An anonymous reader writes: Programmer Michael Fogleman recently built his own emulator for the original Nintendo Entertainment System. He's now put up a post sharing many technical insights he learned along the way. For example: "The NES used the MOS 6502 (at 1.79 MHz) as its CPU. The 6502 is an 8-bit microprocessor that was designed in 1975. ... The 6502 had no multiply or divide instructions. And, of course, no floating point. There was a BCD (Binary Coded Decimal) mode but this was disabled in the NES version of the chip—possibly due to patent concerns. The 6502 had a 256-byte stack with no overflow detection. The 6502 had 151 opcodes (of a possible 256). The remaining 105 values are illegal / undocumented opcodes. Many of them crash the processor. But some of them perform possibly useful results by coincidence. As such, many of these have been given names based on what they do." It's an interesting look at how software and hardware interacted back then, and what it takes to emulate that in modern times. Fogleman released the source code on GitHub.