Computer operation

is of how the different parts of a computer system work together to perform a task.

Central Processing UnitThe central processing unit (CPU) is the brain of a computer system.All processing is carried out by the CPU.

Input UnitData are input through the input unit. An input device puts the datainto a form that is acceptable to the computer.

Main MemoryThe Main memory stores the programs and data to be processed by the CPU. It also holds the processed information before it is output.

Output UnitThe results are output by the output unit. An output device outputs results in a form that can be understood by humans.

Secondary StorageAs the main memory is temporary and of limited capacity, some data and programs are stored in the backing store for future use.

Communication UnitThe communication unit connects a computer system to other computers so that data and programs can be sent to and received from them.

All devices connected to the system unit are called peripherals.

Flow of dataThe components of a Computer System

System Unit

Registers Both the ALU and Control Unit contain high-speed temporary storage locations called registers. They hold program instructions and data temporary during processing.

Central Processing Unit (CPU)

The CPU consists of Arithmetic and Logic Unit (ALU) Control Unit.(see next slide for their functions)

Arithmetic and Logical Unit (ALU)The ALU performs arithmetic and logical operations (AND, OR, NOT, =, > , < , …).It contains a register called Accumulator that temporarily holds results of ALU operations.

Control Unit (CU)The control unit retrieves repetitively an instruction from the main memory and stores it temporarily in a register called Instruction Register (IR). It then interprets (decodes) the instruction in IR and sends out control signals to other parts of the computer to carry out the instruction. By means of a register called Program Counter (PC), the Control Unit is able to keep track of the sequence of instructions that are to be processed. PC contains the address of the memory location storing the next instruction to be executed and will be automatically increased to point to that location when an instruction is fetched.

Calculating

Circuitry

PC (Program Counter)

IR (Instruction Register)

[5] Acc

Acc (Accumulator)

18

[6] +[Acc] Acc

CU (Control Unit)

ALU (Arithmetic and Logic Unit)

CPU (Central Processing Unit)

Instruction D

ecoder

Main memory is divided into memory locations. Each location has a memory address. By specifying its memory address, a memory location can be identified.

0

1

2

3

4

5

6

7

010110101001010101111010010101011011010010001101110000100100010110010100001010100000010100010100110000101000011000100001000001100010111100000110

A memory location holding a piece of data or an instruction. The address for this location is 1.Memory

addresses

Note: Memory addresses are in binary. Decimal numbers are used here for simplicity.

Main Memory

Calculating

Circuitry

0

1

2

34

5

6

7

PC (Program Counter)

IR (Instruction Register)

[5] Acc

Acc (Accumulator)

18

[6] +[Acc] Acc

CU (Control Unit)

ALU (Arithmetic and Logic Unit)

CPU (Central Processing Unit)Main Memory

Instruction D

ecoder

Machine Instructions

Machine instructions are in binary. Symbolic forms are used hereto tell the meanings of the instructions used.

Instruction Action [5] Acc load the contents of location 5 to the

Accumulator

[6]+[Acc] Acc Add the contents of location 6 and the Accumulator leaving the result in the Accumulator

[Acc] 7 Store the contents of the Accumulator to location 7

Note: [x] means the content of x

Here are the instructions for adding two numbers:

The example in the next slide shows how a program is executed in acomputer.

Note the following:

1. The program and data to be processed are stored in the main memory.

2. The instructions are fetched to and decoded by the CU one by one repetitively.

3. The CU sends out control signals to instruct other parts to carry out the instruction.

Calculating

Circuitry

0 [5] Acc

1 [6]+[Acc] Acc

2 [Acc] 7

3 Stop4

5 18

6 26

7

PC (Program Counter)

IR (Instruction Register)

0

[5] Acc

Acc (Accumulator)

18

+ 18

44

1

[6] +[Acc] Acc

26

2

[Acc] 7

CU (Control Unit)

ALU (Arithmetic and Logic Unit)

CPU (Central Processing Unit)Main Memory

Instruction D

ecoder

44

Stop

3

Machine Cycle

The process that a CPU executes an instruction is referred as a machine cycle. The machine cycle is repeated each time the computer executes an instruction.

