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Other computer families will have machine instructions that differ in detail, due to the differences in the
designs of the computers (number of registers, word size, etc.), but they all do the same, simple, basic things.
The instructions manipulate the bits of the words mathematically and logically. In general, instructions fall into
these categories: data transfer, input/output, arithmetic operations, logical operations, control transfer, and comparison.
Upon such simple functions all else is built.
MEMORY
Computer memory is organized into addressable units, each of which stores multiple bits. In the early days
of computing (meaning up until the 1970s), there was no agreement on the size of a memory unit. Different
computers used different size memory ???cells.??? The memory cell size was also referred to as the computer??™s
???word size.??? The computer word was the basic unit of memory storage. The word size of the IBM 704 was
36 bits; the word size of the Digital Equipment PDP-1 was 18 bits; the word size of the Apollo Guidance
Computer was 15 bits; the word size of the Saturn Launch Vehicle Computer was 26 bits; the word size of the
CDC 6400 was 60 bits. These machines existed during the 1950s, 1960s, and 1970s.
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