Prerequisite – 8085 program to find the factorial of a number

**Problem –** Write an assembly language program for calculating the factorial of a number using 8086 microprocessor

**Examples –**

Input : 04H Output : 18H as In Decimal : 4*3*2*1 = 24 In Hexadecimal : 24 = 18H Input : 06H Output : 02D0H as In Decimal : 6*5*4*3*2*1 = 720 In Hexadecimal : 720 = 02D0H

**Assumptions –**

Starting address of program: 0400

Input memory location: 0500

Output memory location: 0600 and 0601

**Important –**

If the Given Number is a 16-bit number, the AX register is automatically used as the second parameter and the product is stored in the DX:AX register pair. This means that the DX register holds the high part and the AX register holds the low part of a 32-bit number.

In 8086 microprocessor, user have direct instruction (MUL) to multiply two numbers, so we don’t have to add Multiplicand by Multiplier times like in 8085

**Advantage of 8086 over 8085 (In case of Multiply):**

- Don’t have to write a bulky code as 8086 has a small code
- Easy to remember
- Already have multiplication Instruction

**Algorithm –**

- Input the Number whose factorial is to be find and Store that Number in CX Register (Condition for LOOP Instruction)
- Insert 0001 in AX(Condition for MUL Instruction) and 0000 in DX
- Multiply CX with AX until CX become Zero(0) using LOOP Instruction
- Copy the content of AX to memory location 0600
- Copy the content of DX to memory location 0601
- Stop Execution

**Program –**

ADDRESS | MNEMONICS | COMMENTS |
---|---|---|

0400 | MOV CX, [0500] | CX <- [0500] |

0404 | MOV AX, 0001 | AX <- 0001 |

0407 | MOV DX, 0000 | DX <- 0000 |

040A | MUL CX | DX:AX <- AX * CX |

040C | LOOP 040A | Go To [040A] till CX->00 |

0410 | MOV [0600], AX | [0600]<-AX |

0414 | MOV [0601], DX | [0601]<-DX |

0418 | HLT | Stop Execution |

**Explanation –**

**MOV CX, [0500]**loads 0500 Memory location content to CX Register**MOV AX, 0001**loads AX register with 0001**MOV DX, 0000**loads DX register with 0000**MUL CX**multiply AX with CX and store result in DX:AX pair**LOOP 040A**runs loop till CX not equal to Zero**MOV [0600], AX**store AX register content to memory location 0600**MOV [0601], DX**store DX register content to memory location 0601**HLT**stops the execution of program

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