Section 1 - Components Of A Computer

Question 1

What is the purpose of the CPU?

Answer 1

Central Processing Unit:
The purpose of the CPU is to process data and instructions by constantly repeating the fetch - decode - execute cycle.

Question 2

What are the components of a CPU?

Answer 2

Registers
Control Unit
Buses
Cache
Clock

Question 3

What is the Control Unit?

Answer 3

The Control Unit controls and coordinates the activities of the CPU, directing the flow of data between the CPU and other devices.

Question 4

What are the types of Buses?

Answer 4

Data Bus
Address Bus
Control Bus

Question 5

What is an Address Bus?

Answer 5

The address bus transmits the memory addresses of words that are used as operands in program instructions, so that the data can be retrieved and sent back to the processor.

Question 6

What is a Data Bus?

Answer 6

A data bus, typically consisting of 8, 16, 32 or 64 separate lines, provides a bi-directional path for moving data and instructions between system components.

Question 7

What is a Control Bus?

Answer 7

A control bus is a bi-directional bus meaning signals can be carried in both directions. The data and address buses are shared by all components of the system. Control lines must therefore be provided to ensure that access to and use of the data and address buses by the 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.

Question 8

What are the Control Lines?

Answer 8

Bus Request: Indicates that a device is requesting the use of a data bus
Bus Grant: Indicates that the CPU has granted access to the data bus
Memory Write: Causes data on the data bus to be written into the addressed location
Memory Read: Causes data from the addressed location to be placed on the data bus
Interrupt Request: Indicates that a device is requesting access to the CPU
Clock: Used to synchronize operations

Question 9

What is the ALU?

Answer 9

Arithmetic Logic Unit:
The ALU performs arithmetic and logical operations on the data. It can perform instructions such ass ADD, SUBTRACT, MULTIPLY, DIVIDE on fixed or floating point numbers. It can also perform shift operations, shifting bits to the left or right within a register. It can carry out boolean logic operations such as AND, OR, NOT, XOR.

Question 10

What are Registers?

Answer 10

Registers are special memory cells that operate at very high speed. Results of all arithmetic, logical or shift operations are temporarily stored in registers. Typically there is up to 16 general purpose registers in the CPU.

Question 11

What are the types of Registers?

Answer 11

Accumulator (ACC)
Program Counter (PC)
Current Instruction Register (CIR)
Memory Address Register (MAR)
Memory Data Register (MDR)

Question 12

What is the Accumulator (ACC)?

Answer 12

The accumulator stores the result of calculations and logical operations from the ALU.

Question 13

What is the Program Counter (PC)?

Answer 13

The program counter holds the address of the next instruction to be executed.

Question 14

What is the Current Instruction Register (CIR)?

Answer 14

The current instruction register hold the current instruction being executed, divided into operand and opcode.

Question 15

What is the Memory Address Register (MAR)?

Answer 15

The memory address register holds the address of the memory location from which data (or an instruction) is to be fetched or to which data is to be written.

Question 16

What is the Memory Data Register (MDR)?

Answer 16

The memory data register is used the temporarily store the data read from or written to the memory. It is sometimes known as the memory buffer register.

Question 17

What is the fetch-decode-execute cycle?

Answer 17

The FDE cycle is the sequence of operations involved in executing an instruction, divided into three phases. Fetching, Decoding and Executing. This cycle is repeated over and over as each program is executed.

Question 18

How are the registers used in the FDE cycle?

Answer 18

Fetch:
1. The address of the next instruction is coped from the program counter to the memory address register
2. The instruction held at that address is copied to the memory data register. Simultaneously, the content of the PC is incremented so that it holds the address of the next instruction
3. The contents of the MDR and copied to the current instruction register

Decode:
4. The instruction held in the CIR is decoded. The instruction is split into opcode and operand and the opcode is used to determine the type on instruction and what hardware to use to execute it.
The operand holds either:
The address of the data to be used with the operation, which is then copied to the MAR
The actual data to be operated on, which will be copied to the MDR
The data to be operated on may be passed to the ALU/Accumulator

Execute:
5. The appropriate instruction/opcode is carried out on the operand

Question 19

What are the main factors affecting processor performance?

Answer 19

Clock speed
Number of cores
Cache memory

Question 20

How does the clock speed affect processor performance?

Answer 20

All processor activities begin on a clock pulse, although some activities may take longer than one clock cycle to complete. One cycle per second = 1 Hertz (Hz) and clock speed is measured in Gigahertz (GHz), about 1 billion cycles per second. Typical speeds for a PC are between 2 and 4 GHz. The greater the clock speed, the faster instructions will be executed

Question 21

How does the number of cores affect processor performance?

Answer 21

Each core is theoretically able to process a different instruction at the same time with it’s own FDE cycle, making the processor two or even four time faster with a quad-core chip. However, although a dual core processor has twice the power, it does not always perform twice as fast because the software may not always be able to take full advantage of both processors

Question 22

How does cache memory affect processor performance?

Answer 22

When an instruction is fetched from main memory it is copied into the cache so if it is needed again soon after, it can be fetched from cache which is much quicker than going back to main memory. As cache fills up, unused instructions or data still being held are replaced with more recent ones

Question 23

What is pipelining?

Answer 23

Pipelining is a technique used by some processors to improve performance. Without pipelining, the steps in the FDE cycle take place on after the other. While the next instruction is being fetched, the ALU, the arithmetic part of the process is idle. Pipelining allows the next instruction to be fetched at the same time as the processor is performing arithmetic or logic operations, holding them in a buffer close to the processor until the instruction can be performed

Question 24

What is Von Neumann Architecture?

