FREE online courses on Information Technology - Chapter 6 IT - FUNDAMENTALS
- THE COMPONENTS OF A PERSONAL COMPUTER
The computer you are most likely to encounter first is a
personal computer. There are millions of these devices in schools, homes, and
offices. Generally, they feature a keyboard for entering data, a CRT or television like output device for
displaying data, and some form of storage. A typical schematic for such a
computer is shown in Figure below. In this section, we present an overview of
the parts of the computer; in subsequent sections, we discuss the components in
more detail.
The heart of the computer is the central processing unit or CPT. It contains the logic that controls
the calculations done by the computer. In most personal computers, the central
processing unit is connected to a bus. The bus is a communications device,
really a connection, among various parts of the computer. The bus carries (1)
instructions from programs telling the computer what to do and (2) data. On most
modern computers, there is a separate bus between the CPU and random access memory (RAM) or what is
often called primary memory. A
separate, high-speed bus is needed here because contemporary CPU and memory
chips are so fast, the main bus into which the keyboard and secondary-storage
devices are plugged is too slow for the CPU and memory.
Primary memory of the computer holds two kinds of
information. The first is data, as one might expect. For example, if we want to
add two numbers together, such as 178 and 256, these numbers are stored in
computer memory. Once added, their sum, 434, would also be placed in memory.
Instructions in the form of software programs are also stored
in primary memory. The instructions or software tell the CPU what to do.
Instructions provide the logic of the computer and enable it to perform
calculations and manipulate data. One main feature of memory is its passive
nature as memory is only a storage place for information. Instructions are
executed in the CPU and data are moved between the CPU and memory when a
calculation is being performed. Another way to look at memory is to consider
that primary memory has no logic capability. It is a repository for data and
instructions.
Figure to be inserted
Figure: Schematic of a personal Computer.
The diskette drive or disk is
another form of storage. This secondary
storage is usually larger than primary memory and is less costly. Although
the CPU contains the most logic in the computer, we can see there are other
functions in which a component must display some logic, such as the disk
controller. Similarly, there is a keyboard processor to interface the keyboard
with the computer, and a video driver to control the CRT. Finally, we have an input/output (I/O) processor, which is
dedicated to controlling devices such as printers.
The last component in Figure is called read-only memory (ROM). Read only memory is used to store
instructions to the computer that are in essence, contained in hardware. Your
personal computer has something called a ROM BIOS which contains the Basic
Input/Output System of the computer and loads when you turn the computer on.
There, then, are the components of a typical personal
computer. What the user sees in the keyboard, a systems unit, the monitor or
CRT, and the printer. Inside the systems box are all other components, as well
as the diskette and disk drives.
Although the central processing unit controls the computer,
we need to discuss primary memory before examining the CPU to demonstrate how
the computer stores data and instructions. In the next sections, we see how the
CPU processes the stored program and data to produce next.
How does a computer actually store data? Remember that all we
need is to distinguish between two states to represent a 0 or a 1. From these
binary numbers, we can build an alphabet of symbols and numbers using other
number basis. Today's computers all use semiconductor technology for primary or
random access memory (RAM). A typical memory cell consists of electronic
elements, including the transistor and capacitor. The designer of the computer would represent a 1 in
memory by the presence of a voltage on the capacitor and a 0 by the absence of
such a voltage. An important characteristic of RAM is the fact that it is
volatile When power is turned off, the contents of RAM memory are lost.
As stated earlier, the CPU controls the operation of the
computer as it contains most of the logic circuitry for the machine. Program
instructions are stored in memory along with data. In a basic computer system,
the instructions are stored sequentially beginning at some location in memory.
By convention, the CPU always fetches the next instruction in sequence and
executes it unless the program instructs it to do otherwise.
The central processing unit or CPU on most computers is found
on a single chip. The advanced CPU chip contains the following components: The
control unit manages the CPU, initiating instruction fetch and execute
cycles. The
bus interfaces the cache memory on the chip with random access memory chips
(RAM). Note that there is a 64 bit wide bus to move data back and forth between
the CPU and memory. The address bus is 32 bits wide and it transmit the address
between RAM and the CPU to fetch instructions and to fetch and store data.
The code cache is
an 8K byte portion of very fast memory on the CPU chip. The chip copies a series
of program instructions here form RAM so that it can reach them faster than it
could if they were only on memory chips.
The data cache is
also an 8K byte memory for keeping small amounts of data of faster access than
is available form RAM memory chips.
