Why a Beeb?

Post date: 20-Dec-2009 23:59:29

Most 6502 fans have an attachment to some early computer they had: a Compukit, Superboard, KIM, PET, C64 or Atari. Or the Beeb - the BBC Micro from Acorn.

Other than emotional attachment, and being in possession of 3 beebs, it's a good platform for our purposes because

• You can still buy them
• The OS is well-documented and extensible
• Many chips are socketed
• There are 4 slots for ROMs, all socketed (and RAM expansions are also possible in that space)
• There's room in the case for a CPU replacement board
• There's an asynchronous message-passing co-processor interface called the TUBE, with OS support, which allows for higher clock speeds and wider busses on the 'parasite' side.
• It runs at 2MHz (slowing to 1MHz for peripherals) which means we get a meaningful speedup from even a 4MHz replacement CPU
• It has a stable PSU with headroom for additional circuits
• There are 16MHz, 8MHz and 4MHz free-running clocks easily accessible on the motherboard

The awkward aspects include

• The system clock is generated on the motherboard - a processor replacement cannot stop or modify the clock, and must cope with transitions between 2MHz and 1MHz
• The interrupt handler is not 65816 compatible, and the 65816 extended vectors overlap with OS call locations