8085 program to subtract two 8-bit numbers with or without borrow

In this program we will see how to subtract two 8-bit numbers using 8085 microprocessor.

Problem Statement

Write 8085 Assembly language program to subtract two 8-bit numbers with or without borrow and store the result at locations 8050H and 8051H.

Discussion

In 8085, the SUB instruction is used 2’s complemented method for subtraction. When the first operand is larger, the result will be positive. It will not enable the carry flag after completing the subtraction. When the result is negative, then the result will be in 2’s complemented form and carry flag will be enabled.

We are using two numbers at location 8000H and 8001H. When the numbers are 78H and 5DH, then the result will be (78 – 5D = 1B) and when the numbers are 23H and CFH, then the result will be (23 – CF = 154) Here 1 indicates the number is negative. The actual result is 54H. It is in 2’s complemented form.

Input

first output

Address	Data
. . .	. . .
8000	78
8001	5D
. . .	. . .

second output

Address	Data
. . .	. . .
8000	23
8001	CF
. . .	. . .

Flow Diagram

Address	HEXCodes	Labels	Mnemonics	Comments
F000	0E, 00		MVI C,00H	Clear C register
F002	21, 00, 80		LXI H,8000H	Load initial address to get operand
F005	7E		MOV A,M	Load Acc with memory element
F006	23		INX H	Point to next location
F007	46		MOV B, M	Load B with second operand
F008	90		SUB B	Subtract B from A
F009	D2, 0D, F0		JNC STORE	When CY = 0, go to STORE
F00C	0C		INR C	Increase C by 1
F00D	21, 50, 80	STORE	LXI H,8050H	Load the destination address
F010	77		MOV M, A	Store the result
F011	23		INX H	Point to next location
F012	71		MOV M, C	Store the borrow
F013	76		HLT	Terminate the program

Output

first output

Address	Data
. . .	. . .
8050	1B
8051	00
. . .	. . .

second output

Address	Data
. . .	. . .
8050	54
8051	01
. . .	. . .