An emulator of the 6502 CPU found in the Commodore 64, NES, Apple II, and more
Opcodes and other useful info: https://codebase64.org/doku.php?id=magazines:chacking1#opcodes_and_quasi-opcodes
Exact memory layout: https://sta.c64.org/cbm64mem.html
Lots of detailed info: https://rosettacode.org/wiki/Category:6502_Assembly
6502 virtual assembler: https://www.masswerk.at/6502/assembler.html
Assembly specs and table: https://www.masswerk.at/6502/6502_instruction_set.html