3.2 Inside the CPU

Question 1

Arithmetic Logic Unit (ALU)

Answer 1

1. Carries out calculations (arithmetical processing) - to find the numerical answer to mathematical problems
2. Carries out logical operations - to find the true/false answer to logical deductions
3. Holds temporary/interim values during calculations in a register called the accumulator (ACC)

Question 2

Control Unit

Answer 2

• Controls operation of memory, processor and input/ output
• INSTRUCTIONS are interpreted
• Sends signals to other components telling them “what to do”
• Has a timer to send control signals in the right order, giving each action time to finish before sending the next signal

Question 3

Registers

Answer 3

Small areas of fast memory inside the CPU

Question 4

Accumulators

Answer 4

Used as fast memory to hold partial results in a calculation

Question 5

Program Counter (PC)

Answer 5

Holds address of the next instruction to fetch

1. Can change to a different location by some instructions
2. Peripherals can change PC to interrupt CPU

Question 6

Memory Address Register (MAR)

Answer 6

Holds address to read from/write to

Question 7

Instruction Register (IR)

Answer 7

Holds instruction currently being executed

Question 8

Memory Data Register (MDR)

Answer 8

stores data/instructions THAT IS IN USE from address in MAR

Question 9

Buses

Answer 9

Short wire connections

Question 10

Address bus

Answer 10

• carries an address/location…
• … of the next item to be fetched
• data travels in one direction (unidirectional)

Question 11

Data bus

Answer 11

• carries data…
• … that is currently being processed
• data travels in both directions (bidirectional)

Question 12

Control bus

Answer 12

• carries signals
• controls the actions of the CPU/processor
• can be either unidirectional or bidirectional

Question 13

Describe the purpose of a logic gate in a logic circuit.

Answer 13

• to carry out a logical operation
• to control the flow of electricity through a logic circuit
• to alter the output from given inputs

Question 14

7 stages in a Von Neumann fetch-execute cycle

Answer 14

1. PC (program counter) contains the address of the next instruction to be fetched
2. Address contained in the PC is copied to the MAR (memory address register) via the address bus
3. The instruction is then copied from the memory location contained in the MAR and is placed in the MDR (memory data register) using the data bus
4. The entire instruction is then copied from the MDR and placed in the CIR (current instruction register)
5. Value in the PC is then incremented so that it points to the next instruction to be fetched
6. The address part of the instruction, if any, is placed in the MAR
7. The instruction is finally decoded and is then executed

Question 15

Stored program concept

Answer 15

• Program instructions and data are stored temporarily in main memory
• Instructions are then fetched, decoded and executed by the processor ONE AFTER ANOTHER

Question 16

Fetch-execute cycle (3 steps)

Answer 16

1. The Program Counter (PC) in the CPU specifies where the next instruction to be executed is stored. The control unit (CU) fetches the next instruction to be executed from main memory.
2. The CU decodes the instruction so it knows what action to carry out.
3. The CU sends the instruction to the ALU. The ALU carries out the action. The PC is updated to point to the next instruction.
   - The process is repeated.

Question 17

Handshaking

Answer 17

The exchange of pre-determined signals between a computer and input/output device to assure proper synchronisation

Question 18

Polling

Answer 18

The checking of the status/readiness of the external device

Question 19

Buffer

Answer 19

A temporary storage area in the main memory for data before the data is processed

Question 20

Interrupt

Answer 20

A signal sent from a device that halts the running of the processor to service the interrupt
- interrupts have different priorities

Question 21

Checksum

Answer 21

A verification check used to detect errors in data after transmission