jsbeeb - Javascript BBC Micro emulator

A 32K BBC Model B micro computer emulator in Javascript. Runs on Firefox and Chrome. Key mappings you may find useful:

• BBC F0 is F10
• BBC Break key is F12
• On a US keyboard, BBC star is on ". For UK keyboards this is on 2 (sorry!)

To play right now, visit http://bbc.godbolt.org/

Getting set up to run locally

Fire up a local webserver and load it up. I use python for this as it has a built-in webserver. So:

$ cd jsbeeb
$ python -mSimpleHTTPServer
Serving HTTP on 0.0.0.0 port 8000 ...

Then visit http://localhost:8000/ and off you go.

URL parameters

• autoboot - fakes a shift break
• disc1=XXX - loads disc XXX (from the discs/ directory) into drive 1
• disc2=XXX - as above
• disc1=!YYY - creates a local disk YYY which will be kept in browser local storage
• disc1=|ZZZ - loads disc ZZZ from the Stairway to Hell archive
• tape=XXX - loads tape XXX (from the tapes/ directory)
• tape=|ZZZ - loads tape ZZZ from the Stairway to Hell archive
• patch=P - applies a memory patch P. See below.

Patches

Patches can be applied by making a patch=P URL parameter. P is a sequence of semicolon separated patches of the form @XXXX,YYYY:ZZZZZ,... where the @XXXX specifies a PC address to breakpoint, the YYYY is the address to patch and the ZZZZ is the data to write at address YYYY. The @ part is optional, but is handy to ensure the code you want to patch has actually loaded. For example: patch=@31a6,0769:6e4c4d48465a which is a patch for the default Elite image. Once the PC has reached $31a6, the bytes at 0769 are replaced with 6e4c4d48465a.

TODO

If you're looking to help:

• Testing
  • Play lots of games and report issues either on github or by email (matt@godbolt.org).
• Core
  • Save state ability
  • Get the "boo" of the boot "boo-beep" working
  • Optimization
• Save disc support
  • I've started dropbox support, but it's not quite there yet.
  • Google Drive support would be nice
  • Local discs need to be made more workable and need an "export" feature
• git grep -i todo

Timings

In the tests directory is 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 discs/ directory. The emulation now agrees 100% with this and there are tests to keep it in line.

Tests

There are some simple tests of correctness against a few timing sources. Visit the /tests URL to run them.

Tests can be run automatically if you have node installed - just run make and it'll ensure the relevant libraries are installed, then it'll run the tests.

Thanks

Based on Tom Walker's C B-Em emulator -- thanks to him for his hard work and for open sourcing his code. Also huge thanks to Richard Talbot-Watkins for his advice and help along the way.

More information

I've written a lot of how the innards work on my blog in the emulation section.