mdj2812 / 6502emulator

Learning how a CPU works by emulating one

6502Emulator

Learning how a CPU works by emulating one.

This code was written during the youtube video : https://youtu.be/qJgsuQoy9bc

11/2020 NOTES / TODO

• All 6502 legal opcodes emulated
• Decimal mode is not handled
• Test program /Klaus2m5/6502_65C02_functional_tests - will succeed if decimal is disabled.
• Counting cycles individually for each part of an instruction is cumbersome and probably should just deduct the correct number at the end of the instruction.
• There is no way to issue and interrupt to this virtual CPU
• There are no hooks for debugging.
• There is is no dissasembler or UI, this is just the CPU emulator & units test.
• There are no asserts if you write memory outside of the bounds (it will overwrite memory)
• Illegal opcodes are not implemented, the program will throw an exception.

Issues

• Does the BRK command break when interrupts are disabled? that needs testing.
• PLP clears the break flag when executed? is this correct?
• INS_JMP_ABS, see notes about bug in 6502 for correct emulation

What did we learn from this?

• A program is just big array bytes
• Writing a CPU emulator is time consuming
• Some features are undocumented!
• Emulating a CPU is harder you than you think, because you have emulate things you got for free in a real CPU (like the clock)
• Writing Unit tests for the CPU was a good idea.
• Big switch state works for small instruction set processors, probably not for bigger ones, like the 68000.