8085 program to access and exchange the content of Flag register with register B

Problem – Write an assembly language program in 8085 microprocessor to access Flag register and exchange the content of flag register F with register B.

Example –

Assumptions – Initial values of flag register, register B and stack pointer are is 00, 3F, and 3FFF respectively.

PSW stands for PROGRAM STATUS WORD. PSW combines accumulator A and flag register F.

Algorithm –

1. Push the value of PSW in memory stack by help of PUSH instruction
2. Pop the value of Flag register and store it in register H by help of POP instruction
3. Move the value of register H in register C
4. Move the value of register B in register H
5. Move the value of register C in register B
6. Push the value of register H in memory stack by help of PUSH instruction
7. Pop the value of PSW from memory stack using POP instruction

Program –

MEMORY ADDRESS	MNEMONICS	COMMENT
2000	PUSH PSW	Push value of accumulator and flag in stack
2001	POP H	Pop value from TOP of memory stack in H
2002	MOV C, H	C <- H
2003	MOV H, B	H <- B
2004	MOV B, C	B <- C
2005	PUSH H	Push the value of register H
2006	POP PSW	Pop value of flag register and Accumulator
2007	HLT	END

Explanation – Registers used A, B, C, H, F

1. PUSH PSW instruction performs the following task:

       SP <- SP - 1
       M[SP] <- A
       SP <- SP - 1
       M[SP] <- F

2. POP H instruction performs the following task:

       H <- M[SP]
       SP <- SP + 1

3. MOV C, H – moves the value of H in register C
4. MOV H, B – moves the value of B in register H, hence H is updated
5. MOV B, C – moves the value of C in register B, hence B is updated
6. PUSH H performs the following task:

       SP <- SP - 1
       M[SP] <- H

7. POP PSW performs the following task:

    
       F <- M[SP]
       SP <- SP + 1
       A <- M[SP]
       SP <- SP + 1

8. HLT – stops executing the program and halts any further execution

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