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Almost all NES games were hand-written in 6502 assembly, the same as used in the Commodore 64, the Apple ][e, etc. The very few which were written in C had a reputation for being terribly slow because the NES is only 2Mhz and has 2Kb of onboard RAM (with an 8Kb window for the cartridge to patch more in). Careful assembly designed to take full advantage of the NES's peculiar architecture was much more effective.

Stop and think about that... games like Super Mario Bros 3 and Kirby's Adventure ran on 2Mhz. Compare how rich they are to today's games which swallow gigahertz and hundreds of megabytes of RAM...