mini6502

An attempt to make the smallest standalone NES CPU (Ricoh 6502) simulator in JavaScript (< 1kb zipped)

Developed for my mini NES emulator: https://github.com/xem/jsnes-lite

Features:

Initialization of memory (m), registers (A, X, Y, S, PC, P), flags (C, Z, I, D, B, V, N) and cycle counter (c) as global vars
Memory helpers (read: r(addr), write: w(addr,value), push: h(value), pop: g())
Emulation of the 151 official 6502 opcodes and 3 interrupts
Emulation of the hardware bugs and quirks (JMP, cross-page reads...)
Cycle-accurate
Optimized to run properly at 1.76M cycles/second

Features not included:

NES specific memory mirrorings (showing the same data at different addresses)
NES specific I/O registers (to handle graphics, sound, joypads...)
NES Mappers (cartridge models, allowing ROM bank switching, save slots, ...)
Decimal mode (which is present on the original MOS 6502 CPU but not in Ricoh 6502 used by the NES)

How to use:

Include 6502.js or 6502.min.js in your project
Put the contents of a ROM in the memory m (an array of integers between 0 and 255)
Reset the CPU with op(2) or set PC at the address where the program starts
Call op() repeatedly to simulate opcodes one by one
Call op(1) for a NMI interrupt, op(2) for a Reset and op(3) for an IRQ

How to compress after golfing:

zip: add 6502.min.js in a zip file, run ./ect-0.8.3.exe -9 -zip .\6502.min.zip
gzip: run zopfli --gzip -i1000000 6502.min.js
reshuffle opcode list: <in.txt npx --node-args --experimental-modules dict-tempering --type=newline >out.txt

Useful resources: