Optimal Page Replacement Algorithm - C Program

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/** * Optimal Page Replacement Algorithm - C program * @author Mangilal Sharma * Initial Edit: November 27, 2011 for OS lab * Last Modified: October 27, 2016 for http://world-of-c-programming.blogspot.in/ * * Description: * A page, memory page, or virtual page is a fixed-length contiguous block of virtual memory. * A page frame is the smallest fixed-length contiguous block of physical memory into which memory pages are mapped by the os. * A transfer of pages between main memory and an virtual memory happens. * The optimal page replacement algorithm, also known as Look Forward Technique, simply says that the page with less urgency in future should be removed. * * Example: * Given number of frames: 3 * Given number of pages: 12 * Required pages access sequence: 7, 0, 1, 2, 0, 3, 0, 4, 2, 3, 0, 3 * 7 0 1 2 0 3 0 4 2 3 0 3 * Frame1 7 7 7 2 2 2 2 2 2 2 0 0 * Frame2 0 0 0 0 0 0 4 4 4 4 4 * Frame3 1 1 1 3 3 3 3 3 3 3 * Faults * * * * * * * * Number of page faults = 7 */ #include<stdio.h> #define MAX_PAGE 25 // Define a constant, program can support only this MAX_PAGE number of pages. #define MAX_FRAME 25 // Define a constant, program can support only this MAX_FRAME number of frames. /** * Function main returns 0 on success. */ int main() { // Declare arrays for pages and frames and distance_from_future_occurance int pages[MAX_PAGE], frames[MAX_FRAME], distances[MAX_PAGE]; // Number of pages, frames, page faults variables int numberPages, numberFrames, numberPageFaults = 0; // Other variables int i, j, temp, flag, found, lastFilledFrame, index, highestDistance; // Get number of frames from user printf("Enter number of frames (max limit is %d): ", MAX_FRAME); scanf("%d", &numberFrames); printf("You provided number of frames: %d\n", numberFrames); // Initialize frames array with -1 values and set lastFilledFrame value to -1 for(i = 0; i < numberFrames; i++) frames[i] = -1; lastFilledFrame = -1; // Get pages from user printf("Enter pages (enter -999 to exit, max number of pages limit is %d):\n", MAX_PAGE); numberPages = 0; for(i = 0; i < MAX_PAGE; i++) { scanf("%d", &temp); if(temp == -999 || numberPages == MAX_PAGE) break; pages[i] = temp; numberPages++; } printf("You provided number of pages: %d\n", numberPages); // Traverse the sequence of pages according Optimal Page Replacement Algorithm for(i = 0; i < numberPages; i++) // For every page { flag = 0; // Flag to show availability of page in frame // Get the availability of required page in frame for(j = 0; j < numberFrames; j++) // For every frame { if(frames[j] == pages[i]) // If page found in frame { flag = 1; // Set flag to 1, showing that page is available in frame printf("\t"); // Print tab space instead "FAULT" word break; // Break the loop } } // If page is not available in frame, replace some page by required page and print "FAULT" word if(flag == 0) { if (lastFilledFrame == numberFrames-1) // If frames fully filled { // For every frame containing page, calculate distances to future occurance for(j = 0; j < numberFrames; j++) { for(temp = i + 1; temp < numberPages; temp++) // For every future page { distances[j] = 0; // Set this page_occurances_in_future to zero, showing no occurance if (frames[j] == pages[temp]) // If frame containing page matches to future occuring page { distances[j] = temp - i; // Store distance break; } } // For every future page loop ends here } // Loop for calculating distances ends here // Choose best candidate index for page replacement in frame, this best candidate is not occuring in future found = 0; for(j = 0; j < numberFrames; j++) // For every frame { if(distances[j] == 0) // If page's distance value is 0, means if no occurance in future { // Or if frame value is -1, means empty frame index = j; // Set this frame index to index variable found = 1; // Set to 1, means a page not occuring in future is found break; } } } else // If frames has not fully filled, best candidate is blank frame { index = ++lastFilledFrame; // Set blank frame's index to index variable found = 1; // Set to 1, means a suitable frame index found } // If still not choosed best candidate, get best candidate that is having highest distance in future if(found == 0) { highestDistance = distances[0]; index = 0; for(j = 1; j<numberFrames; j++) // For every frame { if(highestDistance < distances[j]) { highestDistance = distances[j]; index = j; // In last, index will contain farest distanced element's index in current frames } } } // Do the page replacement frames[index] = pages[i]; // Replace the identified index located page by required page printf("FAULT\t"); // Print "FAULT" numberPageFaults++; // Increment number of page faults } // Print the pages that are present in current frames for(j = 0; j < numberFrames; j++) { if(frames[j] != -1) printf("%d\t", frames[j]); } printf("\n"); } // Print number of page faults. printf("Number of page faults = %d\n", numberPageFaults); return 0; }

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