instructions
  • 8271 Floppy Disc Controller emulated (double drive, double sided, 80 track, read/write)
  • 1770-based Floppy Disc Controllers from various manufacters emulated including Acorn, Opus, Solidisk, Watford Electronics (double drive, double sided, 80 track, read/write)
  • Supports following disc formats - .ssd, .dsd, .adf, .adl, .img, .fdi and variants thereof for the non-Acorn DFSes.
  • Supports the following tape formats: .uef and .csw
  • Can run many protected disc and tape games.
  • SCSI and IDE hard disc emulation
  • Sound emulation, including sample playback
  • BeebSID emulation
  • Hybrid Music System emulating including Music 500o (synth), Music 4000 (keyboard, emulated via MIDI) and Music 2000 (MIDI).
  • Lots of video tricks, such as overscan, raster splitting, rupture, interlace, mid-line palette and mode splits, etc.
  • Video NuLA extended pallete ULA emulation.
  • Sideways RAM emulation
  • Joystick emulation
  • AMX Mouse emulation

New Features

New With V2.2

  • MOS 3.50 emulation
  • Fixed CRTC bug when programmed with stupid values (MOS 3.50 startup)
  • ADFS disc corruption bug fixed (Carlo Concari)
  • Fixed ACIA bug - Pro Boxing Simulator tape version now works
  • Fixed bug which created endless blank hard disc images
  • Printer port DAC emulation
  • AMX mouse emulation
  • Master 512 mouse now works properly
  • Master Compact joystick emulation
  • IDE emulation available in non-Master models
  • UI fixes (some from Carlo Concari)
  • Improvements to VIA emulation
  • PAL video filter
  • Bugfixes in ARM and 65816 coprocessors
  • Debugger fixes
  • Tidying up of code
  • Windows version can now build on MSVC as well as GCC

New Since Version 2.2


  • Implement RTC for Master
  • Working 32016 co-processor (shared with PiTubeDirect)
  • Emulate a SCSI hard disk.
  • VDFS - selective access to host filesystem as a standard Acorn filing system.
  • Music 5000 emulation (from Beech, via Hoglet)
  • Music 4000 emulation via MIDI
  • Load ROMs into specific slots
  • Debugging on all current tube processors
  • Debugger "step over subroutine"
  • Debugger: optional refresh screen in single-step/when breakpoint hit.
  • Debugger: tracing instructions to a file
  • Video NuLa
  • Add ROMs to an existing model from anywhere on your PC via menu and file picker.
  • Save state to a snapshot file for a machine running with Tube processor (except 32016)
  • Emulate the external and internal 6502 tube processors with the correct ROMs and speeds
  • Get the visualisation of memory access when debugging working cross-platform
  • Cross-platform keyboard mapping
  • Cross-platform tape catalogue
  • Replace diverged Windows/Linux GUI with single GUI

Bug Fixes

  • potential crashes when loading tapes
  • CSW files now work on 64-bit Linux
  • Unix: Fix fullscreen handling
  • Add missing SBC zero page indirect X on tube 6502
  • Fix SBC overflow (V) in binary mode on main and tube 6502
  • Fix aparent error with carry flag in undocumented instructions.
  • i8271: fix emulation always reporting drive as ready
  • i8271: ensure spindown happens on disk fault and on closing a disk image.
  • 65816: Fix failure to remember 65816 is enabled
  • mouse: Fix not working Y direction in 80186 co-pro Gem.
  • 6502: fix BCD errors on both main and tube 6502 (but not 65C02)
  • 65c02: Added missing BIT zp,X (0x34) instruction
  • 65c02 core/tube: Correted NOP lengths
  • 65c02 core/tube: Fixed ZP wrapping issue with inditect addressing
  • 6502tube: implement Rockwell instructions RMB/SMB and BBR/BBS
  • debugging: fix disassembly of 6502 opcode 24, BIT zp
  • Fix 256 byte transfer over Tube hangs
  • video: fix loadstate/savestate inconsistency

Default keyboard mapping

BBC key PC key
*: @'
+; :;
-= -_
^~ +=
f0 f0 (function keys are based on keycaps, not positioning)
3# 3
6& 6
7' 7
8( 8
9) 9
0 0
Shift lock - ALT

The PC key Page Up acts as a speedup key and Page Down as pause.


The options are:


Option Meaning
Hard reset resets the emulator, clearing all memory.
Load state load a previously saved savestate.
Save state save current emulation status.
Save Screenshot save the current screen to a file
Exit exit to OS.


