understanding a system computer)
TRANSCRIPT
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14 Part A Computer Systems
In this chapter, we shall study the system unit of a personal computer, the
CPU, the factors that affect the performance of a CPU and the classification
of main memory.
2.1 THE SYSTEM UNIT OF A PC
A computer system consists of both hardware and software. Hardware
refers to the actual machinery, for instance, the microprocessor, keyboard,
mouse and monitor etc. Software refers to programs or data.
The hardware components of a basic computer system consists of
1. Central processing unit (CPU)
2. Main memory
3. Input devices
4. Output devices
5. Secondary storage
6. Communication devices.
Components other than the CPU and main memory are collectively knownas peripheral devices.
Fig.2.1 The hardware components of a basic computer system
System Unit
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microprocessor
memory module
interface card
backview of a system unit System unit with lid open
Fig.2.2 The system unit of a personal computer
A personal computer can be viewed as made up of a system unit and
some peripheral devices. For a desktop computer, the system unit appears in
the form of a metal or plastic case which houses the motherboard, CPU, main
memory, interface cards, power supply and storage devices. For a notebook
computer, the system unit encases almost all its electronic components in
a metal or plastic case. Note that storage devices are regarded as peripheral
devices.
Components on the Motherboard
The motherboard is the main circuit board that includes all essential
chips, connecting circuitryand interfacing facilities.
n Chip A chip is an integrated circuit that contains many transistors on asmall piece of semi-conducting material. On the motherboard, there are
sockets which allow certain chips to be removed for repairs or upgrades.
Both the microprocessorand main memoryare chips that can be plugged
into the motherboard. Besides these important chips, other housekeeping
chips also exist on the motherboard.
n Connecting circuitry The connecting circuitry links between the chipson the motherboard. For example, the connecting circuitry provides
channels for data transfer between the CPU and main memory. Similar
channels also exist between the main memory and peripheral devices.
Each channel is called a bus and is made up of a number of conducting
wires, allowing a series of electronic bits to transmit at the same time.
ports
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16 Part A Computer Systems
Each bus is a composition ofaddress bus,data busand control lines. For
example, when a piece of data is fetched from the memory, the address
is sent over the address busto signal a memory location, and the data isthen copied from the memory location to the data bus, which passes the
data to the destination.
n Interfacing facilities The interface facilities of a motherboard appear inthe form ofportsand expansion slots.
A port is built on the motherboard and is designed to connect to a
peripheral device or another computer. There are various types of ports
designed for different peripheral devices (See Table 2.1). Usually, a
cable is needed to connect a port to a peripheral device, like a printer,
a keyboard or a mouse. There are also ports which is an EM-Wave
transceiver that allows connection to peripherals using infrared or
microwaves. A transceiver is a device that can send and receive signals.An example is the IrDA port which sends and receives infrared.
Fig.2.3 The address bus and data bus between the CPU and memory of acomputer. MAR and MDR are registers in the CPU (see Chapter 10).
Port Name Peripheral devices
1. Keyboard and mouse port Keyboard, mouse
2. Serial port Mouse, modem, graphic tablet
3. Parallel port Printer
4. SCSI port SCSI devices, e.g. hard disk
5. USB port Large range of devices6. IEEE 1394 port DV camcoder, digital camera
7. IrDA port (Infrared DataAssociation)
Devices that use infrared (Not Bluetooth)
8. PCMCIA port (for notebookcomputers only)
PC cards, e.g. Network interface card, harddisk, sound cards, radio transceiver etc.
Table 2.1 Common ports and their corresponding peripheral devices
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Forms of chips
A chip consists of a super thin slice of semi-conducting material packed with elements like wires,
transistors, capacitors and resistors. Semi-conducting material, such as Silicon or Germanium, are
substances with properties between that of a conductor and an insulator.
A chip may be a microprocessor, memory module or supporting circuitry. It is packaged in a
protective carrier that also provides connectors to other computer components. There are at least
three forms of chip carriers: rectangularDIP (dual in-line package), circuit-board-like DIMM (dual
in-line memory module), or pin-cushion-like PGA (pin-grid arrays).
An expansion slot allows an interface card to be inserted into the
motherboard. An interface card is a separate circuit board which is an
add-on component to increase the function of a computer. Examples are
the network interface card (NIC), video card, sound card and modem card.
Some interface cards come with their own ports, allowing connection to
other peripheral devices.
