NES TDD

009. Cpu step

Execute the first opcode: immediate LDA ($A9).

Lesson 9 of 356 · tests/chapter_01_cpu/test_009_cpu_step.py

File to update

emulator/cpu/cpu.py

Location

CPU.step

Reference

https://www.nesdev.org/wiki/Instruction_reference#LDA

Why this step exists

CPU.step introduces the minimum fetch-decode-execute loop. Immediate LDA reads the byte directly following opcode $A9 and stores it in accumulator A.

Complete example implementation

class CPU:
    # Keep the state, fetch, and reset behavior from earlier tests.

    def step(self) -> None:
        opcode = self.fetch_byte()

        if opcode == 0xA9:
            self.a = self.fetch_byte()
            return

        raise NotImplementedError(
            f"Opcode {opcode:02X} not implemented"
        )

Execution timeline

PC=$8000 -> fetch $A9 -> PC=$8001
PC=$8001 -> fetch operand $42 -> PC=$8002 -> A=$42

Important invariants

  • one step executes exactly one instruction
  • an unknown opcode fails visibly
  • immediate LDA consumes two bytes in total

Common misconception

The operand $42 is a value, not an address. Immediate mode does not perform another bus lookup using $0042.

Out of scope

  • Zero and Negative flag updates, introduced in Test 010
  • other LDA addressing modes
  • opcode tables and cycle counts

Run this lesson

uv run pytest tests/chapter_01_cpu/test_009_cpu_step.py -v