Option Meaning
Paste via Keyboard send the contents of the clipbaord as keystoked
Printer to Clipboard capture printer output and copy to clipboard


Option Meaning
Autoboot disc 0/2 load a disc image into drives 0 and 2, and boot it.
Load disc 0/2 load a disc image into drives 0 and 2.
Load disc 1/3 load a disc image into drives 1 and 3.
Eject disc 0/2 removes disc image from drives 0 and 2.
Eject disc 1/3 removes disc image from drives 1 and 3.
New disc 0/2 creates a new DFS/ADFS disc and loads it into drives 0 and 2.
New disc 1/3 creates a new DFS/ADFS disc and loads it into drives 1 and 3.
Write protect disc 0/2 toggles write protection on drives 0 and 2.
Write protect disc 1/3 toggles write protection on drives 1 and 3.
Default write protect determines whether loaded discs are write protected by default
IDE Hard disc Enables emulation of an IDE hard disc
SCSI Hard disc Enables emulation of a SCSI hard disc
Enable VDFS Enable a subset of host OS files to be visible as an Acorn filing system
Choose VDFS Root Chose the directory on the host that is visible via VDFS


Option Meaning
Load tape load a tape image.
Eject tape removes a tape image.
Rewind tape rewind the emulated tape.
Show tape catalogue shows the catalogue of the current tape image.
Tape speed select between normal and fast tape speed.


This menu shows the contents of the Sideways ROM/RAM banks and enables these to be loaded with a different ROM, enabled as RAM or cleared.


This lists the models in alphabetical order and enabled the model being emulated to be changed. Be aware this causes a reset.


This lists the available second processors and enabled one to be chosen. The choice in this menu is overridden if the model picked in the model menu is always supplied with a 2nd processor, for example the Master 512. For models like this the bundled 2nd processor is always used.



Display Type

Option Meaning
Line Doubling stretch the BBC screen by doubling every line.
Scanlines stretch the BBC screen by blanking every other line
Interlaced emulate an interlaced display (useful for a handful of demos). Allows high resolution mode 7.
PAL use PAL filter!
PAL interlaced use PAL filter, with interlacing. Slow! Allows high resolution mode 7.


Option Meaning
None Remove all the black space around the image area.
Medium borders intended to be attractive rather than accurate
Full The full borders as seen on an old TV
Option Meaning
Fullscreen enters fullscreen mode. Use ALT-ENTER to return to windowed mode.


Option Meaning
Internal sound chip enable output of the normal BBC sound chip.
BeebSID enable output of the SID emulation.
Music 5000 enable output from an emulated Music 5000 synth
Printer Port DAC enable output of 8-bit DAC connected to the printer port.
Disc drive noise enable output of the disc drive sounds.
Tape noise enable output of the cassette emulation.
Internal sound filter enable bandpass filtering of sound. Reproduces the poor quality of the internal speaker.
Internal waveform choose between several waveforms for the normal BBC sound chip. Square wave is the original.

reSID configuration

Option Meaning
Model choose between many different models of SID. Many tunes sound quite different depending on the model chosen.
Simple method Choose between interpolation and resampling. Resampling is in theory higher quality, but I can't tell the difference.
Disc drive type choose between sound from 5.25" drive or 3.5" drive.
Disc drive volume set the relative volume of the disc drive noise.


Option Meaning
Redefine keys redefine keyboard setup. Map CAPS/CTRL to A/S - remaps those 2 keys. Useful for games where CAPS/CTRL are left/right.


Option Meaning
AMX mouse enables AMX mouse emulation.


Choose an soeed relative to a real model of that type.


Option Meaning
Debugger Enters debugger for debugging the main 6502. Type '?' to get list of commands.
Debug Tube Enters debugger for debugging the current 2nd processor.
Break break into debugger.

Command Line Options

b-em [discimage|tapeimage|snapshot] [-u name.uef] [-mx] [-tx] [-i] [-c] [-fx]

discimage name.ssd/dsd/adf/adl/img etc. tapeimage name.uef/csw snapshote name.snp (previously saved snapshot) -u name.uef - load UEF image name.uef

-mx - model to emulate, where x is

0 - BBC A with OS 0.1
1 - BBC B with OS 0.1
2 - BBC A
3 - BBC B with 8271 FDC
4 - BBC B with 8271 FDC + loads of sideways RAM
5 - BBC B with 1770 FDC + loads of sideways RAM
6 - BBC B US with 8271 FDC
7 - BBC B German with 8271 FDC
8 - B+ 64K
9 - B+ 128K
10 - Master 128 w/ MOS 3.20
11 - Master 512
12 - Master Turbo
13 - Master Compact
14 - ARM Evaluation System
15 - Master 128 w/ MOS 3.50
16 - BBC B with no FDC + loads of sideways RAM
17 - BBC B w/Solidisk 1770 FDC
18 - BBC B w/Opus 1770 FDC
19 - BBC B w/Watford 1770 FDC
20 - BBC B w/65C02, Acorn 1770
21 - BBC B with 65C02, no FDC

-tx - enable tube, where x is:

0 - 6502 (internal)
1 - ARM (Master only)
2 - Z80
3 - 80186 (Master only)
4 - 65816 (if ROMs available)
5 - 32016
6 - 6502 (external)

-i - enable interlace mode (only useful on a couple of demos)

-c - enables scanlines

-fx - set frameskip to x (1-9, 1=no skip)

-fasttape - speeds up tape access

IDE Hard Discs

To initialise a hard disc, use the included HDINIT program. Press 'I' to investigate the drive - this will set up the default parameters for a 50 mb drive. If you want a different size press 'Z' and enter the desired size - it does not matter that this does not match the size given in the emulated hardware.