Card Name Peripheral devices
1. Network interface card (NIC) Switch or hubs of local area network or Internet
2. AGP video card Visual Display Unit
3. Sound card Microphone, loudspeaker
4. Modem card Telephone network
Fig.2.4 Three common forms of chip carriers
DIP has two parallel rows of pins
DIMM is mainly used for memory module.The number of pins can be 72, 144, 168, 184(DDR), or 284 (DDR-II).
PGA is mainly used formicroprocessors. The pins arearranged in concentric squares
Table 2.2 Common interface cards and the corresponding peripheral devices
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18 Part A Computer Systems
2.2 THE CPU
A central processing unit (CPU) is designed to process instructions that
operate a computer. The CPU of a personal computer is a microprocessor,
which is a single integrated circuit that contains millions of transistors. A
microprocessor performs essentially the same tasks as the CPU of a classic
mainframe. It is sometimes described as a CPU on a chip.
A CPU consists of two major components: the CU and ALU. It also
includes circuitry for devices such as registers, cache memory and various
execution units.
n CU
The control unit (CU) controls the overall operations of the computer.
It repetitively interprets an instruction from a program and starts
appropriate action. It also controls the operations of the peripheral
devices.
n ALU
The arithmetic and logic unit (ALU) performs arithmetic operations,
such as addition and subtraction of integers. It also performs logical
operationssuch as comparing two numbers to see if they are the same.
n Registers
Registers are special memory locations inside a CPU, allowing fast
access. Some of them are needed by the ALU in computation to hold data
to be processed. Others are needed by the CU to control the execution of
instructions.
A typical CPU includes memory address register (MAR), memory data
register (MDR), instruction register (IR), program counter (PC), general
purpose registers, and so on. Registers are accessed by machineinstructions. While an address of a main memory location involves
several bytes, the address of a register may require only several bits.
Fig.2.5 The CPU and otherhardware components
Note
Integer arithmetic are handled
by the ALU. A similar unit
called foating-point unit(FPU) is designed to handle
real numberarithmetic. An
integer unit consists of an ALU
and its own CU. A modern CPU
has several integer units and at
least one FPU.
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2.3 PROCESSOR PERFORMANCE FACTORS
The performance of a CPU is affected by several factors: clock rate, word
length, cache size, instruction setand processing techniques.
A. Clock rate
A CPU works with a timing device, called clock, which generates ticks
(or clock cycles) regularly. The clock rate is the number of ticks produced per
second. It determines the pace for executing instructions.
The unit of clock rate is hertz (Hz). Mega (M) is a prefix representing
one million (106). Giga (G) represents one billion (109). For example, a 800 MHz
PowerPC microprocessor produces 800 million ticks in one second and a 3.0
GHz Intel Pentium 4 microprocessor operates at a speed of 3 billion cycles per
second. All other things being equal, a computer of the same familywith a 3.0
GHz is faster than a computer with 2.0 GHz. Clock rate should notbe used to
compare computers of different brands (See Chapter 10).
It is important to understand that the clock rate of a CPU is not equal to
the number of instructions that the CPU can execute in one second. The reason
is that most instructions take several ticks to complete. The execution of an
instruction involves several stageseach requires at least one tick. We shall see
that clock rate is not the only factor that determines the CPU performance.
Table 2.3 Development of Intel processors (Information extracted from www.intel.com)# Itanium was introduced in 2001 + Pentium class was introduced in 1993 and Pentium 4 was introduced in 2*Date of introduction See Appendix for further details.
Note
A tickis the smallest un
of time in the universe o
microprocessor. In a sing
processor system, every
performed by a CPU is in a multiple number of t
Itanium 2 Pentium 4 Intel486TM DX Intel386TM DX 80286 8086
Typical use
Demanding
enterprise-classservers
Desktops and
entry-levelworkstation Desktops Desktops Desktops Desktops
Clock rate 1.6 GHz 3.80 GHz 50 MHz 33 MHz 10 MHz 4.77 MHz
Word length 64 bits 32 bits 32 bits 32 bits 16 bits 16 bits
No. of transistors 592 million 125 million 1.2 million 0.275 million 0.134 million 0.029 mil
As of date Nov 2004# Nov 2004+ Oct 1989* Oct 1985* Jan 1982* Aug 1978
Intel clone
Today, Intel is the world's largest chipmaker and supplies a signicant percentage of the microprocessors
that power PCs. Other companies, like Advanced Micro Devices (AMD), also make Intel-compatible
processors, known as Intel clones, but at a lower price. AMD's Athlon processors are direct competitors
to Intel's Pentium class CPU and have a slight performance advantage according to somesoftware
benchmarks. (Note: Cyrix which once produced Intel486TM compatible CPU has been bought by VIA)
Clock rate is one of many factors that determine the performance of a CPU. Today, only Intel still uses
clock rate as a selling point in her marketing strategy. As of the print of this book, it is generally agreed
that, given the same clock rate, AMD Athlon is about 30% faster than Intel Pentium 4.
