////////////////////////////////////////////////////////////////////Operating System pratical 2 - Paging Simulation//Student:Chin Peng, TSANG ; ID:1101151//****************************************************************//Page replacement algorithm: extended second-chance and//                            clock algorithm//////////////////////////////////////////////////////////////////public class Paging{    Vector pageTable = new Vector();//store Page table entries    Vector accessList = new Vector();//store tasks    int[] counter = {0,0,0,0,0,0,0};//for summary report    int F=0,D=1,W=2,S1=3,S2=4,S3=5,S4=6;    //counter[faults][discards][writes][s1][s2][s3][s4]    int pageSize;//store given Page size    int numFrames;//Number of page frames    Pte currentPte;//current page entry    int pointer = 0;//Pointer position    Task task = null;    boolean READ = false;    boolean WRITE = true;    public static void main(String inputfile){	read_input(inputfile);	run();//run accessList	//report summary	System.out.println("faults "+counter[F]+			   "discards "+counter[D]+			   "writes "+counter[W]+			   "states "+counter[S1]+			   " "+counter[S2]+			   " "+counter[S3]+			   " "+counter[S4]);    }//end of main()    ///////////////////////////////////////    //read_input() reads input file    //store accesses into accessList    public void read_input(String file){    try{	FileReader fr = new FileReader(file);	StreamTokenizer st = new StreamTokenizer(fr);	st.nextToken();//skip "PageSize"	if(st.nextToken()==st.TT_NUMBER){	    pageSize = (int)st.nval;//read pageSize	    st.nextToken();//skip "MemorySize"	}	if(st.nextToken()==st.TT_NUMBER){	    numFrames = (int)st.nval;//read numFrames	}	//read operations	while(st.nextToken()!=st.TT_EOF){	    if(st.ttype==st.TT_WORD){		if(st.sval.equals("write")){		    if(st.nextToken()==st.TT_NUMBER)			task = new Task(WRITE,(int)st.nval);		}else{		    if(st.nextToken()==st.TT_NUMBER)			task = new Task(READ,(int)st.nval);		}	        //add task into accessList		accessList.add(task);	    }	}	fr.close();    }catch(IOException ioe){	System.out.println("File:"+file+" not found!");    }    }//end of read_input    ///////////////////////////////////////    //return Page table entry if found    //return null if not found    public Pte getPage(int address){	currentPte = null;	for(int i=0;i<numFrames;i++){	    currentPte = (Pte)pageTable.get(i);	    if(currentPte.addr==address){		return currentPte;	    }	}	return currentPte;    }//end of getPage    ///////////////////////////////////////    //run() runs submitted tasks    public run(){	while(accessList.size()>0){	    task = accessList.get(0);	    currentPte = getPage(task);	    if(currentPte==null){		counter[F] += 1;//page fault		System.out.println("fault "+task.memAddr);	//++++++++++++++++++++++++++++++        if(pageTable.size()<numFrames){	    read_disk();//read data from disk	}else{//elseif pageTable is full	    Pte currentPte=getVictim();//get a victim	    if(currentPte.mbit)//if modify bit = true		write_disk(currentPte);	    else{		discard(currentPte);	    }	    //read page from disk	    int pageNo=pointer*pageSize*numFrames;	    currentPte=new Pte(pageNo,true,task.write);	    read_disk(currentPte);	}//end if pageTable not full	//+++++++++++++++++++++++++++++	    }else{//elseif Page is accessed		currentPte.rbit = true;		currentPte.mbit = task.write;	    }	    accessList.remove(0);	}//end of while accessList not null    }//end of run()    ///////////////////////////////////////    //getVictim() gets lowest state victim    //implements extended second chance algorithm    public Pte getVictim(){	boolean found = false;	int modifiedPage = 0;	pointer = pageTable.indexOf(currentPte);	while(!found && modifiedPg<=numFrames){	    //implement clock algorithm	    if(pointer==(numFrames-1))		pointer = 0;	    else pointer++;	   	    Pte victim = (Pte)pageTable.get(pointer);	    switch(victim.state()){	    case 0: {		found = true;		currentPte = victim;		break;	    }	    case 1: {		currentPte = victim;		++modifiedPg; 		break;	    }	    default :{		victim.rbit = false;	    }	    }	}//end of while	return currentPte;    }    /////////////////////////////////////////////    public void discard(Pte victim){	counter[D] += 1;	pageTable.remove(victim);	System.out.println("discard "+victim.addr);    }    /////////////////////////////////////////////        public void write_disk(Pte victim){	counter[W] += 1;	pageTable.remove(victim);	System.out.println("write "+victim.addr);    }//end of write_disk    /////////////////////////////////////////////    public void read_disk(){	pageTable.add(currentPte);	System.out.println("read "+currentPte.addr);    }//end of read_disk}//end of class////////////////////////////////////////////////////////////////////Page table entry:mem_address,reference_bit,modify_bit//////////////////////////////////////////////////////////////////class Pte{    int addr;//page number = address of the first byte of page    boolean rbit;//reference bit    boolean mbit;//modify bit    public Pte(int address,boolean r,boolean m){	addr = address;	rbit = r;	mbit = m;    }        public int state(){	if(rbit){	    if(mbit) return 3;	    else return 2;	}else{	    if(mbit) return 1;	    else return 0;	}    }}////////////////////////////////////////////////////////////////////Task : operation , physical memory address//////////////////////////////////////////////////////////////////class Task{    boolean write;    int memAddr;    public Task(boolean operation,int address){	write = operation;	memAddr = address;    }}//end of class Task