Data are encoded in different ways, depending on the type of data. We described integer, floating-point and
character encodings. The program interprets the bit pattern in a computer word depending on what it expects to
find in that memory location. The same bit pattern can be interpreted in different ways when the program
expects one data type or another. The programmer is responsible for insuring that the program correctly
accesses its data.
The CPU consists of two parts. The control unit is responsible for implementing the steady cycle of
retrieving the next instruction, decoding the bit pattern in the instruction word, and executing the instruction.
CHAP. 3] COMPUTER ORGANIZATION 41
The arithmetic and logic unit (ALU) is responsible for performing mathematical, logical, and comparison
functions.
The instruction set of a computer is the list of primitive operations that the computer hardware is wired to
perform. Modern computers have between 50 and 200 machine instructions, and instructions fall into the categories
of data movement, arithmetic operations, logical operations, control transfer, I/O, and comparisons. Most
programmers today write in higher-level languages, and so are isolated from direct experience of the machine
instruction set, but at the hardware level, the machine instruction set defines the capability of the computer.
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