davepoo/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.

6502emulator's Issues

Cycles

As wrote on your youtube, "Cycle" is only know when the instruction is know by the processor.

How to compile 6502Test with GoogleTest

What is the license of this project?

Hi, first of all, thank you for posting the youtube videos and the source files. I'm learning a lot from your work.
Anyway, as written in the issue title, what is the license of this code? I hope you make it clear someday.

Shifts should set Z flag if A register is zero

ASL, ROL and ROR should set zero flag base on A register every time (in every address mode).