jennings

A py65-compatible 6502/6510 MPU, assembler, and disassembler with first-class support for all 105 documented NMOS 6510 illegal opcodes. Drop-in for py65: change import py65… to import jennings….

Where py65 stubs the illegals (inst_not_implemented — a 0-cycle skip), jennings executes them byte- and cycle-exact via deity-informant's hardware-validated lift + PcodeVM engine, and its assembler/disassembler decode them as real mnemonics instead of ???.

Install

pip install jennings          # pulls deity-informant (pure-Python engine)
pip install jennings[dev]     # + test/lint tooling (pytest, black, pylint, py65)

Use

from jennings.devices.mpu6502 import MPU
from jennings.memory import ObservableMemory
from jennings.disassembler import Disassembler
from jennings.assembler import Assembler

mpu = MPU(memory=ObservableMemory())     # same API as py65.devices.mpu6502.MPU
mpu.memory[0:2] = [0xAF, 0x10]           # LAX $10 — an illegal py65 cannot run
mpu.step()                               # a, x <- mem[$10]; cycle-exact

Assembler(MPU()).assemble("SLO $20")     # [0x07, 0x20]
Disassembler(MPU()).instruction_at(pc)   # (len, "SLO $20")

Compatibility

Mirrors the py65 API surface used in practice: devices.mpu6502.MPU (.step/.reset/.irq/.nmi, a/x/y/sp/p/pc/memory/processorCycles, ByteAt/WordAt, disassemble/instruct/cycletime/extracycles), memory.ObservableMemory, disassembler.Disassembler, assembler.Assembler, utils.addressing.AddressParser.

Every legal opcode is validated byte-, flag-, and cycle-exact against py65 in CI; the illegals are validated by NMS decomposition and (via the engine) byte-exact against sidplayfp. See docs/design.md, docs/illegal-opcodes.md, and docs/migrating-from-py65.md.

Limitations

No BCD (decimal-mode ADC/SBC), matching the deity-informant engine and the C64 target domain (playroutines run CLD). py65.monitor is not reimplemented.