8085 program to subtract two BCD numbers

Here we will see how to perform BCD subtractions using 8085.

Problem Statement

Write 8085 Assembly language program to perform BCD subtractions of tow numbers stored at location 8001 and 8002. The result will be stored at 8050 and 8051.

Discussion

To subtract two BCD numbers, we are going to use the 10s complement method. Taking the first number and storing into B, Load 99 into A then subtract the number to get the 9’s complement. After that add 1 with the result to get 10’s complement. We cannot increase using INR instruction. This does not effect on CY flag. So we have to use ADI 01. Then DAA instruction will be used to adjust the decimal. Then if the result is negative we are storing FF as upper byte, otherwise 00 as upper byte.

Input

Address	Data
. . .	. . .
8000	01
8001	97
8002	88
. . .	. . .

Flow Diagram

Program

Address	HEX Codes	Labels	Mnemonics	Comments
F000	21, 01, 80		LXI H,8001H	Point to get the choice
F003	46		MOV B,M	Load operand to B
F004	3E, 99		MVI A,99H	Load A with 99H
F006	23		INX H	Point to next operand
F007	96		SUB M	Subtract M from A
F008	C6, 01		ADI 01H	Add 01H to get 10's complement
F00A	80		ADD B	Add B with A
F00B	27		DAA	Adjust decimal
F00C	6F		MOV L,A	Store A to L
F00D	DA, 3A, F0		JC SKP2	If CY = 1, jump to SKP2
F010	26, FF		MVI H,FFH	Load H with FFH
F012	C3, 62, F0		JMP STORE	Store result
F015	26, 00	SKP2	MVI H,00H	Clear HL
F017	22, 50, 80	STORE	SHLD 8050H	Store result from HL
F01A	76		HLT	Terminate the program