Milestone #2: Basic CPU

Last updated: 2026-06-19

Before this one: from here on we're imitating the hardware, not yet understanding it. If that framing bothers you, I wrote about why it has to be this way in Imitate first, understand later.

The second milestone is a working 6502 CPU — the processor at the centre of the NES.

What it takes

• The CPU memory map — where the CPU reads and writes, so an instruction can actually fetch and store.
• The 6502 instruction set — the opcodes, their addressing modes, and the flags they touch.
• Checkpoint — the fetch–decode–execute loop and the state-logging that lets you diff against nestest.log.