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 with the help of PUSH instruction 
   
2. Pop the value of the Flag register and store it in register H with help of POP instruction 
   
3. Move the value of register L in register C  [ H -> ACCUMULATOR VALUE, L-> FLAG REGISTER VALUE]
   
4. Move the value of register B in register L 
   
5. Move the value of register C in register B 
   
6. Push the value of register H in-memory stack with the help of PUSH instruction 
   
7. Pop the value of PSW from the memory stack using POP instruction 
   

Program –

MVI B , 3FH
PUSH PSW
POP H
MOV C , L
MOV L , B
MOV B , C
PUSH H
POP PSW
HLT

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, L	C <- L
2003	MOV H, L	L <- B
2004	MOV B, L	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

1. POP H instruction performs the following task: 
   

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

1. MOV C, L – moves the value of L in register C as we know that H contains the accumulator value and L contains flag register value 
   
2. MOV L, B – moves the value of B in register L, hence L is updated 
   
3. MOV B, C – moves the value of C in register B, hence B is updated 
   
4. PUSH H performs the following task: 
   

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

1. POP PSW performs the following task: 
   

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

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

Advantages:

• The program is a useful utility program for accessing and exchanging the contents of the Flag register with another register.
  
• It can be customized to exchange the Flag register contents with other registers as well, by modifying the instructions to load and store the register values.
  

Disadvantages:

• The program is not optimized for speed, as it uses multiple instructions to move each byte of data and push/pop the stack.
  
• The program requires the use of a stack, which can be a limited resource in some applications.
  
• The program does not check for errors or boundary conditions, such as when the register contents exceed the valid range or when the stack overflows.

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