module 4: basic computations in computers

Question 1

Briefly describe the main function of the ALU and how it carries out those functions.

Answer 1

Performs operations (arithmatic, logic, binary)

carries out using control signals from the control unit

Question 2

What are flags? Where and how are they stored?

Answer 2

Flags = Single Bit status indicators that indicate a condition or status

flags are stored in the status/ flag register within the ALU

Question 3

Briefly explain the function of the following flags and how the ALU works out their
value:
a. C flag
b. N flag
c. V flag
d. Z flag

Answer 3

a: Carry flag - if there is an extra bit, C = 1 (carry is true).

b: negative flag - N = 0 (answer is positive)

c: overflow flag - V = (1 XOR 1) = 0 {number is in valid range}

z: zero flag - set to 1 if ALU operation result is 0

Question 4

Interpret each of the following binary numbers as (i) unsigned integers and (ii) signed 2-s complement integers

a. 0101 1100
b. 1100 1010

Answer 4

a. 0101 1100
signed = +92
unsigned = 92

b. 1100 1010
signed = -54
unsigned = 202

Question 5

Explain what AND-masking and OR-masking are and how this is applied.

Answer 5

And-masking: To selectively “clear” specific bits of a data word

Or-masking: OR instructions can be used to selectively “set” specific bits of a data word

Question 6

Briefly explain why the multiplication function in an ALU would require a 16-bit register to store the result of two 8-bit numbers.

Answer 6

largest number you can represent with an 8 bit unsigned is 255 so you would need double that for 2 8 bit numbers.

Question 7

name each and their function:

a. SHL
b. SHR

Answer 7

a: shift left. moves each bit one destination to the left. good for multiplication

b: shift right: moves each bit 1 place to the right. good for division.