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Question 1

What does ALU stand for?

Answer 1

An Arithmatic Logic Unit

Question 2

What does an ALU do?

Answer 2

> Preforms arithmetic and logical operations on binary numbers
Typically there are 2 input numbers and a single output
Control bits are used to select which operation to carry out on the inputs

Question 3

What is a BUS?

Answer 3

An example of a parallel connection

Question 4

What are the conventions for drawing busses?

Answer 4

> Thick line
Black Arrows
Single lines crossed with diagonal lines

Question 5

What are some Logic operations that can be accomplished with an ALU?

Answer 5

> Bitwise operations
Negation operations
Bit inverse clear operations

Question 6

What are bitwise operations?

Answer 6

> Different logic gate functions. e.g. AND, OR etc

> Applied to bit pairs

Question 7

What are negation operations?

Answer 7

> Inversion or complementing

> This can be done with logic gates in parallel

Question 8

What are bit inverse clear operations?

Answer 8

> This is for ARM Processors
This is used to set any bit in a binary number to 0.
Two inputs: A (Binary number) & B (Used to select which bit is to be set to 0)
How it works:
B is inverted
An AND function is applied to both bits
A.!B

Question 9

What are some Arithmatic operations that can be done on an ALU?

Answer 9

> Addition
Subtraction
Most ALU’s will implement a carry look-ahead adder because they are quicker than a ripple adder
Some microprocessors may also have hardware multipliers

Question 10

How is binary subtraction done?

Answer 10

> Using ripple adders and 2s complement
If 2’s complement is applied to the second value then the two number can be added together and this produces the subtracted numbers.
Y = A + (-B)
Y = A + B ̅ + 1

Question 11

How can 2’s complement be applied to a binary number for subtraction?

Answer 11

The second binary number is inverted (1’s Complement) and then when added with the first binary number the carry in on the ripple adder is set to 1

Question 12

What is the 3rd type of operation that an ALU can preform?

Answer 12

A data movement operation

Question 13

What decides what function an ALU preforms?

Answer 13

The control bits

Question 14

What do the ALU control bits do?

Answer 14

Control various multiplexers in the ALU to route the bus to the correct operation

Question 15

LEARN THE MULTIPLEXER ARCHETECTURE OF AN ALU

Answer 15

DO IT

Question 16

What are the common operations of an ALU?

Answer 16

> A+B
> A-B
> B-A
> A AND B
> A OR B
> A XOR B
> A AND !B
> B
> !B

Question 17

What are OP codes?

Answer 17

> Operation codes

> This is the binary number that controls the ALU and select which operation is to be preformed

Question 18

How are these operations named?

Answer 18

With a mnemonic

Question 19

What are the different mnemonics of an ALU?

Answer 19

> ADD - A+B
> SUB - A-B
> RSB - B-A
> AND - A AND B
> ORR - A OR B
> BIC -  A XOR B
> EOR - A AND !B
> MOV - B
> MVN - !B

Question 20

What are flags?

Answer 20

These indicate particular situations that can occur from preforming an operation on an ALU

Question 21

What are 4 typical flags?

Answer 21

C – A final carry-out has been generated
V – Overflow has occurred
N – The ALU output is negative
Z – The ALU output is zero

Question 22

What does the carry flag indicate?

Answer 22

> From the final carry out of the ripple adder
This is meaningless when considering signed numbers
This shows that the output number is too large for the ALU

Question 23

What does the overflow flag indicate?

Answer 23

> If the result is out of the range for that number of bits then
This is meaningless for unsigned numbers
It occurs when the sign bit changes
Overflow occurs when for the most significant bit, the carry-in and carry-out bits are different

Question 24

What does the zero flag indicate?

Answer 24

> To detect whether the output is zero

> This is done by having a NOR gate with multiple inputs connected to the ALU output

Question 25

What does the negative flag indicate?

Answer 25

> This is the sign bit of the output bus (most significant bit)
This is irrelevant when considering unsigned numbers
This is also meaningless for purely logical operations