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

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=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 tapes/ directory)
• tape=sth:ZZZ - loads tape ZZZ from the Stairway to Hell archive
• patch=P - applies a memory patch P. See below.
• loadBasic=X - loads 'X' (a resource on the webserver) as text, tokenises it and puts it in PAGE as if you'd typed it in to the emulator
• autorun - types *TAPE then */ to run from tape. In conjunction with loadBasic it types RUN.
• autochain - types *TAPE then CH."" to run from tape.a
• embed - Adjust the navigation entries to make the page clearer within a 921x733 iframe in a third-party site.

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 ([email protected]).
• Core
  • Save state ability
  • Get the "boo" of the boot "boo-beep" working
• Save disc support
  • Local discs need to be made more workable and need an "export" feature
• git grep -i todo

Tests

For general correctness there are several tests in the tests 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 discs/ directory.
• 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.

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. Please note it can take a while to run the whole test suite.

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 in fathoming out the instruction timings, interrupt fun 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 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.