BBC Micro emulator for Windows, OS X, and Linux
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Latest commit 95815db Nov 4, 2018

Windows Build Status: status

OS X Build Status: status


A cross-platform BBC Micro emulator. Use your Windows/Linux PC or macOS computer to play your old BBC games or develop new BBC software.

If you have feedback, please add it to the b2 GitHub issues page.


For Windows and OS X, you can get a binary release from the b2 GitHub releases page. Releases are tagged with their date, time, and GitHub commit. You probably want the latest one, which will be at the top.


Unzip to a folder of your choice and run b2.exe.

Windows 7

If you have trouble with slow startup and poor performance on Windows 7, try running b2.exe from the command line with the --timer option: b2 --timer.

This setting is sticky, and will be saved on exit for future runs. So after you've done this once, you can just run it from Windows Explorer in future.


Open the dmg, drag b2 to your Applications folder and run it from there.


Please follow the building instructions.

Guided tour

Double-click the icon: b2 on OS X, b2.exe on 64-bit Windows, or b2_32bit.exe on 32-bit Windows.

Everything you need to get started is provided, and the familiar BBC Computer 32K message should appear straight away.

Load a disc

Go to Drive 0 or Drive 1 on the File menu to load a disc. Use the Disc image option to select an SSD file, then hit Shift+Break (PC Shift + PC F11) to start it.

Changing the BBC disc image doesn't affect the file on the PC disk - after loading a disc image, there are additional Save disc image and Save disc image as... options you can use to do this manually.

Change config

The default setup is a BBC Model B with Acorn 1770 DFS and 16K sideways RAM. The File > Change config submenu lets you select something different.

Change keyboard mapping

The default keyboard layout maps PC keys to their BBC equivalent by position - typically what you want for games. Some games might be better with the Default (caps/ctrl) layout, which sets PC Left Ctrl to BBC Caps Lock, PC Left Alt to BBC Ctrl, and PC Caps Lock to BBC Ctrl.

There are also two character map options, designed for typing, which try to map PC keys to BBC keys based on the character, so that PC Shift+0 gives you ) and so on. Default UK is for UK-style keyboards, and Default US is for US-style keyboards. For US-style keyboards, press ` to get the pound sign.


Use File > Save state to save your place, and File > Load last state to reload it. All state is saved, including disc contents.

Use Tools > Timeline... to visualise the saved states. The timeline is shown as a tree of rounded boxes (saved states) and square boxes (open windows), with arrows showing the relationships.

Click Load to load an old state, Clone to create a new window starting from that state, Replay to watch a replay from that state to the current state, or (Windows/Linux only) Video to save a video replay. (Video is MP4: 800Kbps H264 video + 128Kbps AAC audio.)

If the timeline becomes too full of junk, click Delete to delete a saved state.


Use Tools > Options... to bring up the options dialog, letting you fiddle with screen size and speed limiting and stuff.

Display options

Tick Auto scale to have the Beeb display automatically scaled based on the window size, or untick it and use the Manual scale filter to choose your preferred scale.

Correct aspect ratio makes the display slightly narrower, better matching the output from a real BBC.

Filter display lets the GPU smooth the display a bit when it's being stretched.

For best results, leave these options on.

For (inauthentic!) 1:1 pixel output, switch them off and set a manual scale of 1.0, or 2.0, and so on.

Turbo disc

Click Turbo disc to activate turbo disc mode. Turbo disc mode improves disc read/write throughput by 2-3x, and further improves speed by turning off emulation of seek times and internal disc delays.

Turbo disc mode is experimental

Customize keyboard layout

Click Tools > Keyboard layout... to bring up the keyboard layout dialog, showing a map of the BBC keyboard. Hover over a key to see which PC keys correspond to it. (For character maps, some keys have two parts, mapped separately.)

Each keymap has its own section, though initially only the active keymap is shown. Click the little disclosure arrow thing to reveal the others.

The default keymaps are read-only. Click the Copy button to make a modifiable copy, which you can give a name. It will appear in the Keymap submenu along with the others.

(To create a new positional map, copy one of the positional maps; to create a new character map, copy one of the character maps.)

(The BBC keyboard map always shows the Master 128 keypad, but this only has an effect in Master 128 mode.)

At the top of the list you can also configure the (limited number of) keyboard shortcuts available for emulator functions.

Customize configurations

Tools > Configurations lets you customize the configurations list.

To create a new config, use the Copy button to copy an existing one that has the disc interface you want. You can give it a name.

Click the ... button next to each ROM slot to load the ROM image for that slot.

Check the box in the RAM column to make that sideways slot writeable.

To add a new ROM slot, use one of the Add ROM buttons and select a file.

To add a new sideways RAM slot, use one of the Add RAM buttons. Sideways RAM slots are by default empty but you can use the ... button to load a ROM image on startup.

External memory

Check the External memory box to add a 16MByte paged RAM 1MHz bus device. Paging registers are at &FC00 (LSB) and &FC01 (MSB), and the corresponding page of the memory appears in page &FD.

(The external RAM can't be enabled in conjunction with theñ Opus Challenger disc interface, as both devices use page &FD.)

Paste from clipboard

Paste text from the clipboard to the BASIC prompt using OSRDCH Paste and OSRDCH Paste (+Return). The (+Return) version effectively presses Return at the end, which is sometimes necessary when copying and pasting BASIC listings.

This is intended for pasting in BASIC listings at the BASIC prompt. No guarantees it will work properly anywhere else, but you might get lucky...

To make it easy to paste text in from modern applications, newlines (CR LF, LF CR, LF) are translated into CR (ASCII 13), and £ is translated into BBC-style £ (ASCII 95). ASCII characters between 32 and 126 are passed on as-is.

Other characters are not currently supported.

Copy to clipboard

Copy text output using OSWRCH Copy Text. This works a bit like *SPOOL, in that once activated it captures anything printed via OSWRCH until deactivated.

It's explicitly described as Copy Text, because it strips out VDU control codes and normalizes line endings. You stand a good chance of being able to paste the result into a word processor or text editor or what have you.

(A future version will probably sport a Copy Binary version, which grabs everything. Though I'm not entirely sure how useful this will be.)

Debug version - BBC development, debugger, HTTP API, etc.

The debug version has some additional functionality that might prove useful for developing BBC software.

Extra instructions for the debug version.


This repo has submodules, so you have to clone it to build it. The source code archives in the releases page won't work. (This is a GitHub bug and there doesn't appear to be any way around it.)

The submodules are referred to by https. Before cloning, you can use git config --global url.ssh:// to have them cloned over SSH instead, if you have a GitHub login.

To clone the repo:

git clone --recursive

If you're reading this after already cloning it:

git submodule init
git submodule update

Regarding branches, master should always build (and shouldn't contain anything too outrageously half-baked) and build is the branch used by the CI servers to prepare releases. master is periodically merged into build when things are settled down.

wip/* is stuff that's being worked on.

Once you're all set up:

Instructions for building on Windows.

Instructions for building on OS X.

Instructions for building on OS X or Linux.

Notes about the 6502 test suites.


etc, submodules

Please consult folders and files for more info.

The contents of etc is all stuff that's freely available and distributable, included in the repo so it's self-contained, since not every dependency can be added as a git submodule.

experimental, src

Copyright (C) 2016-7 by Tom Seddon

Licence: GPL v3.