vm.c

//command line arguments "vm <file>” 

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
#define SIZE 256

int main(int argc, char *argv[])
{

//	VARIABLE DECLARATIONS
	int addressFile,backingStore; 				//	file descriptors
	char *file= argv[1];	

	char ch,ct, input[1000], output;
	int logicalAddress, physicalAddress;	
	int i=0, j=0;


//	LOGICAL MEMORY
	int p;										//	page-number: used as an index into a page table 	
	int d; 										//	page-offset	

//	PHYSICAL MEMORY
	int f; 										//	frame-number: base address of each page in physical memory
	char frames[SIZE*SIZE]; 					//  physical memory where frame 0 is from frames[0] to frames[255]
	int frametable[SIZE];						// 	one free (-1) or allocated (0) flag entry for every frame 0 to 256
	int start, current;							//	var to handle page faults
	int offset, pagefault=0;	
	int freeFrame=-1;	


//	PAGE TABLE
	int pagetable[SIZE]; 						//	page table where table[p] = f	
	for (j=0;j<SIZE;j++)						//	flushing page table, frametable
	{
		pagetable[j] = -1;
		frametable[j] = -1;
	}	

//	READING LOGICAL ADDRESS FROM FILE
	
	addressFile = open(file,O_RDONLY); 
	backingStore = open("BACKING_STORE.txt",O_RDONLY); 

	if(addressFile != -1)
	{
		while(read(addressFile, &ch, sizeof(char)) != 0)	//  read() returns the number of bytes read and 0 for end of file (EOF) 
		{
			if(ch != '\n')
			{
				input[i] = ch;
				i++;
			}
			else
			{
				logicalAddress =atoi(input);
//				EXTRACT PAGE NUMBER AND OFFSET
				p = (logicalAddress & 0x0000ff00UL) >>  8;
				d = (logicalAddress & 0x000000ffUL);

				printf("\nlogicalAddress: %d, p: %d, d: %d", logicalAddress,p,d);

//				ADDRESS TRANSLATION THROUGH PAGE TABLE

//				pagetable-hit, obtain frame number	
				if(pagetable[p]!= -1){

					f = pagetable[p];
					physicalAddress = (f * SIZE) + d;
					printf("\nphysicalAddress: %d, f: %d", physicalAddress,f);

				}
//				pagetable-miss, page-fault	
				else
				{
					pagefault++;
//					locate free frame (-1) in physical memory 
					for (j=0;j<SIZE;j++)
					{
						if(frametable[j]==-1)
						{
							freeFrame = j;
							break;	
						}
					}	
//					read page from backing-store into the available frame in the physical memory 					
					if(backingStore != -1)
					{						
						
						offset=0;
						start = SIZE * p;
						current=lseek(backingStore, start, SEEK_SET);
						while((offset < SIZE)&&(current))
						{
							current = read(backingStore, &ct, sizeof(char));
							frames[freeFrame*offset] = ct; 
							offset++;														
						}
					}
					else
					{
						printf("Backing-Store Does not exist!");
						close(backingStore);
						close(addressFile);	
						return 0;	
					}
//					update pagetable, frametable
					pagetable[p] = freeFrame;	
					frametable[freeFrame] = 0;

					physicalAddress = (freeFrame * SIZE) + d;
					printf("\nphysicalAddress: %d, freeFrame: %d", physicalAddress, freeFrame);
				}

// 				READ CHAR STORED AT THE PHYSICAL ADDRESS
				output = frames[physicalAddress];
				printf("\nByte value stored at physicalAddress %d: %c\n",physicalAddress, output);
				
				memset(input,0,sizeof(input));
				i=0;

			}
		}
		printf("\nTotal Page Faults: %d",pagefault);		
	}
	else
		printf("Addresses File Does not exist!");

	close(backingStore);
	close(addressFile);	
	return 0;	
}