8085 program to find the factorial of a number

Problem – Write an assembly language program for calculating the factorial of a number using 8085 microprocessor.

Example –

Input : 04H
Output : 18H 
as 04*03*02*01 = 24 in decimal => 18H

In 8085 microprocessor, no direct instruction exists to multiply two numbers, so multiplication is done by repeated addition as 4×3 is equivalent to 4+4+4 (i.e., 3 times).
Load 04H in D register -> Add 04H 3 times -> D register now contains 0CH -> Add 0CH 2 times -> D register now contains 18H -> Add 18H 1 time -> D register now contains 18H -> Output is 18H

Algorithm –

1. Load the data into register B
2. To start multiplication set D to 01H
3. Jump to step 7
4. Decrements B to multiply previous number
5. Jump to step 3 till value of B>0
6. Take memory pointer to next location and store result
7. Load E with contents of B and clear accumulator
8. Repeatedly add contents of D to accumulator E times
9. Store accumulator content to D
10. Go to step 4

Address	Label	Mnemonic	Comment
2000H	Data		Data Byte
2001H	Result		Result of factorial
2002H		LXI H, 2000H	Load data from memory
2005H		MOV B, M	Load data to B register
2006H		MVI D, 01H	Set D register with 1
2008H	FACTORIAL	CALL MULTIPLY	Subroutine call for multiplication
200BH		DCR B	Decrement B
200CH		JNZ FACTORIAL	Call factorial till B becomes 0
200FH		INX H	Increment memory
2010H		MOV M, D	Store result in memory
2011H		HLT	Halt
2100H	MULTIPLY	MOV E, B	Transfer contents of B to C
2101H		MVI A, 00H	Clear accumulator to store result
2103H	MULTIPLYLOOP	ADD D	Add contents of D to A
2104H		DCR E	Decrement E
2105H		JNZ MULTIPLYLOOP	Repeated addition
2108H		MOV D, A	Transfer contents of A to D
2109H		RET	Return from subroutine

Explanation –

1. First set register B with data.
2. Set register D with data by calling MULTIPLY subroutine one time.
3. Decrement B and add D to itself B times by calling MULTIPLY subroutine as 4*3 is equivalent to 4+4+4 (i.e., 3 times).
4. Repeat the above step till B reaches 0 and then exit the program.
5. The result is obtained in D register which is stored in memory

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