Answer 24

In Von Neumann architecture the same data bus is used to transfer both data and instructions. Similarly, a single address bus is used to transfer the addresses of data and instruction. The same word length is used for all memory, whether it holds data or instructions

Question 25

What is Harvard Architecture?

Answer 25

Harvard Architecture has physically separate memory for instructions and data. Harvard Architecture can be faster than Von Neumann because data and instructions can be fetched in parallel instead of competing for the same bus

Question 26

What are the differences between Von Neumann and Harvard Architecture?

Answer 26

Von Neumann:
Used in conventional processors in PCs, servers and embedded systems
Data and programs share the same memory
One bus is used to transfer the data and instructions
Programs can be optimized in size

Harvard Architecture:
Used in digital signal processing and in embedded systems, mobile communications, audio, speed and image processing
Instructions and data are held in separate memories
Parallel data and instruction buses may be used
Programs tend to be large

Question 27

What is contemporary processor architectures?

Answer 27

Modern high performance CPU chips incorporate aspects from both Von Neumann and Harvard architecture. In one design there is main memory for holding both data and instructions but CPU cache memory is divided into an instruction cache and a data cache. Harvard architecture is used as the CPU accesses the cache.

Question 28

What is CISC?

Answer 28

CISC: Complex Instruction Set Computers
A large instruction set is used to accomplish tasks in as few lines of assembly language as possible. A CISC processor has in its instruction set a single instruction that will do loading, multiplication and storing of the result.

Question 29

What is an advantage of CISC?

Answer 29

One advantage of CISC is that the computer has very little work to do to translate a high level language statement into machine code. Because the code is relatively short, very little RAM is required to store the instructions.

Question 30

What is a disadvantage of CISC?

Answer 30

A disadvantage of CISC was that many specialized instructions had to be built into the hardware even though only about 20% of them were used in the average program.

Question 31

What is RISC?

Answer 31

RISC: Reduced Instruction Set Computers
RISC is where only simple instructions, each taking one clock cycle can be executed. RISC has largely replaced CISC as a processor design. But CISC is still used for microcontrollers and embedded systems

Question 32

What is an advantage of RISC?

Answer 32

One advantage of RISC is greater performance due to a simplified instruction set

Question 33

What is a disadvantage of RISC?

Answer 33

One disadvantage of RISC is that the compiler has to do more work to translate high level code into machine code, and more RAM is required to store the machine code instructions.

Question 34

What are Co-processor systems?

Answer 34

Co-Processor systems are an extra processor used to supplement the functions of the primary processor. It may be used to perform floating point arithmetic, graphics processing, digital signal processing and other functions. It generally carries out only a limited range of functions

Question 35

What are multi-core and parallel systems?

Answer 35

Multi-Core CPUs are able to distribute workloads across multiple CPUs, thus achieving significantly higher performance. Many personal computers and mobile phones are dual/quad core CPUs.

Question 36

What is a GPU?

Answer 36

GPU: Graphics Processing Unit
GPU is a specialized electronic circuit which is very efficient at manipulating computer graphics and image processing. A CPU has a few cores optimized for sequential serial processing, a GPU has a massively parallel architecture consisting of thousands of smaller, more efficient cores designed for handling multiple tasks simultaneously. It’s highly parallel structure makes is suitable for tasks where processing large blocks of visual data is done simultaneously

Question 37

What are examples of input devices?

Answer 37

Keyboards, mouse, scanners, cameras, microphones

Question 38

What are examples of output devices?

Answer 38

Screens, monitors, speakers, printers, projectors

Question 39

What is the hard disk?

Answer 39

HDD (Hard Disk Drive)
HDD is a storage device, it uses rigid rotating platters coated with magnetic material. Ferrous particles on the disk are polarised to become either north or south state. This represents 1 and 0. The disk is divided into tracks in concentric circles and each track is subdivided into sectors. The disk spins very quickly at around 10,000 RPM and data is read from or written to the disk as it passed under the drive head.

Question 40

What is an optical disk?

Answer 40

CDs (Optical Disks)
Optical disks come in three different formats, read-only (CD-ROM), recordable (CD-R) and rewritable (CD-RW) An optical disk works by using a high powered laser to “burn” sections of its surface, making them less reflective. A laser at low power is used to read the disk by shining light onto the surface and a sensor is used to measure the amount of light reflected back. A CD-ROM pressed during manufacturing has pits in its surface, the areas not pitted are called lands. Reflected and non-reflected areas are read as 1s and 0s.

Question 41

What is solid state disk?

Answer 41

SSD (Solid State Disk)
Solid state memory comprises of millions of NAND flash memory cells and a controller that manages pages and blocks of memory. Each cell works by delivering a current along the bit and word lines to activate the flow of electrons from the source towards the drain. The current on the world line however is strong enough to force a few electrons across an insulated oxide layer into a floating gate. Once the current is turned off, these electrons are trapped. The sate of the NAND cell is determined by measuring the charge in the floating gate. No charge (with no electrons) is considered a 1 and some charge is considered a 0.

Question 42

What is RAM?

Answer 42

RAM (Random Access Memory)
RAM is used to store programs and data that are currently being used. It is volatile, meaning that its contents are lost when the computer is switched off

Question 43

What is ROM?

Answer 43

ROM (Read Only Memory)
ROM is used to hold information that needs to be permanently in memory. The bootstrap loader, for example has to be held in ROM.

Question 44

What is virtual storage?

Answer 44

Virtual Storage is used when the computer’s RAM may not be large enough to store all the program simultaneously. The operating system may copy a process e.g. word software and it’s data to the hard disk to free up RAM for something else e.g. browser software. When you close the browser software, the operating system will reload into memory.