machine code/fetch execute cycle

Question 1

What voltages are typically used for binary values?

Answer 1

1 = 5 volts
0 = 0 volts

Question 2

What is a machine code instruction?

Answer 2

A binary code that a machine can interpret and execute

Question 3

What is assembly language?

Answer 3

A mnemonic form of machine code (easier for people to read but still a one to one relationship)

Question 4

What are compiled high-level language programs?

Answer 4

Programs translated into machine code before they can be executed on a digital computer

Question 5

What is an executable file (.exe) written in?

Answer 5

machine code

Question 6

What parts does a machine code instruction consist of?

Answer 6

the op-code (operation code) and operand

Question 7

What is the op-code?

Answer 7

The part of a machine code instruction that includes the basic machine operation
(eg ADD)

Question 8

What is the operand?

Answer 8

The part of a machine code instruction that represents one item of data or its address to be operated upon

Question 9

What does the instruction LOAD #3 mean?

Answer 9

place the value 3 into accumulator

Question 10

What does the instruction STORE 13 mean?

Answer 10

put what is in the accumulator into the memory loacation with address 13

Question 11

What does the instruction ADD 13 mean?

Answer 11

Add up the contents of the accumulator and the contents of memory location 13 (the result goes in the accumulator)

Question 12

Which register do ADD, LOAD and STORE use?

Answer 12

The accumulator

Question 13

How are data and addresses represented in assembly code?

Answer 13

In decimal or hexadecimal

Question 14

What is a memory dump?

Answer 14

A copy of the main memory contents - viewed by programmers to see where an error has occurred

Question 15

What is the instruction set?

Answer 15

The set of bit patterns or binary codes for the machine operations that a processor has been designed to perform
i.e. every possible operation that can be carried out by a particular processor

Question 16

If there are 4 bits allocated to the op-code then how large will the instruction set be?

Answer 16

2^4 = 16 so there are 16 possible operations in the instruction set

Question 17

How can an op-code be divided up?

Answer 17

Into the basic instruction and which addressing mode will be used

Question 18

What is an addressing mode?

Answer 18

Specifies in the op-code part, shows how the operand part should be interpreted

Question 19

What are the three addressing modes?

Answer 19

-register transfer involving main memory
=main memory address will be specified
-operand to register transfer
=A value not address will be specified
-operand part not used

Question 20

Give an example of an instruction where the operand part is not used

Answer 20

HALT

just stops the program executing, does not need any data or addresses

Question 21

What is the name for fetching, decoding and executing an instruction repeatedly in a stored program computer?

Answer 21

fetch-execute cycle

Question 22

Which registers are always involved in the fetch execute cycle?

Answer 22

• program counter
• memory address register
• memory buffer register
• current instruction register

Question 23

What is an alternative name for the memory buffer register?

Answer 23

memory data register

Question 24

What are the four steps in the fetch phase of the fetch execute cycle?

Answer 24

• The address of the next instruction to be executed goes from the program counter to the memory address register
• The instruction held in the address is given to the memory buffer register
• The program counter is incremented by one
• The contents of the memory buffer register is copied to the current instruction register

Question 25

What are the two steps in the execute phase of the fetch execute cycle?

Answer 25

• Instruction i the current instruction register is decoded

- Instruction is executed

Question 26

What is register transfer notation?

Answer 26

used to show how things are passed between different registers in a processor

Question 27

How would the contents of program counter be written in register transfer notation?

Answer 27

[PC]

contents of = square brackets

Question 28

How is assigned to represented in register transfer notation?

Answer 28

<—-

the same way as in pseudocode

Question 29

What does x;y mean in register transfer notation?

Answer 29

instruction x and instruction y should be executed at the same time

Question 30

What does SP stand for?

Answer 30

stack pointer register

Question 31

What does PC stand for?

Answer 31

program counter register

Question 32

What does SR stand for?

Answer 32

status register

Question 33

What does ACC stand for?

Answer 33

accumulator

Question 34

What does CIR stand for?

Answer 34

current instruction register

Question 35

What does MAR stand for?

Answer 35

memory address register

Question 36

What does MBR stand for?

Answer 36

memory buffer register

Question 37

Give a basic description of the steps taken in the fetch execute cycle

Answer 37

• Memory address register takes the value of the program counter (ie. the address of the next instruction to be executed)
• control circuit sends down the control bus whether read or write from that address is needed
• Data from the memory address goes up the data bus to the memory buffer register
• Instructions go to the current instruction register
• program counter is incremented
• Instructions are decoded and executed
• Data goes to accumulator/general purpose registers (the top free register)