Component 1 - Key Definitions

Question 1

Register

Answer 1

A small, fixed width piece of memory inside the CPU. Usually between 16-128 bits wide. Registers are used to hold data or instructions as they are being worked on. Registers can also be used to represent the status of various CPU components.

Question 2

Program Counter

Answer 2

A register used to hold the address of the next instruction to be executed. It is possible to switch tasks that the CPU is carrying out by changing the address held in the PC.

Question 3

Accumulator (ACC)

Answer 3

A general purpose register, usually used to hold the result of the last calculation or instruction carried out

Question 4

Memory Address Register (MAR)

Answer 4

Used to hold the address in memory which will be accessed next.

Question 5

Memory Data Register (MDR)

Answer 5

Used to hold the data which ahs just been fetched from RAM or is about to be written to RAM

Question 6

Current Instruction Register (CIR)

Answer 6

Where the instruction about to be run is decoded – held in the form Operator, Operand

Question 7

Data Bus

Answer 7

The connection between the CPU and RAM – literally a collection of wires. Used to send data between CPU and RAM

Question 8

Control Bus

Answer 8

Used to connect the various components in the CPU and also to RAM. Used to send control signals to co-ordinate the timing/workings of the CPU and also to tell RAM whether a read/write operation is needed.

Question 9

Address Bus

Answer 9

Used to connect the CPU and RAM so that addresses which need to be read from or written to can be sent to the memory controller.

Question 10

Control Unit

Answer 10

Used to coordinate the actions of the CPU during the FDE cycle. Sends signals down the control bus to components.

Question 11

FDE Cycle

Answer 11

The purpose of the CPU – the Fetch, Decode, Execute cycle. Happens continuously whilst power is on. The number of FDE cycles per second is called the “clock speed” and is measured in Ghz (Gigahertz)

Question 12

Performance - Speed

Answer 12

The number of FDE cycles a CPU can carry out per second. Has a direct impact on the speed at which program instructions can be executed. Measured in Ghz – billions of cycles per second.

Question 13

Performance - Cores

Answer 13

A core is a discrete processing unit inside a CPU – The more cores a CPU has, the more instructions can be executed simultaneously. Has a direct impact on the multitasking ability of a CPU

Question 14

Performance - Cache

Answer 14

Cache is a small amount of memory inside a CPU. Usually comes in different “levels” which differ in speed and size. L1 cache is the fastest and smallest, L3 is the slowest and largest. All are much, much faster than accessing RAM and therefore it is used to hold frequently accessed instructions to speed up program execution. Uses predictive algorithms to pre-fetch what the CPU thinks it will need to execute next.

Question 15

The use of pipelining in a processor to improve efficiency.

Answer 15

Pipelining is a method of processing instructions more efficiently. Instructions are ordered by the CPU in such a way as to avoid idle cycles where nothing is being executed. In simple terms, the CPU executes one instruction, whilst decoding the second, whilst fetching a third.

Question 16

Von Neumann and Harvard Architecture

Answer 16

The Von Neumann architecture is the basic concept on which all modern computers are based. It loosely consists of input, output, a memory store to hold programs and a CPU which has an ALU, Control Unit and Registers. One common criticism of it is the fact that instructions and data are stored in the same memory space. The Harvard architecture seeks to remove that bottleneck by storing instructions separately to data.