NES TDD

004. Cpu init

Create basic CPU state and fetch helpers.

Lesson 4 of 356 · tests/chapter_01_cpu/test_004_cpu_init.py

File to update

emulator/cpu/cpu.py

Location

class CPU

Reference

https://www.nesdev.org/wiki/CPU_registers

Why this step exists

The CPU needs explicit registers and a bus dependency before it can fetch or execute instructions. Fetching advances the program counter because instruction bytes are consumed sequentially.

Complete example implementation

from dataclasses import dataclass

from emulator.bus.cpu_bus import CpuBus


@dataclass
class CPU:
    bus: CpuBus
    a: int = 0
    x: int = 0
    y: int = 0
    pc: int = 0
    s: int = 0
    p: int = 0

    def fetch_byte(self) -> int:
        value = self.bus.read(self.pc)
        self.pc += 1
        return value

    def fetch_word(self) -> int:
        low = self.fetch_byte()
        high = self.fetch_byte()
        return low | (high << 8)

Important invariants

  • CPU retains the exact bus object supplied by its caller
  • fetch_byte advances PC by one
  • fetch_word consumes low byte first and advances PC by two

Common misconception

Little-endian storage changes byte order in memory, not the numeric result. Bytes $34 then $12 produce the integer $1234.

Out of scope

  • reset-vector behavior
  • opcode decoding
  • hardware-accurate power-up register values

Run this lesson

uv run pytest tests/chapter_01_cpu/test_004_cpu_init.py -v