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B. Word length
Word length refers to the number of bits that a microprocessor can
manipulate at one time. Word length is the size of the general-purpose
registersin the CPU.
In general, a longer word length will give better performance. Suppose
we wish to perform an addition of two 32-bit numbers. If the registers are all
16 bits, each value will have to be stored in two registers and the calculation
will involve two separate additions. If the registers were all 32 bits, the amount
of processing would be halved. Today's personal computers typically contain
32-bit. Advanced computers use 64-bit processors.
C. Cache Memory
Data transfer between the CPU and main memory is slow. Very often,
the CPU has to wait for memory access and becomes idle. Cache memory
is a high-speed memory that holds data which duplicate some data stored
in the main memory. When the data needed by the CPU are stored in the
cache memory, the access time will be much smaller than fetching the
same data from the main memory.
Cache memory is divided into at leasttwo levels: Level 1 (L1)cache
and Level 2 (L2) cache, located in different dies. A die is a silicon slice
inside a chip. L1 cache is located in the same die as the processor, whereas
L2 cache is located in a separate die. Current microprocessors include
both L1 and L2 caches in the same protective carrier (i.e. same chip). But,
in the past, L2 appeared as a separate chip.
L1 cache is faster but smaller than L2 cache the L1 capacity isusually measured in KB, while that in L2 is in MB. Since cache memory is
fixed inside the processor, its size is not configurable.
x-bit processor
What doesx in anx-bit processor mean?
x represents the number of bits used in general-purpose registers (i.e. the integer registers) inside the
processor. For example, the size of integer registers in a 32-bit processor is 32 bits.
Normally, a processor with longer word length will also have wider data bus and wider address bus. However,
it is not necessarily true that the widths of these buses are the same as the word length. An example is Intel
Pentium 4, which is a 32-bit processor, but with 36-bit address bus width and 64-bit data bus width.
32-bit CPUs have been common in personal computers since Intels 386 in 1985. In 2001, Intel released
its rst 64-bit CPU Itanium. In 2003, AMD released its rst 64-bit CPU Athlon 64. Using a processor with
longer word length will make working with huge amount of data easier, such as video editing. However,
general computation can beslowerbecause programs will be larger in size and may not be t completely into
the cache memory.
The movement from a 32-bit to a 64-bit CPU is a major alteration, since operating systems must be modied
to take advantage of the new architecture.
Fig.2.6 Cache memory
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D. CISC or RISC
In 1970s, there was a tendency to offer more varied functions and
give more high-level features to machine instructions. The result is an
increase in size of the instruction set and complexity of the instructions. This
tendency continued until mid-80s, when chip designers realised that complex
instructions could not improve the CPU performance, because the differencein time for performing instructions is not suitable for pipelining(see below). A
CPU with complex instructions is referred to as CISC (Complex instruction set
computer) architecture. An example is the Intel 80286 microprocessors.
A CPU that includes a very limited set of instructions is referred to
as RISC (Reduced instruction set computer) architecture. Examples are
Motorolas PowerPC and Suns SPARC.
Because RISC instructions are simple, most of them use the same
set of stages and take equal time to complete. This facilitates pipelining
and parallelism resulting in higher execution speed. Another advantage is
that RISC chips are less complicated and require fewer transistors (though
more registers), reducing the cost of design and production. However, RISC
architecture requires more instructions for the same program.
The trend is that newer CISC CPUs are designed with a subsetof RISC
instructions, and traditional instructions are split into many RISC instructions
so that the CPU can make use of the advantages of RISC. An example is the
Intel Pentium 4 microprocessors.
E. Pipelining
As mentioned earlier, most instructions require several clock cycles to
complete (see Clock rate). In fact, each instruction involves several discrete
stages. In a traditional processor, instructions are executed so that all the stages
of an instruction are completed before the next instruction can be executed.
