194. Pha implied

Lesson 194 of 356 · tests/chapter_01_cpu/test_194_PHA_implied.py

Prerequisite: step 193 added `pha. In this step, change only emulator/cpu/opcodes.py by importing pha and adding its entry to OPCODE_TABLE`.

Why this step exists

PHA gets its source from CPU register A, so opcode $48 requires no addressing mode or operand and can dispatch directly to `pha(cpu)`.

Suggested implementation

from emulator.cpu.instructions import pha

OPCODE_TABLE = {
    # existing entries
    0x48: pha,
}

Invariants: preserve existing mappings; map exactly $48 to the `pha` function object; consume only the opcode byte; leave stack behavior to step 193's function.

Misconception: the byte following $48 is the next opcode, not an accumulator value to push.

Out of scope: implementing `pha` is step 193. PLA starts at step 195; other stack instruction mappings belong to their later numbered steps.

Run this lesson

uv run pytest tests/chapter_01_cpu/test_194_PHA_implied.py -v

Example implementation

Suggested implementation

from emulator.cpu.instructions import pha

OPCODE_TABLE = {
    # existing entries
    0x48: pha,
}

Show test codetest_194_PHA_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 pha
from emulator.memory.fake_rom import FakeROM


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_pha_implied_is_in_opcode_table():
    """Objective: opcode 0x48 is the official PHA opcode."""
    assert opcodes.OPCODE_TABLE[0x48] is pha


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


def test_opcode_48_pha_pushes_accumulator_to_stack():
    """Objective: executing opcode 0x48 pushes A to the current stack slot."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x48)

    cpu.reset()
    cpu.a = 0x42
    cpu.step()

    assert bus.read(STACK_BASE | 0xFD) == 0x42
    assert cpu.s == 0xFC


def test_opcode_48_pha_does_not_fetch_operand_bytes():
    """
    Objective:
    PHA is one byte long. The byte after PHA must not be consumed as an operand.

    If the next byte is $99, PHA must ignore it and push A instead.
    """
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x48)
    rom.write(0x0001, 0x99)

    cpu.reset()
    cpu.a = 0x55
    cpu.step()

    assert bus.read(STACK_BASE | 0xFD) == 0x55
    assert cpu.pc == 0x8001


def test_opcode_48_pha_does_not_modify_accumulator_or_flags():
    """
    Objective:
    PHA is only a stack push. It does not transform A and it does not update flags.
    """
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x48)

    cpu.reset()
    cpu.a = 0x80
    cpu.p = 0x00
    cpu.step()

    assert cpu.a == 0x80
    assert cpu.p == 0x00
    assert bus.read(STACK_BASE | 0xFD) == 0x80

Test functions (5)

test_pha_implied_is_in_opcode_table
test_pha_instruction_signature_takes_only_cpu
test_opcode_48_pha_pushes_accumulator_to_stack
test_opcode_48_pha_does_not_fetch_operand_bytes
test_opcode_48_pha_does_not_modify_accumulator_or_flags