008. Cpu reset vector

File to update

emulator/cpu/cpu.py

Location

CPU.reset

Reference

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

Why this step exists

The CPU does not choose its program start address directly. On reset it reads a little-endian 16-bit vector from CPU addresses $FFFC-$FFFD through CpuBus.

Complete example implementation

class CPU:
    # Keep the constructor and fetch helpers from Test 004.

    def reset(self) -> None:
        low = self.bus.read(0xFFFC)
        high = self.bus.read(0xFFFD)

        self.pc = low | (high << 8)
        self.s = 0xFD
        self.p = 0x04

Important invariants

• vector low byte comes from $FFFC
• vector high byte comes from $FFFD
• reset reads through the bus rather than indexing FakeROM directly

Minimal example

FakeROM offsets $7FFC=$00 and $7FFD=$80 appear at CPU addresses $FFFC-$FFFD and set PC to $8000. The next fetch therefore reads FakeROM offset $0000.

Common misconception

Reset must not increment PC while reading the vector. It assigns PC from fixed bus addresses; instruction fetching begins afterward.

Out of scope

• opcode dispatch
• interrupt entry
• cycle timing