Pipelining is a technology that allows execution of the next instruction to start
before the previous instruction is fully completed. Pipelining increases the
throughput of the system.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Stage 1 I1 I1 I2 I2 I3 I3
Stage 2 I1 I1 I2 I2 I3 I3
Stage 3 I1 I1 I2 I2 I3 I3
Stage 4 I1 I2 I3
Stage 5 I1 I2 I3
Clock cycle
Sequence
Fig.2.7 A pipeline with five stages that can overlap. For example, when the CPU is performing stage 2 ofI1, it is also performing stage 1 of I2. (I1, I2 and I3 represents three instructions, each of 5 stages)
Note
Intel 80286 is a typic
CISC CPU. Starting f
386, features of RI
can be found in Intel
processors.
Note
More registers are nein RISC to reduce me
access. Some may thi
more registers will in
the cost of a CPU. Bu
this is cost-effective a
CPU performance can
enhanced.
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A daily life example of pipelining is serving set lunch in a fast food
caf. Assume that each set-lunch includes soup, bread, main course and drink.
Instead of preparing the whole set in a single counter, the caf may set up four
counters. A customer after getting the soup will proceed to the next counter
which serves breads. The second customer will then be served soup, without
having to wait for the completion of the first customer.
F. Multiple Execution Units Superscalar
An execution unit (or function unit) is a part of the CPU that performs
operations and calculations. Examples are integer unit and floating-point
unit (FPU) which handles integer and floating-point mathematics respectively.
In fact, an integer unitconsists of an ALU and its own CU. A processor with
multiple integer units and one or two FPUs is called superscalar architecture,
allowing more than one instruction to be executed concurrently in the
same chip. Note that concurrent here is not referring to the overlapping
of instruction stages as in pipelining, but complete separation of execution
processes.
G. Multiple processors
A computer system may consist of multiple processors. It means
that a number of separate processors are working in parallel to solve a
single problem. The processing is called multi-processing. Superscalar and
multi-processing are examples of parallel processing (or parallelism). Unlike
pipelining and superscalar, multi-processing requires a special operating
system that supports multiple processors.
Do not confuse multi-processingwith multi-tasking the latter usually
describes the capability of a single CPU (See Chapter 4).
H. Distributed processing
Distributed processing refers to aggregating
the power of several computers to run a single
computational task in a collaborative manner. An
example is the use of the Web. A Web server, routers,
DNS servers and a Web browser together form a
distributed system.
Another example is the UC Berkeley SETI (Search
for Extra-Terrestrial Intelligence) project, SETI@home.
You may contribute your CPU time to help analyse
data collected from the outer space.
Note
1. In Intel386, the FPU is
an optional chip known as
Math co-processor.
2. Intel Pentium 4 has 1
FPU; Intel Itanium and
AMD AthlonXP each has 2
FPU.
3D-NOW!
The 3D-NOW! technology isdeveloped by AMD to perform
multimedia tasks quickly, such
as 3D games. It is characterised
by a set of FPU-specic
instructions that carry out
oating-point arithmetics
needed by 3D graphic display.
Fig.2.8 The programSETI@home running as a screen saver
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2.4 MAIN MEMORY
Besides the microprocessor, main memory is another important
component found on the motherboard. The main memory (or memory),
which includes RAM, ROM and CMOS, stores data, instructions and some vital
information about the computer.
Main memory is classified into: volatile and non-volatile. The contents of
the volatile memory will be lost when the computer power is turned off. On
the other hand, non-volatile memory will retain the contents after the power is
turned off.
A. Types of Memory
There are three types of memory: RAM, ROMand CMOS.
1. RAM
Random access memory (RAM) is a temporary holding area for data,
application programs and the operating system, as long as the computer is
running. RAM is volatile. All personal computers today use dynamic RAM
(DRAM) for the main memory. In a DRAM, capacitors are used to hold the bits
a charged capacitor represents a 1 and discharged capacitor represents a 0.
Since the charge on a charged capacitor will drain away as time goes by, DRAM
needs recharging from time to time. (Static RAM is made oftransistorsand is
used in cache, registers and CMOS)
Speed of RAM indicates how fast the RAM circuitry reacts to update the
data. It can be expressed in nanoseconds (10 -9 s) or MHz (106 Hz). For example,
8 ns RAM means it takes 8 ns to update a piece of data. 8 ns is equivalent to1/(8 10-9 s) = 1.25 108 Hz or 125 MHz. Under equal conditions, 8 ns RAM is
faster than 10 ns RAM.
