Chapter 3 Hardware: Unit 3.1: Computer architecture

Question 1

Define CPU

Answer 1

The central processing unit (CPU) (also known as a microprocessor or processor) is central to all modern computer systems. The CPU is very often installed as an integrated circuit on a single microchip. The CPU
has the responsibility for the execution or processing of all the instructions and data in a computer application.

Question 2

What components make up a CPU?

Answer 2

control unit (CU)
arithmetic and logic unit (ALU)
registers and buses

Question 3

Who created the basis for computer architecture?

Answer 3

John Von Neumann

Question 4

Explain John Von Neumann’s concept.

Answer 4

He developed the ‘stored program computer’.

Question 5

What were the main features of the stored program computer?

Answer 5

• The concept of a central processing unit (CPU or processor)
• The CPU was able to access the memory directly
• Computer memories could store programs as well as data
• Stored programs were made up of instructions which could be executed in sequential order

Question 6

Define ALU

Answer 6

The Arithmetic & Logic Unit (ALU) allows the required arithmetic or logic operations to be carried out while a program is being run; it is possible for a computer to have more than one ALU to carry out specific functions. Multiplication and division are carried out by a sequence of
addition, subtraction and left or right logical shift operations.

Question 7

Define CU

Answer 7

The control unit reads an instruction from memory. The address of the location where the instruction can be found is stored in the Program Counter (PC). This instruction is then interpreted using the Fetch–Decode–Execute cycle. During that process, signals are generated along the control bus to tell the other components in the computer what to do. The control unit ensures synchronisation of data flow and program instructions throughout the computer. A system clock is used to produce timing signals on the control bus to ensure this vital
synchronisation takes place – without the clock the computer would simply crash.

Question 8

Define RAM

Answer 8

The RAM holds all the data and programs needed to be accessed by the CPU. The RAM is often referred to as the Immediate Access Store (IAS). The CPU takes data and programs held in backing store and puts them into RAM temporarily. This is done because read/write operations carried out using the RAM are considerably faster than read/write operations to backing store; consequently, any key data needed by an application will be stored temporarily in RAM to considerably speed up operations.

Question 9

Define current instruction register

Answer 9

The CIR (Current Instruction Register) stores the current instruction being decoded and executed

Question 10

Define accumulator register

Answer 10

The ACC (Accumulator register) is used when carrying out ALU
calculations; it stores data temporarily during the calculations

Question 11

Define memory address register

Answer 11

The MAR (Memory Address Register) stores the address of the memory
location currently being read from or written to

Question 12

Define memory data/buffer register

Answer 12

The MDR (Memory data/buffer register) stores data which has just been read from memory or data which is about to be written to memory

Question 13

Define program counter

Answer 13

The PC (Program Counter) stores the address where the next
instruction to be read can be found

Question 14

What are the 3 system buses?

Answer 14

Control bus
Address bus
Data bus

Question 15

Define computer memory

Answer 15

The computer memory is made up of a number of partitions. Each partition consists of an address and its contents. The address will uniquely identify every location in the memory and the contents
will be the binary value stored in each location.

Question 16

Describe the process of how the MAR and MDR registers can be used
when carrying out a read and write operation to and from memory.

Answer 16

READ:
The address of the location to be read from is first written into the MAR
A ‘read signal’ is sent to the computer memory
The contents of memory location are then put into the MDR

WRITE:
The data to be stored is first written into the MDR
This data has to be written into location with address so this address is now written into the MAR
Finally, a ‘write signal’ is sent to the computer memory and the value will then be written into the correct memory location

Question 17

Define system bus

Answer 17

System buses are used in computers as parallel transmission components; each wire in the bus transmits one bit of data.

Question 18

Define address bus

Answer 18

The address bus carries addresses throughout the computer system. Between the CPU and memory, the address bus is unidirectional; this prevents addresses being carried back to the CPU, which would be an undesirable feature.
The width of a bus is very important. The wider the bus, the more memory locations that can be directly addressed at any given time.