Store the result

Execute the instruction

Decode the instruction

Fetch an instruction

How memory works

000

001

010

011

100

101

110

111

0 1 1 0 1 1 0 1

In the following, each memory location holds 8 bits of data.

Each memory location can store a number of bits of data and has a memory address.

The memory location with memory address 010 holds the data 01101101.

How memory works

Note that a memory address is not a piece of data stored. It corresponds to an electronic signal generated by the address decoder. (next slide)

000

001

011

100

101

110

111

0 1 1 0 1 1 0 1

Address D

ecoder

010

Address Bus

A memory address is not a piece of data stored. It corresponds to an electronic signal generated by the address decoder.

Data Bus

010

. . . . . . . . 0 1 1 0 1 1 0 1

How address decoder works - decoder.swf

Memory size is limited by the number of bits to store an address (related to computer architecture)10 bit address => 210 = 1K memory locations

0

1023

10 bit address

Add

ress

dec

oder

Memory

Cache memory

Cache Memory

Cache memory is high speed memory (RAM) that a processor uses to hold recently used instructions or data temporarily. In case these instructions and data are needed again, the processor can get them directly from the cache memory rather than going back to the much slower main memory.

Computers spend a lot of time in loops, repeating the same sequence of instructions, so using cache memory can greatly improve performance.

Reference reading: cache.doc

L1 (Level 1) Cache: located on the CPU chip- faster - more expensive- smaller capacity

L2 (Level 2) Cache: on nearby chips with high-speed link to the CPU- slower than L1 cache- less expensive- larger capacity

Cache of a CPU, main memory and othersDemo: Use CPU-Z to view

System Bus

Each bus is a shared transmission medium, so that only one device can transmit along a bus at any one time.

Data and control signals travel in both directions (bi-directional) between the processor, memory and I/O controllers.

Addresses travel only one way (uni-directional) along the address bus: the processor sends the address of an instruction, or of data to be stored or retrieved, to memory or to an I/O controller.

The control bus is a bi-directional bus (two-way).

The control lines of the bus send control signals from CU to other system components to control and co-ordinate execution of operations.

Interrupt requests are sent from other system components to the CU.

The data and address buses are shared by all components of the system. Control lines ensure that access to and use of the data and address buses by different components of the system does not lead to conflict.

The purpose of the control bus is to transmit command, timing and specific status information between system components. •Timing signals indicate the validity of data and address information.•Command signals specify operations to be performed. •Specific status signals indicate the state of a data transfer request, or the status of a request by a component to gain control of the system bus.

Refer to Reference Notes – System Bus for more.

I/O controllers

Peripheral devices cannot be connected directly to the processor. Each peripheral operates in a different way and it would not be sensible to design processors to directly control every possible peripheral.

The processor controls and communicates with a peripheral device through an I/O or device controller.

http://www.mitchellwebdesign.com/arm/lecture1/lecture1-3-2.html

An animation showing how control lines, address and data buses and the system clock work together to complete a task:

DRAM (Dynamic RAM)need to be continuously re-charged to maintain the memory

=> slower, more heat generated, cheaper

SRAM (Static RAM)no need to be re-charged to maintain the memory=> faster, less heat generated, more expensive

SDRAM -- synchronous dynamic RAM

EPROM, or erasable programmable read only memory, is a type of memory chip that retains its data when its power supply is switched off. An EPROM can be erased by exposing it to strong ultraviolet light from a mercury-vapor light source.

EEPROM is user-modifiable read-only memory (ROM) that can be erased and reprogrammed (written to) repeatedly through the application of higher than normal electrical voltage generated externally or internally in the case of modern EEPROMs. Flash memory such as Flash USB memory, SD memory cards are examples of EEPROM.

Instruction pipelining