1.1.1 Structure and Function of the Processor

Question 1

What is the Control Unit (CU)?

Answer 1

A component of the CPU which coordinates all its activites

Question 2

what are the main functions of the CU?

Answer 2

• directs flow of data between CPU and other devices
• sends command control signals on the control bus (eg. requests to read and write to memory)
• coordinates and communicates with other CPU components

Question 3

What is the Program Counter (PC)?

Answer 3

Register which holds the address of the next instruction to be processed. Increments by one each clock cycle.

Question 4

What is the Memory Address Register (MAR)?

Answer 4

Register which holds the address of data or an instruction which is to be fetched or that is going to be written to.

Question 5

what does the MAR use to send addresses?

Answer 5

address bus

Question 6

what is the Memory Data Register (MDR)?

Answer 6

Register used to temporarily store data that is read from or written to memory.

Question 7

what does the Current Instruction Register (CIR) do?

Answer 7

holds the current instruction being processed

Question 8

what is an instruction made up of?

Answer 8

opcode + operand

Question 9

what does the Arithmetic Logic Unit (ALU) do?

Answer 9

performs logical or arithmetic operations on data

Question 10

What does the Accumulator (ACC) do?

Answer 10

one of the general purpose registers that stores the result of the ALU temporarily

Question 11

What does the Address Bus carry?

Answer 11

memory addresses that identify where data is being read from or written to

Question 12

what does the Control bus carry?

Answer 12

command and control signals to and from every component in the CPU/computer

Question 13

what does the Data Bus carry?

Answer 13

the binary code (1s and 0s) that make up the information being transmitted around the computer/CPU

Question 14

how is data processed in a computer?

Answer 14

the computer follows a set of instructions called a computer program

Question 15

what happens during the fetch stage of the FDE cycle?

Answer 15

• PC checked as it holds address of next instruction
• address in PC is copied to MAR
• address then sent to memory on address bus. it then waits for a signal from control bus
• CU sends signal (eg. read or write) along control bus
• contents in memory sent along data bus to MDR
• as the data is an instruction, data is then copied from MDR to CIR
• Increment the PC for next instruction

Question 16

what happens during the decode stage of the FDE cycle?

Answer 16

• instruction in the CIR is decoded by the decode unit
• if the instruction uses the ALU, load the contents of the memory address in the ACC

Question 17

what does the opcode do?

Answer 17

tells the computer what to do

Question 18

what is the operand?

Answer 18

tells the computer what the opcode instruction should be performed on (eg. actual data or an address)

Question 19

what happens during the execute stage of the FDE cycle?

Answer 19

which registers are used will depend on instruction:
- If value is being inputted the ACC will store value
- If value is being outputted this will be value currently in ACC
- If value is loaded from RAM this will be sent across the data bus from RAM (in the address location in the MAR) to the MDR
- If a value is to be stored it will take the value from the ACC, send it to MDR then send it across data bus to RAM (to the address location in the MAR)
- If a value is being added to or subtracted from another value, carried out in the ALU
- If the LMC code is to branch the comparison will take place in the ALU

Question 20

what happens in the decode and execute stages if the program branches?

Answer 20

• if instruction in the opcode says branch, replace value in PC with the operand from the CIR
• carry out the fetch cycle as normal

Question 21

what is the clock speed?

Answer 21

the number of FDE cycles per second in Hertz (Hz)

Question 22

how does an increased clock speed increase performance?

Answer 22

a greater number of cycles per second means more instructions can be carried out so the computer runs faster.

Question 23

what is cache?

Answer 23

temporary storage of data and instructions that is read from or written to.
near the CPU so faster to access

Question 24

how does a larger cache improve CPU performance?

Answer 24

more instructions can be stored near the CPU
this means the are quicker to access

Question 25

what does multiple cores on a single chip mean?

Answer 25

a single chip which has more than one processor that can excecute separate instructions.

Question 26

how do multiple cores improve CPU performance?

Answer 26

more than one instruction can be processed simultaneously so a program can run faster

Question 27

why does multiple cores not directly increase CPU performance?

Answer 27

• cores have to communicate
• some programs are not designed for multicore processing

Question 28

what is pipelining?

Answer 28

carrying out instructions concurrently (at the same time)
each instruction is at a different stage of the FDE cycle

Question 29

what happens to pipelining when there is branching?

Answer 29

the pipeline has to be flushed (some instructions have to be removed)

Question 30

how does pipelining improve efficiency and processor performance?

Answer 30

• Pipelining reduces latency
• The CPU is not idle while waiting for the next instruction which increases the speed of execution
• The next instruction is fetched while the current one is decoded/executed
• All parts of the processor can be used at any instance in time

Question 31

what is the structure of the Von Neumann architecture?

Answer 31

• unified memory
• single system bus
• one control unit
• instructions and data stored in same format

Question 32

what is the structure of the Harvard architecture?

Answer 32

• separate memory for instructions and data
• separate system busses for instructions and data
• separate control units
• reading + writing is done at the same time as fetching instructions

Question 33

what are some examples of contemporary processor architectures?

Answer 33

• SIMD - used in grahics processors
• MIMD - multiple instructions on multiple data with multiple cores
• distributed computing - multiple computers on shared network working on a bigger problem