Computer Systems
CS208
Major Components of a
Computer System





Processor (CPU)
 Runs program instructions
Main Memory
 Storage for running programs and current data
Secondary Storage
 Long-term program & data storage (hard disk, CD, etc)
Input Devices
 Communication from the user to the computer(e.g.
keyboard, mouse)
Output Devices
 Communication from the computer to the user (e.g.
monitor, printer, speakers)
Parts of a PC

System Unit - the metal box that houses the
processor, main memory, and secondary
storage devices.

Input and output devices - attached to the
system unit via a device controller.


The terms "input" and "output" tell you if
data flow is into or out of the system unit.
Abbreviated to “I/O”
Computer Systems
System Unit
Output device
Output device
Output device
Input device
Input device
The Processor
 The processor is the "brain" of the computer system.
 Main processor is called the Central Processing Unit (CPU).
 A particular computer will have a particular type of
processor, such as a Pentium or a SPARC chip.
 Co-processors assist the CPU with some of the processing
functions. Examples:
 Math co-processors handle heavy duty math processing
 Graphics coprocessors speed up the display of graphics
onto the monitor
Component Interaction
The CPU controls all of the other resources within the system,
in order to accomplish a task.
The CPU
The
CPU is a silicon chip that contains
millions of tiny electrical components.
The

CPU’s three main parts are:
Control Unit
 Arithmetic

Registers
Logic Unit (ALU)
Central Processing Unit (CPU)
Arithmetic / Logic Unit
Control Unit
Registers
Performs calculations
and decisions
Coordinates
processing steps
Small, fast
storage areas for
instructions and
data
Registers
 Registers are small, fast memory within the CPU
 Different registers hold different things
 instructions and addresses of instructions
 data (operands)
 results of operations
Special Purpose Registers
Special Purpose Registers contain specific
information the CPU needs.
Instruction
Register (IR) contains the
actual instruction which is currently being
executed by the CPU.
The
Program Counter (PC) contains the
address of the next instruction to be
executed by the program.
General Purpose Registers
General
Purpose Registers hold:

the operands for arithmetic and logical
operations (ie. the values on which the
operation will be performed)

So
the results of such operations
General Purpose Registers are used for
holding and manipulating data used by the CPU
Computer Systems
Not All Processors Are Created Equal
Newer machines use specialized processor chips that
have been developed to speed up the processing of
data.

CISC - Complex instruction set computer
 Large instruction set, many formats
RISC - Reduced instruction set computer
 Small instruction set, single or small variations in
format

MPP - Massive parallel processing


Many CPUs working in parallel
Memory
Computer Memory
- millions/billions of on/off charges
Divided into:

Bits
0 or 1

Bytes
Groups of 8 bits
A byte is the smallest unit of storage.
(Can hold one text character)

Words
Groups of bits/bytes (8, 16, 32, 64-bits)
Memory
Storage is usually too large to be expressed in bytes or
words. Instead we use:
(210 bytes)

Kilobyte (KB) = 1024 bytes

Megabyte (MB) = 1024 x 1024 bytes or
one million bytes
(220 bytes)

Gigabyte (GB) = 1024 x 1024 x 1024 bytes or
one trillion bytes
(230 bytes)

Terabyte (TB) = 1024 x 1024 x 1024 x 1024 bytes
one quadrillion bytes (240 bytes)
Main Memory


Each computer has a specific word size

Word sizes vary from computer to computer.

Word size is an even multiple of a bytes.
Each word within memory can hold either

data or

program instructions
Main Memory
Each memory cell
has a numeric
address, which
uniquely identifies
its location
5248
5249 10011010
5250
5251
5252
5253
5254
5255
5256
Each memory cell stores a
set number of bits (some
computers use 8 bits/one
byte, others use words)
A word is stored in
consecutive
memory bytes.
CPU and Memory

CPU can interact with main memory in two ways:

It can write a byte/word to a given memory location.
 The
previous bits that were in that location are
destroyed
 The

new bits are saved for future use.
It can read a byte/word from a given memory location.
 The
CPU copies the bits stored at that location and
stores them in a CPU register
 The
contents of the memory location are NOT
changed.
Main Memory Characteristics
 Very closely connected to the CPU.
 Contents are quickly and easily changed.
 Holds the programs and data that the
processor is actively working with.
 Interacts with the processor millions of times
per second.
 Nothing permanent is kept in main memory.
Secondary Storage
Characteristics
 Connected to main memory through a bus and a
device controller.
 Contents are easily changed, but access is very
slow compared to main memory.
 Only occasionally interacts with CPU.
 Used for long-term storage of programs and data.
 Much larger than main memory (GBs vs. MBs).
Program Instructions

Programs instructions are stored in secondary storage
(hard disks, CD-ROM, DVD).