Types of RAM
RAM can be classied into
1. SRAM Static, used in cache memory, registers and CMOS
2. DRAM Dynamic, used in the main memory
DRAM is further classied into1. EDO RAM Extended data out, less popular
2. SDRAM Synchronous DRAM, most popular
3. RDRAM Rambus DRAM, more expensive
SDRAM can have different effective speeds under the same clock rate,
arranged in ascending order of speed:
1. SDR SDRAM Single data rate SDRAM
2. DDR SDRAM Double data rate SDRAM
3. DDR-2 SDRAM Double data rate two SDRAM, will be the most
popular. There is also DDR-3 SDRAM.
Note
Loading a le means
copying data from the
secondary storage to the
main memory. When
the computer is poweredon, certain operating
system les are loaded
automatically from the h
disk into the RAM.
Saving a le means cop
data from RAM to the
non-volatile secondary
storage.
Fig.2.9 Memory of acomputer
Other RAM
RAM not only exists on
the motherboard, but can
also be found in periphe
devices, like video adap
and hard disk. The RAM
in a video adapter is use
to display images on the
screen of a VDU. Each
pixel corresponds to a pi
of data in the video RAM
So, a screen with higher
resolution needs more v
RAM. The RAM in a ha
disk is used as buffer(s
Chapter 10).
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2. ROM
Read only memory (ROM) stores data permanently and is non-volatile.
The contents in ROM can be read but cannot be changed.
The instructions stored in ROM are known as firmware, which means
that the instructions are stored permanently in a chip instead of disc.
Instructions are said to be hard-wired in ROM.
A. Variations in Non-volatile Memory
There are other types of ROM: PROM, EPROMand EEPROM.
PROM stands for programmable read only memory. It is a memory chip
that can be written once. A special device called PROM programmer (or burner)
is needed to write the user's data and instructions. Once written, the contents
cannot be changed any more. PROM is different from ROM in that a PROM is
manufactured and sold as a blank memory.
PROM is used for storing the specialised or unique programs that
custom-manufacturing of a true ROM would be too costly. A well known use of
PROM is by the video game producers.
EPROM stands for erasable programmable read only memory. It is
similar to PROM, except that the contents can be erased under ultraviolet. An
example of use is in manufacturing industry where other types of storage are
not suitable. The EPROM provides instructions to a machine that performs
repetitive tasks in manufacturing a certain product. When the old production
is finished and a new production is started, the old instructions can be erased.
Then, the EPROM is programmed with a new set of instructions.
EEPROM stands for electrically erasable programmable read onlymemory. It is similar to RAM in that it can be read and written using a
computer. But, unlike RAM, EEPROM is non-volatile, i.e. it does not need
continuous power to retain the data.
Some people called EEPROM as flash memory. But, there is a slight
difference between the two: EEPROM writes one byte at a time, whereas flash
memory writes a block of bytes at a time. Therefore, flash memory is faster.
B. BIOS
The startup program BIOS (basic input/output system) used in computeris stored in an EEPROM (or flash ROM). This explains why BIOS can be updated.
BIOS is part of the system programs. These instructions tell the computer
how to access the hard disk, find the operating system, and load it into RAM.
Once the operating system is loaded, the computer will understand your input,
run software and access your data. See Fig.4.11.
Note
Some computers have dual bios a main BIOS and a backup
BIOS. The purpose of using
two BIOS chips is to help the
motherboard recover from
issues that may happen during
a BIOS update and help protect
the BIOS from any potential
virus, such as CIH in 1999.
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3. CMOS Memory
CMOS memory (Complementary metal-oxide semiconductor memory)
is a type of memory on the motherboard that stores the configuration
information about a computer. It includes information like current time and
date, capacity of hard disk, types of existing ports, keyboard and monitor, and
so on. However, CMOS does not store programs.
CMOS memory is usually powered by a small battery. Unlike ROM, the
computer can change the information in CMOS memory. Unlike RAM, the
contents in CMOS memory are retained even the main power supply is turned
off. This is because the contents in CMOS memory are kept by an independent
battery.
RAM ROM CMOSmemory
Permanence volatile non-volatileContents are kept by a
battery
Flexibility read and write read only read and write
Type of informationdata and instructions
being processedsystem program and
dataconfiguration data
Size largest small smallest
Upgradableyes; memory size can
be increasedno no
Table 2.4 Comparison between RAM, ROM and CMOS
Fig.2.10 CMOS display
Note
It is controversial to say
whether CMOS memory
is volatile or not. From
users point of view, CMis non-volatile because th
data you enter will remai
unchanged after the com
is powered off.
From the engineers poin
of view, CMOS is volatil
because CMOS is made
transistors which require
electrical power to hold t
data.
Therefore, it depends on
volatile is dened.
