Computer Systems

Computer Systems

System Unit

Input device

Input device

Output device

Output device

Output device

Major Components of a Computer System

Processor (CPU)Runs program instructions

Main MemoryStorage for running programs and current data

Secondary StorageLong-term program & data storage (hard disk, CD, etc)

Input DevicesCommunication 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”

Component Interaction

The CPU controls all of the other resources within the system, in order to accomplish a task.

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

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 Logic Unit (ALU)

Registers

Central Processing Unit (CPU)

Arithmetic / Logic Unit

Registers

Control Unit

Small, fast storage areas for instructions and data

Performs calculations and decisions

Coordinates processing steps

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)

the results of such operations

So General Purpose Registers are used for holding and manipulating data used by the CPU

Memory

Computer Memory - millions/billions of on/off charges 

Divided into:

Bits 0 or 1

BytesGroups of 8 bitsA 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:

Kilobyte (KB) = 1024 bytes (210 bytes)

Megabyte (MB) = 1024 x 1024 bytes orone Million bytes (220 bytes)

Gigabyte (GB) = 1024 x 1024 x 1024 bytes orone Billion bytes (230 bytes)

Terabyte (TB) = 1024 x 1024 x 1024 x 1024 bytesone Trillion 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

524852495250525152525253525452555256

A word is stored in consecutive memory bytes.

10011010Each memory cell stores a set number of bits (some computers use 8 bits/one byte, others use words)

Each memory cell has a numeric address, which uniquely identifies its location

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.

InstructionsThe 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-decode-execute cycle.

Fetch-Decode-Execute CycleDuring 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

Program Counter (PC)

3023

Instruction Register

General Purpose Registers

FetchGet instruction and increment PC

CPU…

3020

3021

3022

3023

3024

3025

3026

3027

3028

…

Main Memory

add r3, r1, r2

33

45

add r3, r1, r2

r1

r2

r378

3024

ExecuteIn this case add r1 and r2 and put result in r3.

DecodeDetermine what the instruction is (add)

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

Software that controls the hardware devices

Operating System – - the user interface

Applications e.g. Word, Netscape, etc

How Programs Are RunThe 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 memoryThe CPU runs the instructions in the program, and presents the

initial Word screen

Within Word, the user uses the menu to open a documentThe 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.