To process data, the CPU requires a working area


Uses Main Memory

Also called: RAM (random access memory),
primary storage, and internal memory.
Before a program is run, instructions must first be copied
from the slow secondary storage into fast main memory

Provides the CPU with fast access to instructions to
execute.
Instructions


An instruction is a sequence of bits.
A simple instruction format may consist of an
operation code (op code) and an address or
operands.
Op Code

Operands / Address
Instructions tell the computer’s CPU what to do.
Instructions

The operation code specifies the operation the
computer is to carry out (add, compare, etc)

The operand/address area can store an operand or an
address


An operand is a specific value or a register number

An address allows the instruction to refer to a
location in main memory
The CPU runs each instruction in the program,
starting with instruction 0, using the fetch-decodeexecute cycle.
Fetch-Decode-Execute Cycle

During the fetch part, the CPU fetches the next
instruction from the address contained in the
Program Counter and places the instruction in
the Instruction Register.


When a program starts, the program counter contains 0,
so the instruction at address 0 is fetched.
As soon as an instruction is fetched, the CPU
adds 1 word to the contents of the Program
Counter, so that it will contain the address of the
next sequential instruction.
Fetch-Decode-Execute Cycle

The decode unit within the CPU deciphers the
instruction in the Instruction Register and
determines what operations need to be done and
what type of operands will be used (decode part).

During the execution part, the specified operation
is performed (add, compare, etc).

After execution of the instruction has been
completed the cycle starts all over again (unless
the instruction terminates the program).
Fetch-Decode-Execute
Diagram
CPU
Fetch
Program Counter (PC)
Main Memory
…
3020
3021
3022
add r3, r1, r2 3023
Decode
3024
Determine what the
3025
instruction is (add)
3026
Execute
3027
In this case add r1 and
r2 and put result in r3.
3028
…
Get instruction and
increment PC
3024
3023
Instruction Register
add r3, r1, r2
General Purpose Registers
33
r1
45
r2
78
r3
Then begin again by Fetching the instruction in 3024….
Clocks
 Computer systems have an internal clock, which is
used to synchronize their activities.
 Processors run at a particular clock speed.
 Clock speed is measured in Hertz
 One Hertz is one clock tick per second.
 MHz means mega Hertz
One MHz is one million clock ticks per second.
The clock speed determines how fast instructions
can be run
Access to Instructions
The hard disk is too slow to provide instructions to the CPU.
So programs are first loaded into main memory, which is much faster.
The CPU can then access the instructions more quickly.
Cache Memory

But as CPU speeds became faster, the main
memory couldn’t provide the CPU with the
instructions at a fast enough rate.

So even faster memory ( cache memory) is
now placed between the CPU and main
memory to provide the instructions at an
quicker rate to the CPU.
Cache Memory
When an instruction or data is accessed from main memory, it is placed in
the cache. Second and subsequent use of the same instruction/data will
then be faster, since it is accessed directly from the cache.
Primary and Secondary
Cache Memory

Most modern CPUs now have a cache
memory (L1), on the same silicon wafer as
the CPU, to provide the CPU with
instructions at the same clock speed as the
CPU.

An additional off-the-chip secondary cache
(L2) may also interact with the CPU at a
slower speed.
L1 and L2 Cache Memory
User view of Computer Systems
Applications
e.g. Word, Netscape, etc
Operating System –
- the user interface
Software that
controls the
hardware devices
How Programs Are Run


The operating system presents an interface to the user
(e.g. Windows Desktop)
The user double clicks on an icon to run a program (e.g.
Microsoft Word)



The operating system copies the program (or at least the first part
of it) from the hard disk into main memory
The CPU runs the instructions in the program, and presents the
initial Word screen
Within Word, the user uses the menu to open a document


The application software (Word) asks the Operating System to
open the file.
The Operating System communicates with the hardware to open
the file on the hard disk.