Question 19

Define data bus

Answer 19

The data bus is bidirectional, which means data can be carried from CPU to memory (and vice versa) and to and from input/output devices. It is important to point out that data can be an address, an instruction or a numerical value.
As with the address bus, the width of the data bus is important; the wider the bus the larger the word length that can be transported.
Larger word lengths can improve the computer’s overall performance.

Question 20

Define control bus

Answer 20

The control bus is also bidirectional. It carries signals from the control unit (CU) to all the other computer components. It is usually 8-bits wide. There is no real need for it to be any wider since it only carries control signals.

Question 21

Describe the process of the Fetch-Decode-Execute cycle.

Answer 21

FETCH
Both data and instruction can be stored in MDR. The next instruction is fetched from the memory address currently stored in the MAR and the instruction is stored in the MDR. The contents of the MDR are then copied to the Current Instruction Register (CIR). The PC is then incremented so that the next instruction can be then be processed.

DECODE
The instruction is then decoded so that it can be interpreted in the next part of the cycle.

EXECUTE
The CPU passes the decoded instruction as a set of control signals to the appropriate components within the computer system. This allows each instruction to be carried out in its logical sequence.

Question 22

Describe how the Fetch-Decode-Execute Cycle is used in a Von Neumann computer model.

Answer 22

The PC contains the address of the memory location of the next instruction which has to be fetched
This address is then copied from the PC to the MAR; this is done using the address bus
The contents (instruction) at the memory location (address) contained in MAR are then copied temporarily into the MDR
The contents (instruction) of the MDR are then copied and placed into the CIR
The value in the PC is then incremented by 1 so that it now points to the
next instruction which has to be fetched
The instruction is finally decoded and then executed by sending out signals (via the control bus) to the various components of the computer system

Question 23

What are the 4 factors that affect the performance of a CPU?

Answer 23

• increasing bus width (data and address buses) increases the performance and speed of a computer system
• increasing clock speed will potentially increase the speed of a computer
• a computer’s performance can be changed by altering bus width, clock speed and use of multi-core CPUs
• use of cache memories can also speed up a CPU’s performance

Question 24

What 2 things is an operation made up of?

Answer 24

An opcode and an operand

Question 25

Define opcode

Answer 25

The opcode informs the CPU what operation needs to be done

Question 26

Define operand

Answer 26

The operand is the data which needs to be acted on or it can refer to a
register in the memory

Question 27

Define microcontroller

Answer 27

This has a CPU in addition to some RAM and ROM and other peripherals all embedded onto one single chip (together they carry out a specific task)

Question 28

Define microprocessor

Answer 28

Integrated circuit which only has a CPU on the chip (there is no RAM, ROM or peripherals – these need to be added)

Question 29

Define system on chips (SoC)

Answer 29

This may contain a microcontroller as one of its components (they almost always will include CPU, memory, input/output ports and secondary storage on a single microchip)

Question 30

What are the benefits of using embedded systems?

Answer 30

They are small in size and therefore easy to fit into devices
Compared to other systems, they are relatively low cost to make
They are usually dedicated to one task allowing simple interfaces and often no requirement for an operating system
They consume very little power
They can be controlled remotely using a mobile phone
Very fast reaction to changing input (operate in real time and are feedback orientated)
With mass production comes reliability

Question 31

What are the drawbacks of using embedded systems?

Answer 31

It can be difficult to upgrade some devices to take advantage of new technology
Troubleshooting faults in the device becomes a specialist task
Although the interface can appear to be more simple, in reality it can be
more confusing
Any device that can be accessed over the internet is also open to hackers, viruses, etc.
Due to the difficulty in upgrading and fault finding, devices are often just thrown away rather than being repaired
Can lead to an increase in the ‘throw away’ society if devices are discarded just because they have become out-of-date