Lecture 2

Question 1

What does the PC contain?

Answer 1

The address of the next instruction to be executed in main memory.

Question 2

What does the IR contain?

Answer 2

The instruction currently being processed.

Question 3

What does the MAR contain?

Answer 3

The address of the data in memory to be retrieved or written.

Question 4

What does the MBR contain?

Answer 4

The data word.

Question 5

What do the I/O AR and I/O BR contain?

Answer 5

Address and data word to be retrieved or written to I/O space.

Question 6

What are registers in the CPU? (6)

Answer 6

PC (program counter), IR (instruction register), MAR, MBR, I/O AR and I/O BR.

Question 7

What is an I/O module?

Answer 7

Consists of at least one buffer which provides space for data words or instructions. Each buffer location has unique address.

Question 8

Describe Instruction Cycle (4 phases, two main).

Answer 8

Start, fetch next instruction, decide and execute instruction, halt.

Question 9

Describe fetch cycle (3 parts).

Answer 9

Processor fetches instruction from memory location located in PC.
PC is incremented.
Fetched instruction loaded into IR.

Question 10

Describe execute cycle, 4 instruction types.

Answer 10

Processor to memory (cpu to memory).
Processor to i/o (cpu to i/o module).
Data processing (arithmetic or logical operation on data).
Control (alteration of sequence of operations ie Jump).

Question 11

How many bits is an instruction or data word?

Answer 11

16 bits long or 4 hex digits.

Question 12

What is an op code? Where is it?

Answer 12

The left most hex digit is the op code. The address is the three right most hex digits.

Question 13

Which computer functions are interrupts vital for? (3)

Answer 13

i/o management, error handling and multi-tasking.

Question 14

What are the four classes of program interrupts?

Answer 14

Program (generated by instruction execution - division by zero etc).
Timer (generated by internal timer).
I/O
Hardware Failure.

Question 15

What does ISR stand for?

Answer 15

Interrupt service routine. Also interrupt handler.

Question 16

How many interrupt sources typically?

Answer 16

20+. Can be enabled or disabled.

Question 17

What are steps of interrupt handler?

Answer 17

Suspend program.
Save context.
Set PC to start address of interrupt handler routine.
Process interrupt.
Restore context.

Question 18

What are instructions. What are programs?

Answer 18

Instructions are binary bit patterns (machine code). A program is a sequence of instructions.

Question 19

What is assembly language?

Answer 19

Assembler converts it to binary for cpu, easier for humans. Addresses and operands represented as numbers or symbols. Mnemonics represent op codes.

Question 20

Write a = a + b as assembly language.

Answer 20

ADD A, B

Question 21

How many op codes in 4 bits/hex?

Answer 21

16

Question 22

How many operand references and how many bits each? How many addresses can each reference?

Answer 22

2 operand references of 6 bit width. Each address 64 memory locations.

Question 23

What are the main types of instructions a cpu can perform?

Answer 23

Data transfer (between cpu & memory).
Arithmetic.
Logical.
Conversion. (date from decimal to binary).
Input / Output. (Transfer data between cpu register & I/O device).
System control (read/modify control register).
Transfer of control (?).

Question 24

Difference between more address fields and fewer address fields?

Answer 24

More= more complex instructions, fewer instructions, requires more internal registers.
Fewer= reduce complexity, more instructions, fetched and decided quicker,

Question 25

In the context of interrupts what is an IC?

Answer 25

An interrupt controller/Interrupt Unit.

Receives interrupt requests from internal and external sources.

Question 26

How is an interrupt source connected to an IC?

Answer 26

A physical wire.

Question 27

How do you deal with simultaneous multiple interrupt events?

Answer 27

Disable interrupts (ignore, keep pending, in sequence).
Define priorities (higher given precedence).

Question 28

What is the AC?

Answer 28

The Accumulator. A data register.