010. Lda flags

Update Zero and Negative flags after immediate LDA.

Lesson 10 of 356 · tests/chapter_01_cpu/test_010_LDA_flags.py

File to update

emulator/cpu/cpu.py

Location

CPU.step, inside the existing $A9 immediate-LDA branch

Why this step exists

The 6502 records whether an LDA result is zero and whether bit 7 is set. Later branch instructions consume these status bits, so both setting and clearing must work.

Complete example implementation

class CPU:
    # Keep the existing methods from Tests 004, 008, and 009.

    def step(self) -> None:
        opcode = self.fetch_byte()

        if opcode == 0xA9:
            self.a = self.fetch_byte()

            if self.a == 0:
                self.p |= 1 << 1
            else:
                self.p &= ~(1 << 1)

            if self.a & (1 << 7):
                self.p |= 1 << 7
            else:
                self.p &= ~(1 << 7)

            return

        raise NotImplementedError(
            f"Opcode {opcode:02X} not implemented"
        )

Result table

A=$00 -> Z=1, N=0
A=$7F -> Z=0, N=0
A=$80 -> Z=0, N=1

Important invariant

Only the Zero and Negative bits are changed; reset's Interrupt Disable bit and every other status bit retain their previous values.

Common misconception

"Negative" does not perform signed arithmetic here. It means that bit 7 of the 8-bit result is set.

Out of scope

named flag constants and the shared helper, introduced in Test 012
absolute and zero-page LDA
instruction cycle counts

Run this lesson

uv run pytest tests/chapter_01_cpu/test_010_LDA_flags.py -v

Example implementation

Complete example implementation

class CPU:
    # Keep the existing methods from Tests 004, 008, and 009.

    def step(self) -> None:
        opcode = self.fetch_byte()

        if opcode == 0xA9:
            self.a = self.fetch_byte()

            if self.a == 0:
                self.p |= 1 << 1
            else:
                self.p &= ~(1 << 1)

            if self.a & (1 << 7):
                self.p |= 1 << 7
            else:
                self.p &= ~(1 << 7)

            return

        raise NotImplementedError(
            f"Opcode {opcode:02X} not implemented"
        )

Result table

A=$00 -> Z=1, N=0
A=$7F -> Z=0, N=0
A=$80 -> Z=0, N=1

Show test codetest_010_LDA_flags.py

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

def test_lda_clears_zero_flag():
    """Now it's time configure LDA flags
    Ensure that zero flag is unset (clear)

    If result == 0 -> Flag Zero is set 

    7  bit  0
    ---- ----
    NV1B DIZC
    |||| ||||
    |||| |||+- Carry
    |||| ||+-- Zero
    |||| |+--- Interrupt Disable
    |||| +---- Decimal
    |||+------ (No CPU effect; see: the B flag)
    ||+------- (No CPU effect; always pushed as 1)
    |+-------- Overflow
    +--------- Negative
    
    Pseudocode:
    if LDA:
      if self.a == 0:
        set flag Z
     else: 
        clear flag Z

    """
    rom = FakeROM()
    
    rom.write(0x7FFC, 0x00)
    rom.write(0x7FFD, 0x80)

    rom.write(0x0000, 0xA9)
    rom.write(0x0001, 0x42)

    bus = CpuBus(program_rom=rom)
    cpu = CPU(bus)

    cpu.reset()
    cpu.step() 

    assert cpu.a == 0x42
    assert (cpu.p & (1 << 1)) == 0 # Flag Zero is clear (0)


def test_lda_sets_zero_flag():
    """
    Ensure that zero flag is set

    if self.a == 0 -> set flag Z else Z is cleared
    Pseudocode:
    if LDA:
     if self.a == 0:
        set flag Z
     else: 
        clear flag Z

    """
    rom = FakeROM()
    
    rom.write(0x7FFC, 0x00)
    rom.write(0x7FFD, 0x80)

    rom.write(0x0000, 0xA9)
    rom.write(0x0001, 0x00)

    bus = CpuBus(program_rom=rom)
    cpu = CPU(bus)

    cpu.reset()
    cpu.step() 

    assert cpu.a == 0x00
    assert (cpu.p & (1 << 1)) != 0 # Flag Zero is set

def test_lda_sets_negative_flag():
    """
    Ensure that Negastive Flag is set on bit 7 active
    for cpu.a
    """
    rom = FakeROM()
    
    rom.write(0x7FFC, 0x00)
    rom.write(0x7FFD, 0x80)
                            #        bit 7
    rom.write(0x0000, 0xA9) #         v
    rom.write(0x0001, 0x80) # 0x80 = b1000_0000

    bus = CpuBus(program_rom=rom)
    cpu = CPU(bus)

    cpu.reset()
    cpu.step()

    assert cpu.a == 0x80
    assert (cpu.p & ( 1 << 7 )) != 0 # N 


def test_lda_clears_negative_flag():
    """
    Ensure that Negastive Flag is unset on bit 7 inactive
    for cpu.a
    """
    rom = FakeROM()
    
    rom.write(0x7FFC, 0x00)
    rom.write(0x7FFD, 0x80)
                            #        bit 7
    rom.write(0x0000, 0xA9) #         v
    rom.write(0x0001, 0x7F) # 0x00 = b0111_1111

    bus = CpuBus(program_rom=rom)
    cpu = CPU(bus)

    cpu.reset()
    cpu.step()

    assert cpu.a == 0x7F
    assert (cpu.p & ( 1 << 7 )) == 0 # N = 0

Test functions (4)

test_lda_clears_zero_flag
test_lda_sets_zero_flag
test_lda_sets_negative_flag
test_lda_clears_negative_flag