Computer
Hardware Question 17: What is the central processing unit (CPU)?
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| The CPU and Processing System |
Answer: The central processing unit
is the computer's main processing device. It functions through the interaction
of three different units: (1) the control unit that interprets instructions and
directs the processing, (2) the arithmetic/logic unit that performs arithmetic
operations and makes comparisons, and (3) the primary storage unit that
temporarily stores data during processing (main memory).The central processing
unit is the most complex of the computer's hardware components, directing most
of the information processing activities. Each new generation of CPUs adds new
processing capabilities and, at the same time gets faster. As new processing
methods are invented, new ways of miniaturizing the required circuits are also
devised. This miniaturization has resulted in ever smaller, faster computers.
Microcomputers that fit comfortably on your desk now have more processing power
than mainframe computers that used to fill an entire room.
Today's CPUs are incredibly complex
devices. To understand them, it is best to view them in terms of their
function. Functionally, the CPU is composed of two main parts, the control unit
and the arithmetic logic unit.
The CPU and processing system is illustrated below.
The CPU and processing system is illustrated below.
Answer: The control unit controls
and coordinates all of the CPUs activities. Acting on instructions that it
retrieves one by one from main memory (sometimes known
as primary storage), the control unit interprets each
instruction and carries it out. In addition, it controls input and output
devices and transfers data between the arithmetic/logic unit and main
memory.
Answer: The arithmetic/logic unit (
ALU) performs arithmetic computations and logical operations. The
arithmetic operations include addition, subtraction, multiplication, and
division. The logical operations involve comparisons such as less than, greater
than, or equal to. Although these may seem like simple operations, the ALU can
carry out extremely complex tasks by combining these functions. To a great
degree, the speed of the ALU determines the speed of the computer.
Both the control unit and the ALU
contain registers. They are temporary storage locations for
managing instructions and data as they are being processed. For example, the
ALU might temporarily store the result of one arithmetic calculation in a
register while it performs a second calculation using that result.
Computer Hardware Question 20: What is the instruction
cycle (I-Cycle) and the execution cycle (E-Cycle)?
Answer: When the CPU receives an instruction from main
memory to perform a specified operation, the period of time it takes to
retrieve the instruction and initiate the task is referred to as the instruction
cycle. It may also be referred to as the I-cycle
or I-time. The execution cycle refers
to the period of time in which the instruction is executed and the result is
stored in a register. It may also be referred to as the E-cycle
or E-time. A machine cycle consists of
the instruction cycle and one or more execution cycles. A machine cycle used to
be measured in milliseconds (thousandths of a second). But with the advent of
ever faster CPUs, we now are more likely to measure machine cycles in
microseconds (millionths of a second) or even in nanoseconds (billionths of a
second) and picoseconds (trillionths of a second) in the largest and fastest
computers.
Answer: The CPU has an internal clock
that synchronizes all of the operations in the cycle and the clock speed helps
to determine the speed at which operations are carried out.
Answer: Computers act on instructions provided by
computer programs. These instructions are temporarily stored in a special data
storage area referred to as main memory. Before the CPU can act, it must
retrieve instructions from main memory (also known as temporary or internal
storage). The CPU may also store data in main memory temporarily as it carries
out processing tasks. By far the most common type of temporary storage system
in use today is based on sets of silicon chips. Each chip contains millions of
miniature circuits and each of those circuits can be in one of two states. This
system is known as a binary system, the word binary
referring to two distinct states, on or off, present or absent. Data is stored
in these chips in grouped, coded patterns using this binary method. By setting
some circuits to on and others to off, the computer can store many different
kinds of data. In fact, all the different types of data that can be used on
computers can be stored using complex combinations of this simple on/off binary
coding system.
Data stored on these chips remains
in storage until the computer changes it by changing the pattern or until the
computer is turned off. Without power, the circuits in the chips change back to
their normal off-state and all the data is lost. For that reason, this type of
memory is known as volatile and it is contrasted to more
permanent types of storage systems that are known as nonvolatile.
This type of primary storage is also known as random-access memory
( RAM) and the chips are referred to as random-access memory
chips. However, the term "random" may not be the best way to refer to
this type of memory. While almost all of today's computers use some
random-access method of storing data (that is, the computer can retrieve data
from wherever it is stored, randomly), the term RAM is reserved for the
computer's primary, chip-based memory system.
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