MOS-6502-Emulator

An instruction-accurate 6502 CPU emulator written in C, designed for integration into larger systems.
The 6502 and its derivatives powered systems like the Commodore 64, Nintendo Entertainment System (NES), Apple II, BBC Micro.

features

This emulator uses a jump-table design,
where the opcode value (e.g., 0x80) directly indexes a function pointer table.
Each entry in the table points to a function that replicates the behavior of the corresponding 6502 instruction.

Binary Coded Decimal (BCD) mode is fully supported, along with all 13 addressing modes:

IMPLIED
IMMEDIATE
ACCUMULATOR
ABSOLUTE
ZERO PAGE
INDEXED-X ABSOLUTE
INDEXED-Y ABSOLUTE
INDEXED-X ZERO PAGE
INDEXED-Y ZERO PAGE
ABSOLUTE INDIRECT
INDEXED-X INDIRECT
INDEXED-Y INDIRECT
RELATIVE

The emulator fully implements all 151 official opcodes of the 6502.
The 8-bit opcode design allows for 256 possible values, but many are "illegal opcodes."
These undocumented instructions perform unexpected or unconventional operations,
such as modifying multiple registers or combining valid opcodes.

Illegal opcodes are not implemented, and executing them will only increments the PC by the OP size.

main loop

classic fetch-decode-execute cycle:

while (1)
{
  // fetch
  mos6502->opcode = bus->read(ram, mos6502->PC);

  // decode - execute
  mos6502->cycles = mos6502->opcodes[mos6502->opcode](mos6502);
}

• Fetch: The next opcode is retrieved from memory at the program counter (PC).
• Decode: The opcode is used as an index in the opcode table (function pointer array).
• Execute: The indexed function is called, performing the 6502 instruction and returning the number of cycles it takes.

integration

This emulator is designed to be integrated into larger systems emulating machines that use the 6502 processor,
such as the Commodore 64, NES, Atari, or Apple II.

• dedicated "production" branch, which excludes visualization features.
• bus implementation to connect the CPU with RAM, ROM, and other devices.

typedef	struct _bus {
  uint8_t     ram[ADDR_RANGE];
  uint8_t     rom[ADDR_RANGE];
  // bank switching logic .. 
  // methods ..
  // other devices ..
}	_bus;

visualisation

The main branch has a dedicated thread to run the visualization using the ncurses library,
allowing for real-time observation of the CPU behavior

• PC.
• Accumulator (A).
• Index registers (X/A).
• Stack pointer (SP).
• Status register 8-bit (each bit represents a flag).
• last executed opcode.
• Memory dumps:
  • PC surrounding area: hex dump of memory from PC - 0xFF to PC + 0xFF.
  • SP Surrounding Area: Displays memory from SP - 0x32 to a fixed address (0x200).

upcoming features

• full illegal opcodes implementation (partial progress)
• decimal mode support (now added)