8085 Microprocessor

Question 1

What is the purpose of flags in the microprocessor

Answer 1

Control decision making in program execution

Question 2

Explain the role of the Z-Zero Flag

Answer 2

Indicates that the result in the accumulator and other registers is zero. In this case it is set to one

Question 3

Explain the role of the CY-Carry Flag

Answer 3

Set to one when the result of an arithmetic operation results in a carry, i.e the result is greater than 8-bit

Question 4

Explain the role of the S-Sign Flag

Answer 4

Used with signed numbers. Set to to one if the bit D7 is 1- (means number is negative)

Question 5

Explain the role of the parity flag

Answer 5

Set to one if the number of ones is even

Question 6

Explain the role of the auxiliary carry

Answer 6

Set to one if a carry is generated by bit D3 and D4.
Also used for BCD operations. There are no jump instructions associated with this flag

Question 7

What is the function of the Program Counter

Answer 7

The microprocessor uses PC to sequence execution of instructions
Points to the memory address from which the next instruction (1 byte) is going to be fetched. After the byte is fetched the content of the PC is automatically incremented

Question 8

What is the function of the stack pointer

Answer 8

Points to the read write memory called the stack. The beginning of the stack is defined by loading a 16 bit address to the stack pointer.
The stack is important when dealing with subroutines (functions)

Question 9

Draw the 8085 flag register

Answer 9

*See page 27