FCFS Disk Scheduling Algorithms (original) (raw)

Last Updated : 12 Jul, 2025

Prerequisite: Disk scheduling algorithms.
Given an array of disk track numbers and initial head position, our task is to find the total number of seek operations done to access all the requested tracks if First Come First Serve (FCFS) disk scheduling algorithm is used.

First Come First Serve (FCFS)
FCFS is the simplest disk scheduling algorithm. As the name suggests, this algorithm entertains requests in the order they arrive in the disk queue. The algorithm looks very fair and there is no starvation (all requests are serviced sequentially) but generally, it does not provide the fastest service.

Algorithm:

1. Let Request array represents an array storing indexes of tracks that have been requested in ascending order of their time of arrival. ‘head’ is the position of disk head.
2. Let us one by one take the tracks in default order and calculate the absolute distance of the track from the head.
3. Increment the total seek count with this distance.
4. Currently serviced track position now becomes the new head position.
5. Go to step 2 until all tracks in request array have not been serviced.

Example:

Input: Request sequence = {176, 79, 34, 60, 92, 11, 41, 114} Initial head position = 50

Output: Total number of seek operations = 510 Seek Sequence is 176 79 34 60 92 11 41 114

The following chart shows the sequence in which requested tracks are serviced using FCFS.

Therefore, the total seek count is calculated as:

= (176-50)+(176-79)+(79-34)+(60-34)+(92-60)+(92-11)+(41-11)+(114-41) = 510

Implementation:
Implementation of FCFS is given below. Note that distance is used to store absolute distance between head and current track position.

C++ `

// C++ program to demonstrate // FCFS Disk Scheduling algorithm

#include <bits/stdc++.h> using namespace std;

int size = 8;

void FCFS(int arr[], int head) { int seek_count = 0; int distance, cur_track;

for (int i = 0; i < size; i++) {
    cur_track = arr[i];

    // calculate absolute distance
    distance = abs(cur_track - head);

    // increase the total count
    seek_count += distance;

    // accessed track is now new head
    head = cur_track;
}

cout << "Total number of seek operations = "
     << seek_count << endl;

// Seek sequence would be the same
// as request array sequence
cout << "Seek Sequence is" << endl;

for (int i = 0; i < size; i++) {
    cout << arr[i] << endl;
}

}

// Driver code int main() {

// request array
int arr[size] = { 176, 79, 34, 60, 92, 11, 41, 114 };
int head = 50;

FCFS(arr, head);

return 0;

}

C

// C++ program to demonstrate // FCFS Disk Scheduling algorithm #include <stdio.h> #include <math.h>

int size = 8;

void FCFS(int arr[],int head) { int seek_count = 0; int cur_track, distance;

for(int i=0;i<size;i++)
   {
    cur_track = arr[i];
  
      // calculate absolute distance
    distance = fabs(head - cur_track);
  
      // increase the total count
    seek_count += distance;
  
      // accessed track is now new head
    head = cur_track;
}

printf("Total number of seek operations: %d\n",seek_count);
  
  // Seek sequence would be the same
// as request array sequence
printf("Seek Sequence is\n");

  for (int i = 0; i < size; i++) {
    printf("%d\n",arr[i]);
}

}

//Driver code int main() { // request array int arr[8] = { 176, 79, 34, 60, 92, 11, 41, 114 }; int head = 50;

FCFS(arr,head);

return 0;

}

//This code is contributed by Pratham Kashyap

Java

// Java program to demonstrate // FCFS Disk Scheduling algorithm class GFG { static int size = 8;

static void FCFS(int arr[], int head) { int seek_count = 0; int distance, cur_track;

for (int i = 0; i < size; i++) 
{
    cur_track = arr[i];

    // calculate absolute distance
    distance = Math.abs(cur_track - head);

    // increase the total count
    seek_count += distance;

    // accessed track is now new head
    head = cur_track;
}

System.out.println("Total number of " + 
                   "seek operations = " + 
                    seek_count);

// Seek sequence would be the same
// as request array sequence
System.out.println("Seek Sequence is");

for (int i = 0; i < size; i++)
{
    System.out.println(arr[i]);
}

}

// Driver code public static void main(String[] args) { // request array int arr[] = { 176, 79, 34, 60, 92, 11, 41, 114 }; int head = 50;

FCFS(arr, head);

} }

// This code is contributed by 29AjayKumar

Python3

Python program to demonstrate

FCFS Disk Scheduling algorithm

size = 8;

def FCFS(arr, head):

seek_count = 0;
distance, cur_track = 0, 0;

for i in range(size):
    cur_track = arr[i];

    # calculate absolute distance
    distance = abs(cur_track - head);

    # increase the total count
    seek_count += distance;

    # accessed track is now new head
    head = cur_track;

print("Total number of seek operations = ", 
                               seek_count);

# Seek sequence would be the same
# as request array sequence
print("Seek Sequence is");

for i in range(size):
    print(arr[i]);

Driver code

if name == 'main':

# request array
arr = [ 176, 79, 34, 60,
        92, 11, 41, 114 ];
head = 50;

FCFS(arr, head);

This code contributed by Rajput-Ji

C#

// C# program to demonstrate // FCFS Disk Scheduling algorithm using System;

class GFG { static int size = 8;

static void FCFS(int []arr, int head) { int seek_count = 0; int distance, cur_track;

for (int i = 0; i < size; i++) 
{
    cur_track = arr[i];

    // calculate absolute distance
    distance = Math.Abs(cur_track - head);

    // increase the total count
    seek_count += distance;

    // accessed track is now new head
    head = cur_track;
}

Console.WriteLine("Total number of " + 
                "seek operations = " + 
                          seek_count);

// Seek sequence would be the same
// as request array sequence
Console.WriteLine("Seek Sequence is");

for (int i = 0; i < size; i++)
{
    Console.WriteLine(arr[i]);
}

}

// Driver code public static void Main(String[] args) { // request array int []arr = { 176, 79, 34, 60, 92, 11, 41, 114 }; int head = 50;

FCFS(arr, head);

} }

// This code is contributed by PrinciRaj1992

JavaScript

`

Output:

Total number of seek operations = 510 Seek Sequence is 176 79 34 60 92 11 41 114