1.1.1 Structure and Function of the Processor

What are the 7 internal structures of the CPU?

• Control unit

• Program counter

• Memory address/buffer register

• Memory data register

• Current instruction register

• Arithmetic logic unit

• Accumulator

What is the ALU responsible for?

Performing mathematical calculations and logical operations on data

What are the tasks of the ALU?

• Performing addition, subtraction, multiplication, division

• Performing logical bitwise operations, such as AND, OR, NOT, and XOR

• Performing comparisons between values

• Shifting binary patterns to the left or right

What is the Control Unit?

A component of the processor that directs the operations of the CPU

What are the jobs of the CU?

• Controlling and coordinating the activities of the CPU

• Managing the flow of data between the CPU and other devices

• Accepting the next task, decoding tasks

• Storing the resulting data back in memory

How does the CU work?

Uses control signals to enable data to be read from and written to memory

What is the CPU?

An electronic chip that fetches instructions from main memory, decodes them, executes them and produces an output

What is the Program Counter?

Holds the address of the location of the next instruction to be executed

What does the MAR do?

Temporarily holds the address of the memory location that is to be read from or written to

What does the MDR do?

Temporarily holds the data that has been read from or that needs to be written to main memory

What does the CIR do?

Holds the current instruction being executed - divided up into operand and opcode

What does the ACC do?

Stores the results from calculations performed by the ALU

What is a bus?

A set of parallel wires that connect two or more components

What are the 3 system buses?

• Address bus

• Data bus

• Control bus

What does uni-directional mean?

Information flows in one direction - from the processor to main memory

What does bi-directional mean?

Information flows both ways

What buses are uni-directional?

Only the address bus

What buses are bi-directional?

Data and control bus

What needs to be done for a program’s intructions to be ready to be carried out?

• Translated into machine code

• Instructions have been loaded from secondary storage into main memory

What are external buses?

Connects the peripherals to the processor via serial or parallel connections

What is the purpose of the address bus?

Carries memory addresses that identify where the data is being read from and written to

What is the purpose of the data bus?

Transports data and instructions between components

What is the purpose of the control bus?

Carries command and control signals between internal and external components

What is an instruction set?

A collection of instructions that the CPU uses to carry out its tasks

What is assembly language?

Programming language that is written in a readable format

What is LMC?

Little Man Computer - A simplified instruction set

What are the 11 mnemonics in LMC and what is the translation?

• ADD - Add

• SUB - Subtract

• STA - Store

• LDA - Load

• BRA - Branch Always

• BRZ - Branch if zero

• BRP - Branch if positive

• INP - Input

• OUT - Output

• HLT - End program

• DTA - Data Location

What are the 6 factors that can affect CPU performance?

• Clock speed

• Number of cores

• Cache size

• Size of RAM

• GPU

• Pipelining

What is clock speed?

Number of instructions a single processor core can carry out per second

What is a core?

A complete processing unit that has its own registers, ALU and CU

What is cache?

A small amount of fast RAM that stores regularly used data for fast CPU access

How does the clock speed affect CPU performance?

The greater the clock speed, the faster the CPU can run, resulting in more instructions being processed every second and programs take less time to run

What is a disadvantage of having a high clock speed and how can it be combatted?

Power is used with every clock tick, which generates heat. If overclocking occurs, the heat can damage the chip. A heat sink can be used

What is overclocking?

When the CPU is run faster that its normal design speed

How does having multiple cores affect CPU performance?

• Multi-tasking - Multiple F-D-E cycles can be run at the same time on multiple cores

• Parallel Processing - when a single task is split into two or more parts and each is processed at the same time

What is a dual-core?

A CPU with two cores

What is a quad-core?

A CPU with four cores

What is a disadvantage of increasing the number of cores?

Not all programs take advantage of multiple cores

How does increasing the cache size affect CPU performance?

• Less retrieval from RAM - regularly used instructions will be retrieved from the cache at a faster rate

• More space for data/instructions in cache memory

What is a disadvantage of increasing cache size?

Cache is more expensive than RAM

What are the 3 levels of cache?

• Level 1 (L1) - Very fast but has the smallest capacity

• Level 2 (L2) - Relatively fast with a medium-sized capacity

• Level 3 (L3) - Slow but has the largest capacity

Where is L1 cache found?

Inside the CPU chip

Where is L2 and L3 cache found?

Inside or outside the CPU

What is RAM?

Random Access Memory - Main memory that is used to store computer programs and data currently in use

What happens if there is not enough RAM?

• The computer will perform poorly - some applications may not load at all

• Applications are swapped out to virtual memory

What is virtual memory?

• An area set aside on the hard disk to act as very slow RAM

• Stores some of the currently running programs and data if RAM is full

What is pipelining?

The process of fetching an instruction, whilst the prior one is being decoded and the one before that is being executed

What is instrucion pipelining?

Seperating out the instruction into fetching, decoding and executing

What is arithmetic pipeling?

Breaking down the arithmetic operations and overlapping them as they are performed

What is the fetch, decode, execute cycle?

The sequence of operations that are completed in order to execute an instruction

What is the fetch stage?

1. Contents of the PC is copied into the MAR, which is connected to the address bus. The address of the next instruction to be executed is placed on the address bus

2. CU instructs a memory read operation to allow the contents of the memory location to be transferred to the processor. Instruction stored at that address is transferred using the data bus from RAM to the processor, and is copied into the MDR

3. Contents of the MDR is copied into the CIR and the PC is incremented by 1 simultaneously to hold the address of the next instruction

What is the decode phase?

CU decodes the instruction into operand and opcode in the CIR

What is the execute phase?

Instruction is executed

What is the Von Neumann Architecture?

• The idea of storing program instructions and data in main memory and moving them between memory and the processor

• Shared memory and shared data bus for both data and instructions

What does the Von Neumann Architecture include?

A single CU, ALU, registers and memory units

What are the issues with the Von Neumann Architecture?

• Shared memory for data and programs is a security threat

• Defective programs may fail to release the memory when finished using it

• Von Neumann bottleneck

• Bottlenecked data bus

What is the Harvard Architecture?

Physically seperates memories for instructions and data

What are the isseues with the Harvard Architecture?

• More physical space needed

• Complex to handle two seperate blocks of memory

• Increases potential cost designs

What are the advantages of Von Neumann?

• Cheaper to develop as the CU is easier to design

• Programs can be optimised in size

What are the advantages of Harvard?

• Quicker execution - data and inctructions can be fetched in parallel

• Memories can be different sizes, which can make more efficient use of space

What is contemporary processing?

Uses both Harvard and Von Neumann Architecture.

• Harvard for communication between CU and caches - divides cache into instruction cache and data cache

• Von Neumann is used when working with data and instructions in main memory