SLR 01 - Structure and Function of the CPU

Question 1

What does the CU stand for?

Answer 1

The Control Unit

Question 2

What does the CU do?

Answer 2

Co-ordinates all activities
Directs flow of data
Controls the FDE cycle
Sends memory read/write requests
Uses registers and clock
Communicates with all components

Question 3

What does the PC stand for?

Answer 3

Program Counter

Question 4

What does the PC do?

Answer 4

It is holds the address of the next instruction
This is either:
- The next instruction in a sequence
- The address to jump too when branching
Every FDE cycle the contents of the PC copies into the MAR

Question 5

What does the MAR stand for?

Answer 5

The Memory Address Register

Question 6

What does the MAR do?

Answer 6

Holds the address of the memory location from which a data / instruction to be fetched
Sends the address down the address bus to main memory

Question 7

What does the MDR do?

Answer 7

The Memory Data Register

Question 8

What does the CIR stand for?

Answer 8

The Current Instruction Register

Question 8

What does the MDR do?

Answer 8

Temporary storage of data that is to read from or to be written to memory
Sometimes considered the Memory Buffer Register
All data that interacts with Main memory must pass through the MDR

Question 9

What does the CIR do?

Answer 9

Holds the currently executed instruction
MDR contents is copied into the CIR if it is an instruction
Contains the opcode and operand of an instruction

Question 10

What does ALU stand for?

Answer 10

The arithmetic logic unit

Question 11

What does the ALU do?

Answer 11

Executes all Arithmetic and logic operations
Uses general purpose registers to hold results

Question 12

What does the Accumulator do?

Answer 12

It is a general purpose register to hold data or control information
A CPU with more general purpose registers runs faster

Question 13

What does the Address Bus do?

Answer 13

Carries memory addresses from the MAR to Main Memory

Question 14

What does the Data Bus do?

Answer 14

Carries binary that makes up transmitted information between the MDR and Main Memory

Question 15

What does the Control Bus do?

Answer 15

Carries command and control signals to all components

Question 16

What does the Decode Unit do?

Answer 16

A unit that is presented with a sequence of bits from Main Memory
Uses a lookup table to translate these into instructions understood by the CPU
These commands are specific to each processor

Question 17

What does the Status Register do?

Answer 17

It contains information about the Processor’s State

Question 18

What does the Clock do?

Answer 18

It Synchronises components by generating pulses
This is measured in Hertz (Hz)
The higher the clockspeed the faster the CPU functions.

Question 19

What does the Cache do?

Answer 19

It is a small area of fast memory
L1 is a part of the CPU chip
L2 and L3 exist between the CPU and RAM

Question 20

What does FDE stand for?

Answer 20

Fetch - Decode - Execute

Question 21

What happens in the fetch stage?

Answer 21

The PC is checked as it holds the address of the next instruction
This is copied to the MAR
This is sent along the address bus to main memory
It waits to recieve a signal from the control bus
This can be “read” from the control unit
The contents would then be sent along the data bus to the MDR
The recieved data if an instruction is copied into the CIR
The PC increments to contain the next address

Question 22

What happens in the Decode stage?

Answer 22

The instruction in the CIR is decoded by the decode unit
This splits it into it’s opcode and operand
If the operation is load, the data fron the operands location is loaded into a register

Question 23

What happens in the Execute Stage?

Answer 23

The address is sent down the Address Bus to Main Memory
The control unit releases a signal down the control bus
The contents of the address is sent down the data bus to the MDR
The contents of the MDR is copied into the ACC

Question 24

What happens if a program branches?

Answer 24

If the CIR contains the binary string to branch (e.g 0110 - branch always)
It replaces the value of the PC with that of the CIR
Because the address is differen the program jumps around memory

Question 25

What are the 3 Cs of CPU performance?

Answer 25

Clock Speed
Cache Size
Number of Cores

Question 26

How does Clock Speed affect CPU performance?

Answer 26

It controls the rate at which CPU components function.
When your clock speed is higher more instructions are processed per second.

e.g 3.2GHz is 3.2 Billion Instructions

Question 27

How does Cache Size affect CPU performance?

Answer 27

Because the Cache exists on or near the CPU it can store copies of frequently fetched instructions
It is quicker to access than RAM, meaning if you can avoid fetching from RAM you save time

Question 28

How does the Number Of Cores affect CPU performance?

Answer 28

A core is a complete copy of a CPU
Each has its own components
CPUs with multiple cores can execute multiple FDE cycles at any given time utilising Multi-Threading
Not all programs are designed with this in mind, meaning it only plays a part in specific circumstances

Question 29

What are the features of Von Neumann architecture?

Answer 29

Shared memory space for instructions and data
Instructions and data are stored using the same format
single Control Unit follows a linear FDE cycle
1 Instruction at a time
Registers are used for fast access of data

Question 30

What are the features of Harvard Architecture?

Answer 30

Instructions and Data are stored seperately
Each core has its own buses
Reading and writing Data can happen at the same time (Pipelining)
Used by RISC processors

Question 31

What does SIMD stand for and what uses it?

Answer 31

Single Instruction Multiple Data
Used most often by specialised Processors (e.g a GPU)

Question 32

What does MIMD stand for and what uses it?

Answer 32

Multiple Instruction Multiple Data
Used by CPUs

Question 33

What is Distributed Computing?

Answer 33

Multiple computers on a shared network executing a larger task concurrently.

Question 34

What is Pipelining?

Answer 34

A process involving fetching, decoding and executing instructions simulatenously

Question 35

Why is Pipelining used?

Answer 35

When not used components risk being sat idley while they could be in use?

Question 36

What are the 2 Pipelines?

Answer 36

The Instruction Pipeline

The Arithmetic Pipeline

Question 37

What does the Instruction Pipeline do?

Answer 37

It reads the next instructions from memory while others are being executed

Question 38

What does the Arithmetic Pipeline do?

Answer 38

It divides a problem into seperate parts to be executed simultaneously