Chapter 5 Processor fundamentals

Question 1

What are the basic features of the Von Neumann model of a computer?

Answer 1

• There is a processor, a central processing unit.
• The processor has direct access to a memory
• The memory contains a stored program that is being executed.
• The stored program consists of individual instructions
• The processor executes instructions sequentially

Question 2

Which are the main components of a CPU and what do they do?

Answer 2

• The ALU is the Arithmetic and logic circuit and it is responsible for any arithmetic or logic processing that might be needed when a program is running.
• The control unit that does the following:
  
  • Controls the flow of data through the processor and computer
  • Ensures that computer data is handled correctly

Question 3

Which are the two most important parts of the control unit and why?

Answer 3

The internal clock - controls the cycles inside the processor and its frequency is called clock speed
The system clock - controls the activities outside the processor

Question 4

What are registers, where are they situated and how much can they store?

Answer 4

The registers are storage components very close to the ALU and can store 16, 32 or 64 bits.

Question 5

What are the general purpose registers?

Answer 5

A general purpose register or an accumulator can store only a single value at a given time.

Question 6

What is an accumulator?

Answer 6

An accumulator is a general purpose register that stores a value or instruction before and after the execution of an instruction by the ALU

Question 7

How does the ALU use accumulators?

Answer 7

The ALU uses accumulators to store instructions in them that can be changed to other instructions.

Question 8

Which are the special purpose registers and what do they do?

Answer 8

CIR (Current Instruction Register) - store current instruction while executed
PC (Program Counter) - stores the address of where the next instruction is to be read from.
IX (Index register) - Stores a value for indexed addressing
MAR (Memory Address Register) - Stores the address where the CPU is going to read from or write to a value
MDR (Memory Data Register) - stores data that was read from a memory or is about to be written into a memory because it is closer to the processor.
SR (Status Register) - operates as a flag so it will signal certain errors or situations

Question 9

What is a system bus?

Answer 9

A system bus is a parallel transmission component with each separate wire carrying a single bit.

Question 10

What is crucial to remember about a system bus?

Answer 10

A system bus isn’t a storage device.

Question 11

What does a system bus consist of?

Answer 11

Address bus
Data bus
Control bus

Question 12

What is an address bus do?

Answer 12

The address bus is a component that carries an address to the memory controller from the MAR to identify and address where data is to be read from or written to.

Question 13

What can identify how many addresses an address bus can carry?

Answer 13

The number of wires is defining how many addresses an address bus can carry.

Question 14

What kind of bus is an address bus?

Answer 14

An address bus is a one way bus.

Question 15

What does a data bus do?

Answer 15

Carries data to and from the processor

Question 16

What is the typical word length of a data bus?

Answer 16

16 bits, 32 bits of 64 bits.

Question 17

What kind of bus is a data bus?

Answer 17

A data bus is normally a bidirectional bus.

Question 18

What does the control bus do?

Answer 18

The control bus sends clock cycle signals to the component so that it receives the time when the data was sent.

Question 19

What kind of bus is a control bus?

Answer 19

A control bus is normally a bidirectional bus.

Question 20

How many wires does a control bus normally have?

Answer 20

The control bus typically has just 8 wires, no need for more.

Question 21

What is a Universal serial buses purpose and abbreviation

Answer 21

Universal serial bus is a data bus that allows to connect external devices and its abbreviation is USB.

Question 22

What happens in a fetch execute cycle?

Answer 22

The data or instruction is fetched than decoded and finally executed.

Question 23

What is register transfer notation?

Answer 23

Register transfer notation is a notation protocol to describe the process.

Question 24

What could some reasons for an interrupt to be generated?

Answer 24

Fatal error in the program
Hardware fault
A need of I/O process to begin
User interaction
Timer signal

Question 25

When is the interrupt handled?

Answer 25

The interrupt is handled only at the end of the fetch-execute cycle.

Question 26

What happens at the moment of the interrupt?

Answer 26

At the moment of interrupt the system stores the content of the
program counter and any other register involved.

Question 27

What is an ISR?

Answer 27

ISR is the Interrupt Service Routine program that is loaded after an interrupt has happened.

Question 28

What are the steps taken after the system stores the content of the program counter and any other register involved?

Answer 28

Step 1 The start address of the ISR is loaded to the PC
Step 2 The ISR checks if further interrupts are need to be handled
Step 3 If there are no more interrupts the ISR restores the original PC and the registers that were saved when the intrerrupt kicked in at the beginning.