8085 program to multiply two 16-bit numbers

Problem: Write an assembly language program in 8085 microprocessor to multiply two 16 bit numbers. 

Assumption:

• The starting address of program: 2000 
• Input memory location: 2050, 2051, 2052, 2053 
• Output memory location: 2054, 2055, 2056, 2057 

Example:

INPUT:
       (2050H) = 04H
       (2051H) = 07H 
       (2052H) = 02H 
       (2053H) = 01H
OUTPUT:
        (2054H) = 08H
        (2055H) = 12H
        (2056H) = 07H
        (2057H) = O0H

Result:  Hence we have multiplied two 16-bit numbers. 

Algorithm:

1. Load the first data in the HL pair. 
2. Move content of HL pair to the stack pointer. 
3. Load the second data in the HL pair and move it to DE. 
4. Make H register as 00H and L register as 00H. 
5. ADD HL pair and stack pointer. 
6. Check for carrying if carry increment it by 1 else moves to the next step. 
7. Then move E to A and perform OR operation with accumulator and register D. 
8. If the value of the operation is zero, then store the value else go to step 3. 

Program:

MEMORY ADDRESS	MNEMONICS	COMMENTS
2000	LHLD 2050	Load H-L pair with data of address 2050 and 2051
2003	SPHL	SAVE IT IN STACK POINTER
2004	LHLD 2052	Load H-L pair with data of address 2052 and 2053
2007	XCHG	EXCHANGE HL AND DE PAIR CONTENT
2008	LXI H,0000H	H<-00H,L<-00H
200B	LXI B,0000H	B<-00H,C<-00H
200E	DAD SP
200F	JNC 2013	JUMP NOT CARRY
2012	INX B	INCREMENT BC BY 1
2013	DCX D	DECREMENT DE BY 1
2014	MOV A,E	A<-E
2015	ORA D	OR THE CONTENT OF ACCUMULATOR AND D REGISTER
2016	JNZ 200E	JUMP NOT ZERO
2019	SHLD 2054	L<-2054,H<-2055
201C	MOV L,C	L<-C
201D	MOV H,B	B<H
201E	SHLD 2056	L<-2055,H<-2056
2021	HLT	TERMINATES THE PROGRAM

Explanation: 

Registers B, C, D, E, H, L, and accumulator are used for general purposes. 

1. LHLD 2050: load HL pair with data of address 2050and 2051. 
   
2. SPHL: save the content of HL in the stack pointer. 
   
3. LHLD 2052: load H-L pair with data of addresses 2052 and 2053. 
   
4. XCHG: exchange the content of the HL pair with DE. 
   
5. LXI H, 0000H: make H as 00H and L as 00H. 
   
6. LXI B, 0000H: make B as 00h and C as 00H 
   
7. DAD SP: ADD HL pair and stack pointer. 
   
8. JNC 2013: jump to address 2013 if there will be no carry. 
   
9. INX B: increments BC register with 1. 
   
10. DCX D: decrements DE register pair by 1. 
    
11. MOV A, E: move the content of register E to the accumulator. 
    
12. ORA D: or the content of accumulator and D register. 
    
13. JNZ 200E: jump to address 200E if there will be no zero. 
    
14. SHLD 2054: store the result to memory addresses 2054 and 2055 from HL pair register. 
    
15. MOV L, C: move the content of register C to L. 
    
16. MOV H, B: move the content of register B to H. 
    
17. SHLD 2056: store the result to memory addresses 2056 and 2057 from HL pair register. 
    
18. HLT: terminates the program. 

Disadvantages:

• This program uses a loop to perform the multiplication operation, which can be slow for large inputs.
  
• The program assumes that the input numbers are both positive and within the range of 16 bits, which may not always be the case.
  
• The program does not handle overflow situations, where the result of the multiplication operation is larger than 16 bits.
  
• The program assumes that the memory locations of the 16-bit numbers are known and fixed, which may not always be the case in a dynamic computing environment.
  
• The program is complex and difficult to understand, making it difficult to modify or debug.