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B. Memory Size
Memory size is measured in byte. A byte consists of eight bits. Each
ASCII character requires one byte or eight bits of memory.
Manufacturers always produce memory in multiples of 210 = 1,024 bytes
or one kilobyte, abbreviated as KB or K. For example, a memory of 128 KB is
equal to 128 1,024 = 131,072 bytes.
In recent years, memory size increases drastically. Memory chips are
produced to hold kilo of kilobytes, called megabyte (MB). 1 MB is equal to 220 =
1,024 1,024 bytes = 1,048,576 bytes.
Similarly, larger memory size is measured in kilo of megabyte, called
gigabyte (GB). 1 GB is equal to 230 =1,024 1,024 1,024 = 1,073,741,824
bytes.
Sometimes, for the purpose of simplicity, 1024 is rounded down to 1000.
Therefore, 1 KB is approximately equal to one thousand bytes. 1 MB equals
approximately one million bytes and 1 GB approximates one billion bytes.
The size of main memory affects the efficiency of the computer. A
computer is usually upgraded by increasing the memory size. This explains
why the memory chips are removable from the motherboard.
Term AbbreviationApproximate Memory
Size (bytes)Exact Memory Size
(bytes)
Kilobyte KB / K 103 = 1,000 210 = 1,024
Megabyte MB 106 = 1,000,000 220 = 1,048,576
Gigabyte GB 109 = 1,000,000,000 230 = 1,073,741,724
Table 2.5 Memory and storage size
Note
ASCII uses 7 bits to represent
characters. Since the minimum
accessible unit in a computer is
one byte, in modern computers,an ASCII character is stored
using 8 bits with the most
signicant bit (leftmost bit) set
to 0.
What should be considered in upgrading a microprocessor?
The rst thing to consider is budget. The price of the latest and fastest microprocessor is often much
higher than the second or third latest versions. Sometimes, it can be as high as half the price of the
entire system unit. The second thing to consider is the type of chipset and CPU socket on the
motherboard. The new microprocessor must match with the existing motherboard. The third thing
to consider is that a microprocessor can operate at full efciency only if all of the components in
the computer also can handle the faster speed. It means that you may need to replace the RAM or
motherboard as well.
To nd out whether an upgrade would be worth the cost, you might do a little research on the Web or
in computer magazines.
Note
In 1990s or before,
specications ofstorage mainly
used K for 1024 and M for1048576; whereas specications
forrate of transmission used K
for 103 and M for 106. In recent
years, storage capacity also
using M for 106 and G for 109
become popular. This is perhaps
due to two reasons:
1. New terms like kibi, mebi,
and gibi appear to represent
210, 220 and 230 respectively.
2. The new storage capacity
is so high that consumersdo not care about the small
differences.
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C. Data Access in memory
The main memory can be thought of as a series of locations each of
length one byte. Each location has a unique address, counting from zero
onwards. The address is called memory address.
For example, for a computer with a memory size of 1 K, the memory
address ranges from 0 to 1,023.
Since each byte has a unique address, the CPU can jump directly to the
requested data (see note). Therefore, data fetching and storing can be done
very efficiently. This mode of data access is called direct access, which is in
contrast to sequential access.
In sequential access, address is unknown. Data are accessed by searching
from one end of the storage medium until a match occurs. Example of
sequential access is to locate data from a tape or finding a word in a document.
D. The Maximum Size of Memory
Clearly, the size of a memory is limited by the maximum address. The
maximum address is determined by the number of bits used in each address.
For example, if 8 bits are used as address, the address space is from 0000
00002
to 1111 11112, which is equivalent to 0 to 28-1, or 0 to 127. Therefore, the
maximum size of memory is 128 bytes.
In general, ifn bits are used to represent an address, then
the maximum size of memory = 2n bytes.
In a computer, the number of bits used in each address is determined
by the width of the address bus. For example, in Pentium 4, the address bus
width is 36 bits. It means that it can address up to 236 bytes (or 26 230 = 64 GB
= 6.87 1010 bytes).
How does a computer
to the requested data?
Suppose the CPU needs a
in the memory. The addr
that data is put into the ad
bus. The bus circuitry int
the address and locates th
requested data in the mem
The data is copied to the
bus, which passes the dat
CPU.
Fig.2.11 Memory address
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E. Virtual Memory
As the main memory is limited in size, will a computer run out of
memory? The solution to this problem is virtual memory. Virtual memory is an
area of a hard disk that extends the RAM.
