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292 Stars 59 Forks GNU General Public License v3.0 874 Commits 64 Opened issues


Javascript BBC micro emulator

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jsbeeb - Javascript BBC Micro emulator

A BBC Micro emulator in Javascript. Runs on Firefox, Chrome and Microsoft Edge. Emulates a 32K BBC B (with sideways RAM) and a 128K BBC Master. The BBC had a somewhat different-looking keyboard to a modern PC, and so it's useful to know some of the mappings:

  • BBC F0 is F10
  • BBC Break key is F12
  • BBC star is on " (if it doesn't work for you try shift-2)

To play right now, visit To load the default disc image (Elite in this case), press shift-F12 (which is shift-Break on the BBC).

Getting set up to run locally

jsbeeb needs to be built with webpack. If you're developing locally you'll need a recent

and then run
npm install
to install needed packages.

You can then run

npm start
to get a local webserver which will autoreload on changes in the code - visit http://localhost:8080/ and off you go.

URL parameters

  • autoboot
    - fakes a shift break
  • disc1=XXX
    - loads disc XXX (from the
    directory) into drive 1
  • disc2=XXX
    - as above
  • disc1=local:YYY
    - creates a local disk YYY which will be kept in browser local storage
  • disc1=sth:ZZZ
    - loads disc ZZZ from the Stairway to Hell archive
  • tape=XXX
    - loads tape XXX (from the
  • tape=sth:ZZZ
    - loads tape ZZZ from the Stairway to Hell archive
  • patch=P
    - applies a memory patch
    . See below.
  • loadBasic=X
    - loads 'X' (a resource on the webserver) as text, tokenises it and puts it in
    as if you'd typed it in to the emulator
  • embedBasic=X
    - loads 'X' (a URI-encoded string) as text, tokenises it and puts it in
    as if you'd typed it in to the emulator
  • autorun
    - types
    to run from tape. In conjunction with
    it types
  • autochain
    - types
    to run from tape.
  • autotype
    - types whatever you put after. e.g.
    &autotype=PRINT"Matt is cool"%0a
    (return is URI escaped to
  • embed
    - Adjust the navigation entries to make the page clearer within a 921x733 iframe in a third-party site.
  • cpuMultiplier=X
    speeds up the CPU by a factor of
    . May be fractional or below one to slow the CPU down. NB disc loads become unreliable with a too-slow CPU, and running too fast might cause the browser to hang.
  • sbLeft
    - a URL to place left of, right of, or below the cub monitor. The left and right should be around 648 high and the bottom image should be around 896 wide. Left and right wider than 300 will run into problems on smaller screens; bottom taller than 100 or so similarly.
  • videoCyclesBatch
    - the number of video cycles to batch up befofre running the video emulation. Defaults to zero: anything higher leads to emulation inaccuracies. Useful for showing why accuracy is important, even if less efficient.
  • rom
    - load the given URL or path as an extra ROM. If a URL is provided, that URL must allow cross-site requests. Doesn't support the sth: pseudo URL unlike
    , but if given a ZIP file will attempt to use the
    file assumed to be within.


Patches can be applied by making a

URL parameter.
is a sequence of semicolon separated patches of the form
where the
specifies a PC address to breakpoint, the
is the address to patch and the
is the data to write at address
. The
part is optional, but is handy to ensure the code you want to patch has actually loaded. For example:
[email protected],0769:6e4c4d48465a
which is a patch for the default Elite image. Once the PC has reached
, the bytes at
are replaced with

Loading BASIC files from GitHub gists

  • Create a gist with your code. - here's an example
  • Get the "Raw" link by clicking "raw" and copying the URL . In the case above that's:
  • Add that after "" or similar, for example, this link

Note that every update you make means you need to make a new raw link.

Things left to do

If you're looking to help:

  • Testing
  • Core
    • Save state ability
    • Once we have this I'd love to get some "reverse step" debugging support
    • Get the "boo" of the boot "boo-beep" working (disabled currently as the Javascript startup makes the sound dreadfully choppy on Chrome at least).
  • Save disc support
    • Local discs need to be made more workable and need an "export" feature
    • Multiple discs need a UI
  • git grep -i todo
  • Optimisation
    • While every attempt to make things fast has been made, I'm sure there's some more clever things that can be done without compromising emulation accuracy


For general correctness there are several tests in the

directory, including:
  • Klaus Dormann's exhaustive test of all documented opcodes for 6502 and 65C12. This is brought in as a git submodule from a forked version of Klaus's original as it needed a few tweaks to get 65C12 working.
  • hoglet's Binary Coded Decimal tests.
  • A public domain Commodore 64 6502 test suite which tests every 6502 opcode (documented or otherwise) for every possible input and flags condition.

For timing correctness we have:

  • A timing test program written by Rich. It has been run on a real live BBC B and the results are in the directory. An SSD of the same tests is in the
  • Some of Kevin Edwards' protection systems (stripped of the games themselves). These are extremely timing- and correctness-sensitive when it comes to the timers and interrupts of the BBC.
  • Some 65C12-specific read-modify-write tests written by Ed Spittles.

Tests can be run automatically if you have

installed - just run
and it'll ensure the relevant libraries are installed, then it'll run the tests. Please note it can take a while to run the whole test suite.


jsbeeb was heavily based on Sarah Walker's C B-Em emulator -- thanks to her for her hard work and for open sourcing her code. B-em is now being maintained by a group of enthusiasts - thanks to them too!

Huge thanks to Richard Talbot-Watkins for his advice and help along the way in fathoming out the instruction timings, interrupt fun, video code rewrite and for being such a good pal all these many years!

Thanks to Michael Borcherds for his help; improving the keyboard layouts and handling in Javascript, reporting issues, chasing down game bugs and much more.

Thanks to David Banks (hoglet) for his help in testing the gnarly BCD flag behaviour on real live BBCs.

Cheers to Ed Spittles for testing various interrupt timing code on a real BBC.

Thanks to Chris Jordan for his thorough testing, bug reports, ideas and help.

A lot of the early development used the amazing Visual 6502 as reference for intra-instruction timings. Amazing stuff.

Special shout out the users of the 6502 Forums

More information

I've written a lot of how the innards work on my blog in the emulation section.
I gave a presentation on how it all fits together at work, and posted the video up on YouTube. I presented again at GOTO Chicago 2016, and I'm hoping they post the video up.

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