Information Technology Reference
In-Depth Information
Figure 3.1
Processing device
Control
unit
Arithmetic/
logic unit
Hardware Components
These components include the
input devices, output devices,
primary and secondary storage
devices, and the central
processing unit (CPU). The
control unit, the arithmetic/
logic unit (ALU), and the
register storage areas
constitute the CPU.
Register storage area
Output devices
Input devices
Memory
(Primary storage)
Secondary
storage
Recall that any system must be able to process (organize and manipulate) data, and a
computer system does so through an interplay between one or more central processing units
and primary storage. Each central processing unit (CPU) consists of three associated ele-
ments: the arithmetic/logic unit, the control unit, and the register areas. The arithmetic/
logic unit (ALU) performs mathematical calculations and makes logical comparisons. The
control unit sequentially accesses program instructions, decodes them, and coordinates the
flow of data in and out of the ALU, registers, primary storage, and even secondary storage
and various output devices. Registers are high-speed storage areas used to temporarily hold
small units of program instructions and data immediately before, during, and after execution
by the CPU.
Primary storage , also called main memory or memory , is closely associated with the
CPU. Memory holds program instructions and data immediately before or after the registers.
To understand the function of processing and the interplay between the CPU and memory,
let's examine the way a typical computer executes a program instruction.
central processing unit (CPU)
The part of the computer that con-
sists of three associated elements:
the arithmetic/logic unit, the control
unit, and the register areas.
arithmetic/logic unit (ALU)
The part of the CPU that performs
mathematical calculations and
makes logical comparisons.
control unit
The part of the CPU that sequentially
accesses program instructions,
decodes them, and coordinates the
flow of data in and out of the ALU,
registers, primary storage, and even
secondary storage and various out-
put devices.
Hardware Components in Action
Executing any machine-level instruction involves two phases: instruction and execution.
During the instruction phase, a computer performs the following steps:
Step 1: Fetch instruction. The computer reads the next program instruction to be
executed and any necessary data into the processor.
Step 2: Decode instruction. The instruction is decoded and passed to the appropriate
processor execution unit. Each execution unit plays a different role: The arithmetic/logic
unit performs all arithmetic operations, the floating-point unit deals with noninteger
operations, the load/store unit manages the instructions that read or write to memory,
the branch processing unit predicts the outcome of a branch instruction in an attempt
to reduce disruptions in the flow of instructions and data into the processor, the memory-
management unit translates an application's addresses into physical memory addresses,
and the vector-processing unit handles vector-based instructions that accelerate graphics
operations.
The time it takes to perform the instruction phase (Steps 1 and 2) is called the instruction
time (I-time) .
The second phase is execution. During the execution phase, a computer performs the
following steps:
Step 3: Execute instruction. The hardware element, now freshly fed with an instruction
and data, carries out the instruction. This could involve making an arithmetic
computation, logical comparison, bit shift, or vector operation.
Step 4: Store results. The results are stored in registers or memory.
The time it takes to complete the execution phase (Steps 3 and 4) is called the execution
time (E-time) .
register
A high-speed storage area in the
CPU used to temporarily hold small
units of program instructions and
data immediately before, during,
and after execution by the CPU.
primary storage (main
memory; memory)
The part of the computer that holds
program instructions and data.
instruction time (I-time)
The time it takes to perform the
fetch-instruction and decode-
instruction steps of the instruction
phase.
execution time (E-time)
The time it takes to execute an
instruction and store the results.
 
 
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