If the free space of RAM is less than the size of a new data from a file,
data in RAM not recently used and not frequently used will be put to the virtual
memory to make room for the new data. This process is called swapping. When
the data on virtual memory are needed afterwards, they would be swapped
back to the RAM again.
The process described above is done by the computer automatically and
is transparent to the users. Swapping will slow down the performance of the
computer.
For this reason, the performance of a computer also depends on the size
of RAM, amount of free space of the hard disk and speed of the hard disk (See
Chapter 10 for other factors). Increasing the size of RAM will reduce the chance
of swapping data between RAM and hard disk.
hard disk
Virtual memory
RAM
CPU
Fig.2.11 Virtual memory is used to extend the RAM
Note
In the old days of Windows
3.1, too much swapping
would easily lead to system
crash.
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2Chapter 2 System Unit
Summary
n The hardware components of a basic computer system consists of
Central processing unit (CPU)
Main memory
Input devices
Output devices Secondary storage
Communication devices.
n Peripheral devices are components other than the CPU and main
memory.
n The motherboard is the main circuit board that includes all essential
chips, connecting circuitry and interfacing facilities.
n A chip is an integrated circuit that contains many transistors on a small
piece of semi-conducting material.
n A bus is a channel that connects all components, including the CPU and
main memory and peripheral devices.
n A bus comprises address bus, data busand control lines.
n A port connects the motherboard to peripheral devices or other
computers.
n An expansion slot allows an interface card to be inserted onto the
motherboard.
n An interface card is a separate circuit board which is an add-on
component to increase the function of a computer.
n A central processing unit (CPU) is designed to process instructions that
operate a computer. It consists of CU, ALU, registers, cache memory and
various subsidiary processors.
n The control unit (CU)
controls the overall operations of the computer.
interprets an instruction from a program and starts appropriate
action. It also controls the operations of the peripheral devices.
n The arithmetic and logic unit (ALU) performs arithmetic operations and
logical operations.
n Registers are special memory locations, allowing fast access.
n The performance of a CPU is affected by clock rate, word length, cache
size, instruction set and processing techniques.n The clock rate is the number of ticks produced per second and
determines the pace for executing instructions.
n The clock rate of a CPU is notequal to the number of instructions that
the CPU can execute in one second.
n Most instructions take several clock cycles to complete.
n Word length is the size of the general-purpose registers in the CPU and
determines the number of bits that a microprocessor can manipulate at
one time.
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30 Part A Computer Systems
n Cache memory is a high-speed memory that holds data which duplicate
some data stored in the main memory. Cache memory reduces memory
access time.
n L1 cache is smaller than L2 cache, but faster. L1 cache is built into the
processor die, whereas L2 cache is located in a separate die.
n A die is a silicon subtrate. A microprocessor may have more than one die.
n CISC (Complex instruction set computer) architecture
consists of complex instructions
has a large instruction set
not suitable for pipelining
n RISC (Reduced instruction set computer) architecture
consists of simple instructions with equal length, equal number of
stages and equal execution time
has a smaller instruction set
suitable for pipelining
requires fewer transistors
lower cost of design and production
n Pipelining
allows execution of the next instruction to start before the previous
instruction is fully completed
increases the throughput of the system.
n An execution unit is a part of the CPU that performs operations and
calculations, including integer unit and floating-point unit.
n An integer unit is the combination of a CU and ALU that handles integer
arithmetic.
n A floating-point unit (FPU) handles floating-point mathematics.
n Superscalar architecture is a CPU with multiple integer units and one ortwo FPUs in the same chip.
n Multi-processing means using a computer system with multiple
processors. It requires a special operating system that supports multiple
processors.
n Parallel processing refers to execution of the same task by multiple
processors or execution units at the same time.
n Distributed processing refers to aggregating the power of several
computers to run a single computational task in a collaborative manner.
n In a volatile memory, data will be lost when the computer power is
turned off.
n Random Access Memory (RAM) is a volatile memory for data,
application programs and the operating system while the computer is
running.
n Read only memory (ROM) is a non-volatile memory that stores data
permanently.
n Firmware is the instructions stored in a ROM.
n PROM (programmable ROM) can be written once and sold as a blank
chip. It is used in some video game.
