8086 program to add two 16 bit BCD numbers with carry

Problem – Write an assembly language program to add two 16 bit BCD numbers with carry in 8086 microprocessor. 

Example –

Algorithm –

1. Load the lower part of both the 16 bit BCD numbers in different locations.
2. Add each number by adding first its lower part.
3. Repeat the above step also by adding the carry if any.
4. Make the lower part of register 00 and add the carry. This is done to obtain the carry.
5. Display all the numbers with highest part as carry, middle part as addition of the higher BCD 8 bits and lower part as the lower BCD 8 bits.

Program –

Memory Address	Mnemonics	Comments
0400	MOV AL, [500]	AL ← [500]
0404	MOV BL, [502]	BL ← [502]
0408	ADD AL, BL	AL ← AL+BL
040A	DAA	Decimal Adjust AL
040B	MOV [600], AL	AL → [600]
040F	MOV AL, [501]	AL ← [501]
0413	MOV BL, [503]	BL ← [503]
0417	ADC AL, BL	AL ← AL+BL+CY
0419	DAA	Decimal Adjust AL
041A	MOV [601], AL	AL → [601]
041E	MOV AL, 00	AL ← 00H
0420	ADC AL, AL	AL ← AL+AL+CY
0422	MOV [602], AL	AL → [602]
0426	HLT	Stop Execution

Explanation –

1. MOV AL, [500] moves the value stored at memory location 500 to AL register.
2. MOV BL, [502] moves the value stored at memory location 500 to BL register.
3. ADD AL, BL add the values in AL and BL registers.
4. DAA adds 6 to the digit which is greater than 9.
5. MOV [600], AL display the added value to memory location 600.
6. MOV AL, [501] moves the value stored at memory location 501 to AL register.
7. MOV BL, [503] moves the value stored at memory location 503 to BL register.
8. ADC AL, BL add the values in AL and BL registers and carry (if any).
9. MOV BL, [503] moves the value stored at memory location 503 to BL register.
10. MOV [601], AL display the added value to memory location 601.
11. MOV AL, 00 moves 00 in AL register.
12. ADC AL, AL add the values in AL and AL registers and carry (if any).
13. MOV [602], AL display the added value to memory location 602.
14. HLT stops execution.

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