The 6502 processor was massive in the late seventies and eighties, powering famous computers like the BBC Micro, atari 2600, commodore 64, apple II, and the famicom. Bender in Futurama has a 6502 processor for a brain. Even the Terminator was programmed in 6502.
Many programs of that era were written in 6502 assembly and so it's much easier and more friendly to write than something like x86. It helps to have a strong understanding of binary and hexadecimal.
language
6502 basics
6502 opcodes
6502 branching
tools
https://github.com/skilldrick/6502js
A js 6502 assembler and simulator.
https://wiki.xxiivv.com/site/lin6.html
A linter for 6502 assembly code.