By the way, a ???word??? of memory is the basic unit of
storage for a computer. A 16-bit computer has a word size of 16 bits, or two bytes. A 32-bit computer has
a word size of 32 bits, or four bytes. A 64-bit computer has a word size of 64 bits, or eight bytes. When a computer
accesses memory, it usually stores or retrieves a word of information at a time.
If one looked at a particular memory location, one could not tell whether the pattern of ones and zeros in
that location was an instruction or a piece of data (number). When the computer reads a memory location
expecting to find an instruction there, it interprets whatever bit pattern it finds in that location as an instruction.
If the bit pattern is a correctly formed machine instruction, the computer performs the appropriate operation;
otherwise, the machine halts with an illegal instruction fault.
Each computer is wired to interpret a finite set of instructions. Most machines today have 75 to
150 instructions in the machine ???instruction set.??? Much of the ???architecture??? of a computer design is
reflected in the instruction set, and the instruction sets for different architectures are different. For example,
the instruction set for the Intel Pentium computer is different from the instruction set for the Sun SPARC.
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