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

A writeup is available on: https://xem.github.io/articles/nes.html

Commented source code: https://github.com/xem/mini6502/blob/gh-pages/6502.js

Minified: https://github.com/xem/mini6502/blob/gh-pages/6502.min.js

Zipped: https://github.com/xem/mini6502/blob/gh-pages/6502.min.js.zip (1015b)

Gzipped: https://github.com/xem/mini6502/blob/gh-pages/6502.min.js.gz (928b)

Demo/test: https://xem.github.io/mini6502/demo

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:

• https://wiki.nesdev.com/w/index.php/CPU_unofficial_opcodes
• https://wiki.nesdev.com/w/index.php/Status_flags
• http://wiki.nesdev.com/w/index.php/CPU_interrupts
• https://wiki.nesdev.com/w/index.php/Stack
• https://www.masswerk.at/6502/6502_instruction_set.html
• https://problemkaputt.de/everynes.htm#cpu65xxmicroprocessor
• https://www.npmjs.com/package/dict-tempering
• http://www.6502.org/tutorials/vflag.html
• https://retrocomputing.stackexchange.com/questions/145
• http://forum.6502.org/viewtopic.php?f=8&t=6370
• https://twitter.com/subzey/status/1326227026076971008
• https://twitter.com/subzey/status/1328659977150623750
• https://twitter.com/subzey/status/1328449127471083520