The instruction
location counter always points to the next instruction in a program to be
executed.
The instruction
decoder determines what each instruction means, for example, the ADD
instructions and indicates to the control unit that an add operation needs to
take place and which registers are required. Many instructions in the computer
reference a location in memory, for example, the ADD instruction might say ADD
X, where X is a piece of data in RAM. The
address generator computers the address in memory for this data.
The integer unit
performs integer arithmetic and the floating point unit performs floating point
arithmetic. The arithmetic and logic unit (ALU) performs logical operations such
as comparisons between two numbers. The CPU typically executes an instruction in
two phases. The first is the fetch phase. Its objective is to fetch an instruction, pointed to by
the instruction location counter, and move it to the instruction decoder for
processing. During the execute phase,
the control unit manages the execution of the instruction, for example, the
instruction might say to take data from a certain location in memory and add it
to a number already in the integer unit. A clock on the chip determines the
speed of operations. For example, a chip with a clock speed of 100 MHz has a
clock that produces 100 million cycles per second. Some instructions can be
performed in a single clock cycle, but many require several cycles.
Caching: When
reading product reviews of computers you may see descriptions of different types
of memory caches. The operation of moving data and instructions to and from RAM
takes time (several clock cycles). Memory is available that can move data in a
single clock cycle, but this faster memory costs much more than regular RAM.
Designer build small caches or “holding areas” using this high speed memory to
hold the data and instructions currently used by the CPU. This strategy works
because most programs need only a small number of actual memory locations at any
one time so the cache will usually have the required data or instructions. When
the CPU needs data or instructions not in the cache, additional delays are
encountered while the slower RAM is accessed for them. CPU designers are
responsible for caches within the CPU, which are called L1 caches.
Although the characteristics above will help you select a
computer, they are not enough because the manufactures have come up with some
techniques to make PCs faster. The first option is a cache memory a high-speed
memory that is used to speed up slower memory. Consider a disk drive where it
can take 12 milliseconds on the average to access data. Instead of moving that
data directly into main memory, it is placed in a cache memory. When the
computer reads from the disk, the cache memory is filled with the data requested
and extra data nearby. If the next read is for material in the cache, it can be
transferred to primary memory at memory speeds rather than disk access speeds.
Of course, if there is no ”hit” on the cache it means the data desired are not
there and the computer must get them from the disk.
A cache can be used almost anyplace to speed up a computer.
Many PCs use a cache with primary memory. An 8-megabyte memory might come with a
cache of 128 kilobytes. You can also use a cache or separate memory with the
video controller to improve video speeds. Intel developed a series of chips that
featured clock doubling. The clock rate of the CPU is twice the rate of
processing off the chip. For example, one can buy a clock-doubled 66-megahertz
486 chip in a PC. All operations on the chip take place at 66-megahertz;
operations of the chip (for example, memory access) occur at 33 megahertz. Clock
doubling is popular because the processor can use existing memory chips that run
at up to 33 megahertz.
As described above, a pipelined computer breaks instructions
down into many small steps like an assembly line. Each of these steps or stages
is handled by a separate circuit. When an instruction finishes one stage, it
goes on to the next one, and the stage it just left begins work on the next
instruction.
Also mentioned earlier, the Pentium chip features two integer
execution units each fed by its own instruction pipeline, also called
superscalar architecture. This architecture allows the Pentium chip to execute
two instruction per clock cycle. Programs that translate higher-level user
languages into machine language (discussed in the next chapter) have to be
modified to figure out what user program instructions can be split to operate in
parallel.
Finally, you can purchase a computer with a wider and faster
bus that connects the CPU to peripherals. In the first PCs all components used
the same bus-video, memory, printers, and so on. As we have discussed above,
32-bit CPUs have their own data path of 32 bit (in general) to memory. Computers
today generally come with a bus to peripherals like printers and disks that
carries more data than the standard 16-bit ISA (Industry Standard Architecture)
bus. This is the same but that came on the PC/AT. IBM offers its microchannel
architecture which transfers 32 bit at a time. A group of other manufacturers
banded together (the “Gang of Seven”) to develop their own version of a 32-bit
bus called the EISA (Extended Industry Standard Architecture) to compete with
IBM. Today the newest bus is the PCI or Peripheral Component Interconnect bus
which appears to be replacing the other bus standards.
All of these factors can have a dramatic effect on the
performance of a PC. It is not always the case that a computer with a higher
clock speed is faster than one with a slower clock speed if the latter machine
ahs a number of the features above to increase its power.
