Thus, the length of each instruction may be variable (6502, x86_64) or fixed (ARM) -- a design decision which affects code density, execution speed, and [[Memory Prefetch|memory prefetch]] operations.

Machine language is very hard to write and read, although it can be hand-coded on simple processors (it was not uncommon for 6502 programmers to memorize the complete instruction set, for example). It is particularly difficult to write and read when the ISA uses bit-packing.