8085 program to find 2’s complement of the contents of Flag Register

Problem – Write an assembly language program in 8085 microprocessor to find 2’s complement of the contents of Flag Register.

Example –

Algorithm –

1. Initialize the value of Stack Pointer (SP) to 3999
2. Push the contents of PSW (Register pair formed by Accumulator and Flag Register) into the memory stack
3. Pop the contents from the stack into register pair BC
4. Move the contents of register C to A
5. Take 1’s complement of the contents of A
6. Increment the contents of A by 1
7. Move the contents of A to C
8. Push the contents of register pair BC into the stack
9. Pop the contents of stack into PSW
10. Stop

Program –

MEMORY ADDRESS	MNEMONICS	COMMENT
2000	LXI SP 3999	SP <- 3999
2003	PUSH PSW	PUSH value of Accumulator and Flag into the stack
2004	POP B	POP value from Top of stack into register pair BC
2005	MOV A, C	A <- C
2006	CMA	A = 1’S complement of A
2007	INR A	A = A + 1
2008	MOV C, A	C <- A
2009	PUSH B	PUSH value of register pair BC into stack
200A	POP PSW	POP value from Top of stack into Accumulator and Flag
200B	HLT	Stop

Explanation –

1. LXI SP 3999 is used to initialize the value of Stack Pointer(SP) to 3999.
2. PUSH PSW is used to push the contents of PSW into the memory stack.
3. POP B is used to pop the contents from the top of stack into register pair BC.
4. MOV A, C moves the contents of register C to A.
5. CMA takes 1’s complement of the contents of A.
6. INR A increments the contents of A by 1.
7. MOV C, A moves the contents of A to C.
8. PUSH B is used to push the contents of register pair BC into the stack.
9. POP PSW is used to pop the contents of stack into PSW.
10. HLT is used to end the program.

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