BBC-B

Pet project to learn about 6502 CPU, Rust and BBC micro computer

• ALU does binary ADC/SBC, shifts and rotates, bit wise Boolean, ...
• Addressing modes tested in original prototype
• Stack, register transfers, load & store
• Branch, jump and subroutines
• BRK and IRQ and NMI handling
• 6502 disassembler
• Preliminary benchmark performance ~7e7 instructions / second
• Memory page dispatcher routes 0xFE00-FF to SHEILA mapped I/O (under construction)
• Snoop OSWRCH and pipe output to terminal — allows us to see what's going on despite lack of video circuit emulation
• Quick 'n' dirty MODE 4 frame buffer, using minifb to see what's going on (do proper video ULA and 6845 later)
• (Barely) RUNs a manually typed program in BBC BASIC 2!

notes

• (See also: diary)
• Instruction execution puts PC increment in wrong place
• No timing or cycle counting whatsoever
• Missing 99% peripherals
• Has only 11% of mos6522 logic for system VIA and rudiments of keyboard interface
• Write protect ROM area, paged ROM/RAM bank switching, JIM, FRED, ...
• OMG, Toby Nelson's annotated MOS assembly is a treasure!
• Added somewhat and refactored B-em C implementation of 6522 (system) VIA wrapped in Rust
• Intent is not to keep it there permanently but to compare behaviour to my own implementation and hopefully manage to emulate a working keyboard that way