maxpoletaev / 6502

MOS 6502 CPU emulator

🗿 6502

MOS 6502 CPU emulator made for fun.

Current status

✅ADC	✅AND	✅ASL	✅BCC	✅BCS	✅BEQ	✅BIT	✅BMI	✅BNE	✅BPL	✅BRK	✅BVC	✅BVS	✅CLC
❌CLD	✅CLI	✅CLV	✅CMP	✅CPX	✅CPY	✅DEC	✅DEX	✅DEY	✅EOR	✅INC	✅INX	✅INY	✅JMP
✅JSR	✅LDA	✅LDX	✅LDY	✅LSR	✅NOP	✅ORA	✅PHA	✅PHP	✅PLA	✅PLP	✅ROL	✅ROR	✅RTI
✅RTS	✅SBC	✅SEC	❌SED	✅SEI	✅STA	✅STX	✅STY	✅TAX	✅TAY	✅TSX	✅TXA	✅TXS	✅TYA

The Virtual Machine

A CPU on its own is pretty useless. Because of that, the project comes with a basic virtual machine to play around with. It works at 1MHz and connects the CPU to 64K of RAM and memory-mapped stdout area that can be used to print something to the terminal.

The overall memory layout looks like this:

  +---------------------+
  |   0x0000 - 0x00FF   |
  |      Zero Page      |
  +---------------------+
  |   0x0100 - 0x01FF   |
  |        Stack        | <- Stack Pointer
  +---------------------+
  |   0x0200 - 0x02FF   |
  |       Stdout        |
  +---------------------+
  |                     | <- Reset Vector
  |                     |    (your program starts here)
  |                     |
  |   0x0300 - 0xFFFF   |
  |     ROM memory      |
  |                     |
  |                     |
  |                     |
  +---------------------+

There are several examples of programs in the programs directory that can be compiled using xa65 assembler and then executed as mos6502 hello_world.

Unfortunateley, there are no debugging tools at the moment, so println! is your friend.

Resources

• Ben Eater’s Build a 65c02-based computer from scratch series
• Dave Poo’s 6502 Emulator in C++ series
• Andrew Jabobs’s 6502 Reference
• Amazing NESDev Wiki