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3Chapter 2 System Unit
n EPROM (erasable programmable ROM) can be written and the contents
can be erased under ultraviolet. It is used in controlling robots in
manufacturing and re-programmed when a new production is started
n EEPROM (electrically erasable programmable ROM), also called flash
memory, is non-volatile memory that can be read and written using a
computer.
n BIOS (basic input/output system) is a startup program stored in an EEPROM.
tells the computer how to access the hard disk, find the operating
system, and load it into RAM.
n CMOS memory (Complementary metal-oxide semiconductor memory)
stores the configuration information about a computer, but does
not store programs.
is powered by a small battery
n Memory size is measured in byte.
n A byte consists of eight bits.
n 1 KB = 210 bytes; 1 MB = 220 bytes; 1 GB =230 bytes.
n Each memory location has a unique address.
n Direct access is data access using address. It is more efficient than
sequential access.
n Sequential access is data access without address. Data are accessed by
searching from one end.
n Ifn bits are used to represent an address, then the maximum size of
memory = 2n bytes.
n Virtual memory is an area of a hard disk that extends the RAM.
n Swapping is needed when a new data exceeds the free space of RAM.
Then, data in RAM not recently/frequently used will be put to the virtual
memory to make room for the new data.
n Swapping reduces performance. Too much swapping may lead to system
crash.
n Increasing the size of RAM will reduce the chance of swapping.
Review Exercise
Multiple Choice Questions
1. A register
A. is a high speed memory.B. is a peripheral device.C. carries data between the CPU and main memory.D. carries out additions.
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32 Part A Computer Systems
2. Data from a peripheral device are transmitted to the CPU through
A. the control unit.B. a data bus.C. an address bus.D. an expansion slot.
3. A 32-bit CPU means that
A. the data bus consists of 32 lines.
B. the address bus consists of 32 lines.C. the control line consists of 32 lines.D. the word length is 32 bits.
4. Given that the data bus of a computer consists of 32 lines. How many data can betransmitted at one time?
A. 32 bytesB. 4 bytesC. 232 bytesD. 256 bits
5. Which of the following about cache memory is NOT correct?
A. L1 cache is faster than L2 cache.
B. L1 cache is smaller than L2 cache.C. L1 cache is faster than main memory.D. L1 cache is mounted on the motherboard and separated from the microprocessor.
6. A cache memory stores
A. the booting program.B. the temporary data in an execution.C. the conguration information of a computer.D. a copy of some data in the main memory.
7. Given that the clock rate of a CPU isfHz.
A. Each instruction is nished within 1/fs.B. The CPU can performfinstructions in each second.
C. The CPU can perform more thanfinstructions in each second.D. The CPU can perform less thanfinstructions in each second.
8. Given that the clock rate of a CPU is 3 GHz. The duration of a clock cycle is
A. 3 106 s.B. 3 109 s.C. 3.33 10-10 s.D. 3.33 10-6 s.
9. Pipelining increases
A. the CPU clock rate.B. the instruction execution time.C. the memory size.
D. the throughput of a CPU.
10. Which of the following about a RISC architecture is NOT true when compared withCISC architecture?
A. The instruction set of RISC is smaller than that of CISC.B. All the CISC instructions have the same number of stages.C. The cost of design of a RISC CPU is less than that of a CISC CPU.D. Given the same task, more RISC instructions are needed than CISC instructions.
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3Chapter 2 System Unit
11. The number of bytes in a 512 M RAM is
A. 5.120 108
B. 5.369 108
C. 6.4 107
D. 4.096 109
12. Which of the following is executed when the computer is powered on?
A. BIOSB. DOSC. CMOSD. EPROM
13. Which of the following retain(s) data when the main power is turned off?
(1) Flash ROM(2) CMOS(3) RAM
A. (1) onlyB. (2) onlyC. (1) and (2) onlyD. (2) and (3) only
14. Given that the memory size of a computer is 512 MB. What is the minimum number ofaddress lines need?
A. 29B. 25C. 19D. The answer depends on the width of data bus.
15. The address bus of a computer consists of 12 lines. What is the maximum addressablememory size?
A. 12 bytesB. 4096 bytesC. 4096 bitsD. The answer depends on the width of data bus.
16. A 32-bit computer has address bus width 36 and data bus width 64.
A. The length of the register is 36 bits.B. The maximum memory size is 236 bytes.C. The data bus moves one word of data at a time.D. The computer can process 64 bits of data in each machine instruction.
17. The virtual memory of a computer
A. extends the size of RAM by using the hard disk.B. extends the size of RAM by using the cache memory.C. is a fast memory that improves the performance of a CPU.
D. is an extra hard disk.
18. Which of the following allows both read and write?
(1) EEPROM(2) ROM(3) RAM
A. (1) onlyB. (2) onlyC. (1) and (3) onlyD. (1), (2) and (3) only