C-SCAN Algorithm - C Program by Nived Kannada |
C SCAN algorithm is actually a slightly modified version of SCAN algorithm which are both used for disk scheduling in the operating system.
There is only a few changes from the SCAN algorithm.
If you haven't read our article on the SCAN algorithm, please read and understand the logic of it before reading this post.
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Anyway, the logic is pretty simple,
- Array queue[] is our main array
- Array queue1[] and queue2[] are used to store the requested positions greater than initial head position and less than initial head position respectively.
- Variables temp1 and temp2 are used as the index variables of queue1[] and queue2[] respectively.
- The main difference from SCAN algorithm is that in SCAN algorithm, we sort the first array queue1[] in the ascending order and the second array queue2[] in the descending order, but in C-SCAN algorithm, we sort both of them in their ascending order.
- One slight difference comes in the steps when copying arrays queue1[] and queue2[] to the main array queue[]
- After copying first array queue1[] to queue[], we have to set maxrange position as the next request position. ie. We set queue[i]=maxrange.
- After that, since it is Circular scan, after reaching the outermost cylinder/track, the head moves to the innermost track/cylinder, ie. the position 0. So, after pushing maxrange, increment i and put the value 0 in there.
- We have to copy the next array queue2[] after doing these.
- You can understand the rest of the logic from the comments given on the code itself.
- Please go through the comments and understand them.
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// C Program to Simulate C-SCAN Disk Scheduling Algorithm // Visit www.nanogalaxy.org for more programs. #include<stdio.h> int absoluteValue(int); // Declaring function absoluteValue void main() { int queue[25],n,headposition,i,j,k,seek=0, maxrange, difference,temp,queue1[20],queue2[20],temp1=0,temp2=0; float averageSeekTime; // Reading the maximum Range of the Disk. printf("Enter the maximum range of Disk: "); scanf("%d",&maxrange); // Reading the number of Queue Requests(Disk access requests) printf("Enter the number of queue requests: "); scanf("%d",&n); // Reading the initial head position.(ie. the starting point of execution) printf("Enter the initial head position: "); scanf("%d",&headposition); // Reading disk positions to be read in the order of arrival printf("Enter the disk positions to be read(queue): "); for(i=1;i<=n;i++) // Note that i varies from 1 to n instead of 0 to n-1 { scanf("%d",&temp); //Reading position value to a temporary variable //Now if the requested position is greater than current headposition, //then pushing that to array queue1 if(temp>headposition) { queue1[temp1]=temp; //temp1 is the index variable of queue1[] temp1++; //incrementing temp1 } else //else if temp < current headposition,then push to array queue2[] { queue2[temp2]=temp; //temp2 is the index variable of queue2[] temp2++; } } //Now we have to sort the two arrays //SORTING array queue1[] in ascending order for(i=0;i<temp1-1;i++) { for(j=i+1;j<temp1;j++) { if(queue1[i]>queue1[j]) { temp=queue1[i]; queue1[i]=queue1[j]; queue1[j]=temp; } } } //SORTING array queue2[] in ascending order for(i=0;i<temp2-1;i++) { for(j=i+1;j<temp2;j++) { if(queue2[i]>queue2[j]) { temp=queue2[i]; queue2[i]=queue2[j]; queue2[j]=temp; } } } //Copying first array queue1[] into queue[] for(i=1,j=0;j<temp1;i++,j++) { queue[i]=queue1[j]; } //Setting queue[i] to maxrange because the head goes to //end of disk and comes back in scan Algorithm queue[i]=maxrange; //Moving Disk head to the inner most cylinder, because this is Circular Scan. queue[i+1]=0; //Copying second array queue2[] after that first one is copied, into queue[] for(i=temp1+3,j=0;j<temp2;i++,j++) { queue[i]=queue2[j]; } //At this point, we have the queue[] with the requests in the //correct order of execution as per C-SCAN algorithm. //Now we have to set 0th index of queue[] to be the initial headposition. queue[0]=headposition; // Calculating SEEK TIME. seek is initially set to 0 in the declaration part. for(j=0; j<=n+1; j++) //Loop starts from headposition. (ie. 0th index of queue) { // Finding the difference between next position and current position. difference = absoluteValue(queue[j+1]-queue[j]); // Adding difference to the current seek time value seek = seek + difference; // Displaying a message to show the movement of disk head printf("Disk head moves from position %d to %d with Seek %d \n", queue[j], queue[j+1], difference); } // Calculating Average Seek time averageSeekTime = seek/(float)n; //Display Total and Average Seek Time(s) printf("Total Seek Time= %d\n", seek); printf("Average Seek Time= %f\n", averageSeekTime); } // Defining function absoluteValue int absoluteValue(int x) { if(x>0) { return x; } else { return x*-1; } } // END OF CODE //Code by Nived Kannada
Output
Sample Output of the program
If you have any doubts regarding this program, please let us know in the comments section below.Happy coding!
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