198. Php implied

Lesson 198 of 356 · tests/chapter_01_cpu/test_198_PHP_implied.py

Prerequisite: step 197 added `php. In this step, change only emulator/cpu/opcodes.py by importing php and registering it in OPCODE_TABLE`.

Why this step exists

PHP obtains P and S directly from CPU state. Opcode $08 therefore has no operand and dispatches directly to `php(cpu)`.

Suggested implementation

from emulator.cpu.instructions import php

OPCODE_TABLE = {
    # existing entries
    0x08: php,
}

Invariants: preserve existing mappings; map exactly $08 to the `php` function object; use no addressing wrapper or operand fetch; consume one opcode byte and retain step 197's stack/status semantics.

Misconception: the byte after $08 is not status data; PHP pushes live P.

Out of scope: PHP behavior belongs to step 197, and PLP begins at step 199. Changes to PHP's pushed byte do not belong to this opcode-registration step.

Run this lesson

uv run pytest tests/chapter_01_cpu/test_198_PHP_implied.py -v

Example implementation

Suggested implementation

from emulator.cpu.instructions import php

OPCODE_TABLE = {
    # existing entries
    0x08: php,
}

Show test codetest_198_PHP_implied.py

import inspect

from emulator.bus.cpu_bus import CpuBus
from emulator.cpu import opcodes
from emulator.cpu.cpu import CPU
from emulator.cpu.instructions import php
from emulator.memory.fake_rom import FakeROM


CARRY_FLAG = 1 << 0
BREAK_FLAG = 1 << 4
ONE_FLAG = 1 << 5
STACK_BASE = 0x0100


def make_cpu_with_rom():
    rom = FakeROM()
    rom.write(0x7FFC, 0x00)
    rom.write(0x7FFD, 0x80)
    bus = CpuBus(program_rom=rom)
    return CPU(bus), bus, rom


def test_php_implied_is_in_opcode_table():
    """Objective: opcode 0x08 is the official PHP opcode."""
    assert opcodes.OPCODE_TABLE[0x08] is php


def test_php_instruction_signature_takes_only_cpu():
    """Objective: PHP is implied, so php(cpu) does not need an operand argument."""
    assert list(inspect.signature(php).parameters) == ["cpu"]


def test_opcode_08_php_pushes_status_to_stack():
    """Objective: executing opcode 0x08 pushes P with Break and ONE set."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x08)

    cpu.reset()
    cpu.p = CARRY_FLAG
    cpu.step()

    pushed_status = bus.read(STACK_BASE | 0xFD)
    assert (pushed_status & CARRY_FLAG) != 0
    assert (pushed_status & BREAK_FLAG) != 0
    assert (pushed_status & ONE_FLAG) != 0
    assert cpu.s == 0xFC


def test_opcode_08_php_does_not_fetch_operand_bytes():
    """
    Objective:
    PHP is one byte long. The byte after PHP must not be consumed as an operand.

    The pushed value comes from cpu.p, not from program memory after 0x08.
    """
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x08)
    rom.write(0x0001, 0x99)

    cpu.reset()
    cpu.p = CARRY_FLAG
    cpu.step()

    pushed_status = bus.read(STACK_BASE | 0xFD)
    assert (pushed_status & CARRY_FLAG) != 0
    assert cpu.pc == 0x8001


def test_opcode_08_php_does_not_leave_break_set_in_cpu_state():
    """
    Objective:
    PHP pushes Break in the saved status byte, but does not keep Break set in
    cpu.p afterward in this emulator model.
    """
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x08)

    cpu.reset()
    cpu.p = 0x00
    cpu.step()

    pushed_status = bus.read(STACK_BASE | 0xFD)
    assert (pushed_status & BREAK_FLAG) != 0
    assert (pushed_status & ONE_FLAG) != 0
    assert cpu.flags.get_break_flag() is False
    assert cpu.flags.get_one_flag() is False


def test_opcode_08_php_preserves_existing_cpu_flags_except_temporary_break():
    """Objective: PHP should not damage existing CPU status flags."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x08)

    cpu.reset()
    cpu.p = CARRY_FLAG
    cpu.step()

    assert cpu.p == CARRY_FLAG

Test functions (6)

test_php_implied_is_in_opcode_table
test_php_instruction_signature_takes_only_cpu
test_opcode_08_php_pushes_status_to_stack
test_opcode_08_php_does_not_fetch_operand_bytes
test_opcode_08_php_does_not_leave_break_set_in_cpu_state
test_opcode_08_php_preserves_existing_cpu_flags_except_temporary_break