7. Computer Organisation and Architecture AQA A Level Computer Science

Parts of the CPU

Processor

Main Memory

Address, Control and Data Busses

Input/Output (I/O) controllers

I/O Controller

Interface between peripheral device and computer

Device Drivers

Software that directly interacts directly with I/O controller

Tasks of an I/O controller

Converts signals received from a peripheral device into a format the computer can process.

What does the control bus send?

Control signals between all the components of a CPU.

Control Signals

•Memory read: causes data from the addressed location to be placed on the data bus

•Memory write: causes data on the data bus to be written into the addressed location

•Bus request: indicates that a device is requesting use of the data bus

•Bus grant: indicates that the CPU has granted access to the data bus

•Clock: used to synchronise operations

What does the data bus send?

Sends data between CPU components.

What does the address bus send?

Sends memory addresses from the processor to the other CPU components.

Data Bus Attributes

Bi-directional so data can be sent both ways along the bus.

The width of the bus is defined by the number of lines it contains.

Width affects overall system performance.

Address Bus Attributes

Carries the address of a memory location in one direction from the processor to I/O controllers and memory.

The width of the address bus determines the maximum possible memory addresses of the system

A 32-bit bus can carry 2^32 addresses

Main Memory

Stores data and instructions that are to be processed.

Von Neumann Architecture

The program and data being shared on a single memory bus.

Harvard Architecture

A model that separates the data and instructions into separate buses.

CPU

The brain of the computer, it controls, calculates and executes instructions.

Processor Components

•Arithmetic-Logic Unit (ALU)

•Control Unit

•Clock

•General purpose registers

•Dedicated registers

The Arithmetic Logic Unit (ALU)

The problem solving part of a processor, completing arithmetic, logical and shift operations.

Control Unit

The part of the processor that coordinates the activity of all other components.

The System Clock

A series of ON/OFF signals that synchronise the operations of the processor components.

General Purpose Registers

Stores the results of the ALU

The dedicated registers

Program Counter (PC)

Current Instruction Register (CIR)

Memory Address Register (MAR)

Memory Buffer Register (MBR)

Status Register(SR)

Program Counter (PC)

Holds the memory address of the next instruction to be executed

Current Instruction Register (CIR)

Holds the current in instruction

Memory Address Register (MAR)

Holds the address in memory where the processor is required to fetch or store data.

Memory Buffer Register (MBR)

Temporarily holds data moving between the processor and main memory

Status Register (SR)

Holds information about the current state of operations. It is used to set flags (e.g. carry or overflow) or to detect error conditions

Fetch

The address of the next instruction if fetched from the PC and transferred to the MAR, counter increments by 1, then it is temporarily stored in the MBR and the the CIR.

Decode

Instruction is decoded in the CIR, and all addition data is stored in the MAR and then passed to the registers.

Execute

ALU uses information in register to carry out the instruction, the result then being stored in the general purpose register.

What determines the frequency of the Fetch-Execute cycle?

The clock speed.

Cache Memory?

A small amount of very memory that stores data in the processor.

Factor that affect CPU performance

Clock Speed

Number of Cores

Word Length

Address and data bus width

Instruction Set

The range of commands a processor can perform

How are instructions written?

Machine code, using an operand and opcode.

Opcode

The actual command the processor needs to carry out.

Operand

Items of data that are used in the instruction, and can be values, addresses or registers.

How are instructions formatted.

Opcode (starting with the operation then the addressing mode) followed by the operand.

Addressing Mode

How the instruction should process the operand, whether its a value, address or register.

How is a value represented in the operand?

# (example ADD #5)

How is an address in the memory represented in the operand?

Nothing extra (example: ADD 5)

How is a register represented?

R (example: R5)

Immediate Addressing

Uses operand as its value

Direct Addressing

Uses operand as its address.

Assembly Code

Low Level Language that utilises Opcodes and Operands (like machine code) but its written in mnemonics rather than binary.

Barcode Readers

Light from a laser is directed on the barcode, this light is reflected back with different intensities. The light with less intensity is read as a 0 and the higher intensity is read as a 1.

Digital Camera

Shutter opens to let light come into a lens, this passes through an RGB filter that separates the light into 3 different wavelengths, that then hits a sensor. The intensity of each of these colours are combined to create the colour of one pixel. The picture is formed from a grid of these pixels.

Radio Frequency ID (RFID)

Receiver emits a frequency, the transponder becomes energised by this frequency and can then send data to the reciever.

Steps of laser printing

Data to be printed is first sent to the printer driver.

Printer driver puts data into a format that the printer can process.

Printer driver checks the status of the printer (e.g. is the printer out of toner?).

Data sent to printer and then stored in a printer buffer.

Printing process starts by giving printer drum a negative charge.

As printer drum rotates a laser beam scans across it removing the negative charge in certain areas; this leaves positively charged areas which match the text or images to be printed.

Printer drum is coated with negatively charged toner; it only sticks to the positively charged parts of the drum.

Positively charged sheet of paper is then rolled over the drum.

Toner on the drum now sticks to the paper producing exact copy of the text and images.

To prevent paper sticking to the drum, the electric charge is removed once the page has been printed.

Paper goes through a fuser which bonds the ink making a permanent copy.

At the very end of the printing process, a discharge lamp removes the electric charge from the drum making it ready for the next page.

What is the main difference between Primary and Secondary storage?

Primary is volatile and Secondary is not.

Example of secondary storage?

Magnetic Storage

Optical Storage

Solid state drives

How does Magnetic Storage work?

Positive or Negative polarisation of magnetic particles create a binary pattern on the disk. If the polarity changes, it reads as a 1, everything else as a 0.

How does Optical Disk work?

Disk has pits and land and when a low powered laser is shone at disk, it deflects when it reaches the start or end of a pit, this is read as a 1 and the rest is read as a 0.

Flash Memory Cells

Cells are combined in blocks, electrons trapped in the middle layer reads as a 0 and everything else is as a 1.

Solid State Disk

Comprises millions of NAND flash memory cells.

Advantages of SSD

Faster speed

Lower power consumption

Less susceptible to damage

Silent in operation

Light in weight