**Problem –** Write an assembly language program for determining if a given number is prime or not using 8085 microprocessor.

*If the number is prime, store 01H at the memory location which stores the result, else 00H.*

**Examples:**

Input : 03H Output : 01H The number 3 only has two divisors, 1 and 3. Hence, it is prime. Input : 09H Output : 00H The number 9 has three divisors, 1, 3 and 9. Hence, it is composite.

A prime number is the one which has only two divisors, 1 and the number itself.

A composite number, on the other hand, has 3 or more divisors.

**Algorithm:**

- Take n as input
- Run a loop from i = n to 1. For each iteration, check if i divides n completely or not. If it does, then i is n’s divisor
- Keep a count of the total number of divisors of n
- If the count of divisors is 2, then the number is prime, else composite

**How to find out if i is a divisor or not?**

Keep subtracting i from the dividend till the dividend either becomes 0 or less than 0. Now, check the value of dividend. If it’s 0, then i is a divisor, otherwise it’s not.

**Steps:**

- Load the data from the memory location (2029H, arbitrary choice) into the accumulator
- Initialize register C with 00H. This stores the number of divisors of n
- Move the value in the accumulator in E. This will act as an iterator for the loop from n to 1.
- Move the value in the accumulator in B. B permanently stores n because the value in the accumulator will change
- Move the value in E to D and perform division with the accumulator as the dividend and D as the divisor.
- Division: Keep subtracting D from A till the value in A either becomes 0 or less than 0. After this, check the value in the accumulator. If it’s equal to 0, then increment the count of divisors by incrementing the value in C by one
- Restore the value of the accumulator by moving the value in B to A and continue with the loop till E becomes 0
- Now, move the number of divisors from C to A and check if it’s equal to 2 or not. If it is, then store 01H to 202AH (arbitrary), else store 00H.

202AH contains the result.

ADDRESS | LABEL | MNEMONIC |
---|---|---|

2000H | LDA 2029H | |

2001H | ||

2002H | ||

2003H | MVI C, 00H | |

2004H | ||

2005H | MOV E, A | |

2006H | MOV B, A | |

2007H | LOOP1 | MOV D, E |

2008H | LOOP2 | CMP D |

2009H | JC DIVIDENDLESSTHAN0 | |

200AH | ||

200BH | ||

200CH | SUB D | |

200DH | JNZ LOOP2 | |

200EH | ||

200FH | ||

2010H | DIVIDENDLESSTHAN0 | CPI 00H |

2011H | ||

2012H | JNZ NOTADIVISOR | |

2013H | ||

2014H | ||

2015H | INR C | |

2016H | NOTADIVISOR | MOV A, B |

2017H | DCR E | |

2018H | JNZ LOOP1 | |

2019H | ||

201AH | ||

201BH | MOV A, C | |

201CH | MVI C, 00H | |

201DH | ||

201EH | CPI 02H | |

201FH | ||

2020H | JNZ COMPOSITE | |

2021H | ||

2022H | ||

2023H | INR C | |

2024H | COMPOSITE | MOV A, C |

2025H | STA 202AH | |

2026H | ||

2027H | ||

2028H | HLT |

Store the value of n in 2029H. If 202AH contains 01H, then n is prime, else composite.

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