Then press 'F' to format, and follow the prompts.

Master 512

Master 512 includes mouse emulation. This is used by trapping the mouse in the emulator window - click in the window to capture it, and press CTRL + END to release.

All disc images used by the Master 512 should have the .img extension - the type is determined by size. Both 640k and 800k discs are supported, as well as DOS standard 360k and 720k.

You can use the IDE hard disc emulation with this, run HDISK.CMD in DOS-Plus, then HDINSTAL.BAT. Go make a cup of tea while doing this, it takes forever! HDISK.CMD will create a file 'DRIVE_C' in ADFS so you will need to format the hard disc using ADFS first.

65816 coprocessor

The ROMs for this coprocessor are not included with the emulator. You will need to acquire the file ReCo6502ROM_816 and place it in roms\tube.

The 65816 runs at 16mhz, regardless of what the firmware is set to.

Hardware emulated

Processor Meaning
The 6502 processor All instructions should be emulated. Attempts to be cycle perfect. 65C02 is emulated for Master 128 mode.
The 65C12 tube As a parasite processor.
The 65816 tube As a parasite processor. Emulator from Snem.
The Z80 tube As a parasite processor. Probably a few bugs. Emulator from ZX82.
The ARM processor As a parasite processor for Master 128 only. Emulator from Arculator.
The 80186 tube As a parasite processor for Master 512 only. Emulator from PCem.
The 6845 CRTC Cycle-exact emulation. Runs everything I've tried, with all effects working.
The Video ULA Cycle-exact emulation, though I think palette changes are a cycle off? Might be a delay in the real chip.
The System VIA Keyboard and sound emulated. Also CMOS on Master-based models.
The User VIA Emulated.
8271 FDC Double disc, double sided, 40/80 tracks, read/write. With authentic noise. Supports read-only access of protected FDI images.
1770 FDC Double disc, double sided, 40/80 tracks, read/write. With authentic noise. Supports read-only access of protected FDI images.
IDE hard disc Emulates 2 discs. Emulation from Arculator.
SCSI hard disc Emulates 4 discs. Emulation from BeebEm.
Sound All channels emulated, with sample support and some undocumented behaviour (Crazee Rider). With optional bandpass filter.
BeebSID Emulated using resid-fp, so should be pretty accurate. Is only emulated when accessed, to reduce CPU load.
ADC Real joystick emulation, supporting both joysticks.
6850 ACIA Emulated for cassettes. Read only.
Serial ULA Emulated.
Music 5000 Hybrid synth emulated. Emulation from Beech
Music 4000 Hybrid music keyboard. Emulated via MIDI, Emulation original to B-Em
Music 2000 Hybrid MIDI I/F. Connects to host MIDI, Emulation original to B-Em

Hardware NOT emulated

  • Serial Port
  • Econet


  • Sarah Walker for B-Em up to V2.2
  • David Gilbert for writing Beebem and distributing the sources with it
  • James Fidell for writing Xbeeb and distributing the sources with it
  • Tom Seddon for updating Model B, indirectly motivating me to do v0.6, and for identifying the Empire Strikes Back bug.
  • Ken Lowe for assistance with the Level 9 adventures.
  • Rebecca Gellman for her help with a few things (hardware-related).
  • Thomas Harte for some UEF code - I wrote my own in the end - and for the OS X port.
  • Dave Moore for making and hosting the B-em site
  • Rich Talbot-Watkins and Peter Edwards (also Dave Moore) for testing
  • Robert Schmidt for The BBC Lives!
  • DJ Delorie for DJGPP
  • Shawn Hargreaves for Allegro

Acorn for making the BBC in the first place:

David Allen,Bob Austin,Ram Banerjee,Paul Bond,Allen Boothroyd,Cambridge, Cleartone,John Coll,John Cox,Andy Cripps,Chris Curry,6502 designers, Jeremy Dion,Tim Dobson,Joe Dunn,Paul Farrell,Ferranti,Steve Furber,Jon Gibbons, Andrew Gordon,Lawrence Hardwick,Dylan Harris,Hermann Hauser,Hitachi, Andy Hopper,ICL,Martin Jackson,Brian Jones,Chris Jordan,David King, David Kitson,Paul Kriwaczek,Computer Laboratory,Peter Miller,Arthur Norman, Glyn Phillips,Mike Prees,John Radcliffe,Wilberforce Road,Peter Robinson, Richard Russell,Kim Spence-Jones,Graham Tebby,Jon Thackray,Chris Turner, Adrian Warner,Roger Wilson and Alan Wright for contributing to the development of the BBC Computer (among others too numerous to mention)