056. Adc zero page x

Connect ADC zero page,X (opcode $75) to CPU dispatch.

Lesson 56 of 356 · tests/chapter_01_cpu/test_056_ADC_zero_page_x.py

File to update

emulator/cpu/opcodes.py

Symbols to create/update

opcodes.adc_zero_page_x
OPCODE_TABLE[$75]

Why this step exists

This mode reuses the established zero_page_x resolver, including its page-zero wraparound. The opcode handler then reads the resolved location and passes its value to adc.

Complete example implementation

# emulator/cpu/opcodes.py
def adc_zero_page_x(cpu):
    addr = zero_page_x(cpu)
    value = cpu.bus.read(addr)
    adc(cpu, value)

OPCODE_TABLE = {
    # ...existing entries...
    0x75: adc_zero_page_x,
}

Important invariants

X is added by zero_page_x, not by the handler a second time
(operand + X) & 0xFF keeps the effective address in page zero
the byte at the effective address, not the address, is passed to adc
PC advances by two bytes total

Common misconception

Do not use ordinary 16-bit addition for the index; $FF + $01 must resolve to $0000, not $0100.

Out of scope

absolute indexed and indirect ADC modes
page-crossing timing
changes to the existing addressing helper

Run this lesson

uv run pytest tests/chapter_01_cpu/test_056_ADC_zero_page_x.py -v

Example implementation

Complete example implementation

# emulator/cpu/opcodes.py
def adc_zero_page_x(cpu):
    addr = zero_page_x(cpu)
    value = cpu.bus.read(addr)
    adc(cpu, value)

OPCODE_TABLE = {
    # ...existing entries...
    0x75: adc_zero_page_x,
}

Show test codetest_056_ADC_zero_page_x.py

import inspect

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


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_adc_zero_page_x_handler_exists_and_is_in_opcode_table():
    """Objective: create adc_zero_page_x(cpu) and add 0x75 to OPCODE_TABLE."""
    assert hasattr(opcodes, "adc_zero_page_x")
    assert callable(opcodes.adc_zero_page_x)
    assert list(inspect.signature(opcodes.adc_zero_page_x).parameters) == ["cpu"]
    assert opcodes.OPCODE_TABLE[0x75] is opcodes.adc_zero_page_x


def test_opcode_75_adc_zero_page_x_adds_indexed_value():
    """Objective: 75 10 with X=0x03 adds RAM[$0013] to A."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x75)
    rom.write(0x0001, 0x10)
    bus.write(0x0013, 0x05)

    cpu.reset()
    cpu.a = 0x10
    cpu.x = 0x03
    cpu.flags.set_carry_flag(False)
    cpu.step()

    assert cpu.a == 0x15
    assert cpu.pc == 0x8002


def test_opcode_75_adc_zero_page_x_wraps_inside_page_zero():
    """Objective: Zero Page,X wraps before reading the ADC value."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x75)
    rom.write(0x0001, 0xFF)
    bus.write(0x0000, 0x05)

    cpu.reset()
    cpu.a = 0x10
    cpu.x = 0x01
    cpu.flags.set_carry_flag(False)
    cpu.step()

    assert cpu.a == 0x15
    assert cpu.pc == 0x8002

Test functions (3)

test_adc_zero_page_x_handler_exists_and_is_in_opcode_table
test_opcode_75_adc_zero_page_x_adds_indexed_value
test_opcode_75_adc_zero_page_x_wraps_inside_page_zero