181. Instruction jmp

add addressing-independent JMP behavior.

Lesson 181 of 356 · tests/chapter_01_cpu/test_181_instruction_jmp.py

In this step, add `emulator/cpu/instructions.py::jmp`:

Instruction

JMP -> PC = target_address

Goal

implement jmp(cpu, addr) in instructions.py.

Student guidance

JMP directly changes the program counter. It does not read memory by itself, and it does not modify flags.

Important

PC is a 16-bit register, so keep the assigned address inside 0x0000..0xFFFF:

cpu.pc = addr & 0xFFFF

Why this step exists

Addressing modes resolve the destination; the instruction primitive only installs that address in PC. Invariants: PC is masked to 16 bits, while status, registers, stack, and memory are unchanged. Misconception: `addr` is not a memory location whose byte must be loaded.

Out of scope: importing `jmp into emulator/cpu/opcodes.py` and wiring absolute/indirect handlers and table entries are steps 182-183. JSR begins at step 184.

Run this lesson

uv run pytest tests/chapter_01_cpu/test_181_instruction_jmp.py -v

from emulator.cpu.instructions import jmp from tests.helpers import NEGATIVE_FLAG, ZERO_FLAG, make_cpu CARRY_FLAG = 1 << 0 OVERFLOW_FLAG = 1 << 6 def test_jmp_sets_program_counter_to_target_address(): """Objective: JMP replaces PC with the target address.""" cpu = make_cpu() cpu.pc = 0x8000 jmp(cpu, 0x1234) assert cpu.pc == 0x1234 def test_jmp_wraps_program_counter_to_16_bits(): """Objective: PC remains a 16-bit register.""" cpu = make_cpu() cpu.pc = 0x8000 jmp(cpu, 0x1_0000) assert cpu.pc == 0x0000 def test_jmp_does_not_modify_status_flags(): """Objective: JMP only changes PC; flags are preserved.""" cpu = make_cpu() cpu.p |= CARRY_FLAG cpu.p |= ZERO_FLAG cpu.p |= OVERFLOW_FLAG cpu.p |= NEGATIVE_FLAG jmp(cpu, 0x1234) assert (cpu.p & CARRY_FLAG) != 0 assert (cpu.p & ZERO_FLAG) != 0 assert (cpu.p & OVERFLOW_FLAG) != 0 assert (cpu.p & NEGATIVE_FLAG) != 0