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issues signals that cause other units of the system to execute them.
The Arithmetic Logic Unit (ALU)
The ALU of the CPU is the place where the actual execution of the instructions takes
place during the data processing operation. That is, when the control unit encounters an
instruction that involves an arithmetic operation (such as, add, subtract, multiply, divide)
or a logic operation (such as less than, equal to, greater than), it passes control to the
ALU. As shown in Figure 7.1, the ALU has some special purpose registers (whose
functions are described in a later subsection) and the necessary circuitry to carry out all
the arithmetic and logic operations included in the instructions supported by the CPU. For
example, the control unit might load two numbers into the registers in the ALU. Then, it
might tell the ALU to add the two numbers (an arithmetic operation), or to check if the
two numbers are equal (a logical operation).
In case of a microcomputer, the entire CPU (both the control unit and the ALU) is
contained on a single tiny silicon chip called a microprocessor.
Every CPU has the built-in ability to execute a particular set of machine instructions,
called its instruction set. Most CPUs have 200 or more instructions (such as add, subtract,
and compare) in their instruction set. The machine language designed for a specific CPU
is based on the list of specific instructions supported by the CPU in its instruction set.
Because each processor has a unique instruction set, machine language programs written
for one computer will generally not run on another computer with a different CPU.
CPUs made by different manufacturers have different instruction sets. In fact, different
CPU models of the same manufacturer also often have different instruction sets.
However, manufacturers tend to group their CPUs into "families" that have similar
instruction sets. When a new CPU is developed, it is ensured that its instruction set
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This document was uploaded on 04/07/2014.
- Spring '14