8085 program to add three 16 bit numbers stored in registers

Problem – Write an assembly language program to add three 16 bit numbers stored in register HL, DE, BC and store the result in DE with minimum number of instructions.

Example –

Assumptions –

1. Numbers to be added are already stored in register HL, DE, BC
2. Numbers stored in register are such that final result should not be greater than FFFF

DAD D performs the following task:

  H <- H + D 
  L <- L + E 

DAD instruction take one argument and that argument can be register B, D, H or SP XCHG instruction exchanges the content of register D with H and E with L

Algorithm –

1. Add the content of DE register in HL and store the result in HL by help of DAD instruction
2. Move the content of register B in D and C in E
3. Repeat step 1
4. Use XCHG instruction to swap the content of DE with HL. We will get the result in DE

Program –

MEMORY ADDRESS	MNEMONICS	COMMENT
2000	DAD D	H <- H + D, L <- L + E
2001	MOV D, B	D <- B
2002	MOV E, C	E <- C
2003	DAD D	H <- H + D, L <- L + E
2004	XCHG	Swap content of HL with DE
2005	HLT	END

Explanation –

1. DAD D – adds the content of register D in H and register E in L and store the result in HL
2. MOV D, B – moves the value of register B in register D
3. MOV E, C moves the value of register C in register E
4. Same as step 1
5. XCHG – exchange the content of register H with register D and L with E.
6. HLT – stops executing the program and halts any further execution

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