Outline
TTIT61:
Process Programming and
Operating Systems
Alexandru Andrei
alean@ida.liu.se
phone: 282698, room: B 3D:439
„ Lab 2 : System Calls
„ Introduction to – and – making user programs in Nachos
„ User memory vs Kernel memory in Nachos
„ File & Console related syscalls
„ Synchronizing kernel functions and data structures
„ Lab 3 : Memory Management & System Calls
„ 3.1 Memory management with linear page tables
‰Multiprogramming - multiple user processes
‰Page tables
‰Process handling
„ 3.2 Memory management with software TLB
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System Calls
Four Components of a Computer System
#include <stdio.h>
#include <unistd.h>
#include <string.h>
#include <fcntl.h>
#include <sys/types.h>
/* System calls are written in bold italic. Type “man 2 sys_call_name” for
info in Linux or “man –s 2 sys_call_name” in Solaris */
int fd, n;
char buf[1024];
fd = open(“datafile.txt”, O_RDWR);
n = read(fd, buf, 1024);
printf(“Have read %d bytes: %s\n”, n, buf);
lseek(fd, 0, SEEK_SET);
write(fd, “Some other text”, strlen(“Some other text”) + 1);
write(fd, buf, n);
close(fd);
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User Programs
System Call Execution
„ Located in the Nachos directory: code/test/
„ Compiled with cross-compiler to MIPS machine code
(see code/test/Makefile)
„ MIPS is a RISC architecture, with delayed loads
„ Cross compile – compile in one machine for a
different target machine
„ Emulation – runs on MIPS simulator
„ Class Machine emulates the MIPS processor
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User Program - Example
User Programs (cont.)
„ Simple C programs (any C program that doesn’t use
library functions like printf)
„ In nachos-3.4/code/test
„ For a new test program, add it to the Makefile from
nachos-3.4/code/test
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Test Program Makefile
ƒIn code/testprogram/Makefile
ƒUse copy paste to add another testprogram
#include ”syscall.h”
void main(void) {
int file_id;
char a[10];
for (i=0;i<=8;i++) a[i]=’a’;
a[9]=’\0’;
Create(a);
file_id = Open(a);
Write(”some text”, 10, file_id);
Close(file_id);
Halt();
}
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The User Space Definition of the Syscall Functions
.globl Create
.ent
addiu $2,$0,SC_Create
syscall
j
$31
.end Create
start.o: start.s ../userprog/syscall.h
$(CPP) $(CPPFLAGS) start.s > strt.s
$(AS) $(ASFLAGS) -o start.o strt.s
rm strt.s
TAB
(not spaces)
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Create
Create:
all: halt shell matmult sort
halt.o: halt.c
$(CC) $(CFLAGS) -c halt.c
halt: halt.o start.o
$(LD) $(LDFLAGS) start.o halt.o -o halt.coff
../bin/coff2noff halt.coff halt
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code/test/start.s
.globl Open
.ent
Open
Open:
addiu $2,$0,SC_Open
syscall
j
$31
.end Open
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Program Execution
Under the Hood
ƒThe MIPS processor fetches instructions from memory
and executes them one by one
void Machine::Run() { //
code/machine/mipssim.cc
Instruction *instr = new Instruction;
„ How does Nachos execute the binary code
„ How does Nachos execute system calls
interrupt->setStatus(UserMode);
for (;;) {
OneInstruction(instr);
interrupt->OneTick();
}
}
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2
OneInstruction() (cont.)
OneInstruction()
void Machine::OneInstruction(Instruction *instr){
int raw;
int nextLoadReg = 0;
int nextLoadValue = 0;
if (!machine->ReadMem(registers[PCReg],4,&raw))
return;
// exception occurred
instr->value = raw;
instr->Decode();
pcAfter = registers[NextPCReg] + 4;
switch (instr->opCode) { Ænextslide
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switch
case
case
case
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(instr->opCode) {
OP_ADD: …
OP_DIV: …
OP_SYSCALL:
RaiseException(SyscallException, 0);
return;
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Exception Handling: System Calls
RaiseException()
The processor has detected an exception and passes the
control to the operating system for exception handling
void
Machine::RaiseException(//code/machine/machine.cc
ExceptionType which, int badVAddr){
DEBUG('m', "Exception: %s\n", exceptionNames[which]);
registers[BadVAddrReg] = badVAddr;
DelayedLoad(0, 0);
// finish anything in progress
interrupt->setStatus(SystemMode);
ExceptionHandler(which);
//interrupts are enabled at this point
interrupt->setStatus(UserMode);
// code/userprog/exception.cc
void ExceptionHandler(ExceptionType which) {
int type = machine->ReadRegister(2);
if ((which == SyscallException)&&(type == SC_Halt)){
DEBUG('a', "Shutdown, initiated by user program.\n");
interrupt->Halt();
}
else
printf("Unexpected user mode exception %d %d\n",which,type);
}
}
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Exception Handling: System Calls
void ExceptionHandler(ExceptionType which) {
int type = machine->ReadRegister(2);
if ((which == SyscallException)) {
switch (type) {
case SC_Halt: {
{
break;
break;
break;
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TTIT61 Lab lesson
Exception Handling: System Calls
„ The list of the system call ids (the value from register 2)
is in the file code/userprog/syscall.h
DEBUG('a', "Shutdown, initiated by user program.\n");
interrupt->Halt();
}
case SC_Create:
//your code;
}
case SC_Open: {
//your code;
}
case SC_Read: {
//your code;
}
….
} //end of switch
}//end if
}
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„ For example:
#define SC_Halt
#define SC_Exit
#define SC_Exec
#define SC_Join
#define SC_Create
#define SC_Open
#define SC_Read
#define SC_Write
#define SC_Length
#define SC_Close
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Exception Handling: System Calls
„ ”syscall” machine code instruction
„ The system call id is stored in register 2
„ The arguments of the syscall are stored in the registers,
starting with register 4
„ arg1 -- r4, arg2 -- r5, etc.
„ System calls return values in reg. r2
„ see …/code/test/start.s
„ ExceptionHandler
„ see …/code/userprog/exception.cc
‰SC_Halt already implemented
„ The list of the system calls is in the file
code/userprog/syscall.h
void Write(char *buffer, int size, OpenFileId id);
int Read(char *buffer, int size, OpenFileId id);
void Close(int fileid);
OpenFileId Open(char *filename);
void Exit(int status);
SpaceId Exec(char *name);
int Join(SpaceId id);
void Halt();
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System Calls and Exception Handling
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Exception Handling: System Calls
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Two Types of Arguments
void ExceptionHandler(ExceptionType which) {
…
switch (type) {
case SC_Create:
//read from register 4 the argument
//use machine->ReadRegister(4);
//create the file
//does not return any value
//if it would return, it should write
//the return value in register 2
break;
}
…
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„ Arguments passed by value
‰void Exit(int status);
‰case SC_EXIT:
int status=machine->ReadRegister(4);
„ Arguments passed by address (pointers)
‰void Create(char *filename);
‰case SC_EXIT:
int address_of_pointer=machine->ReadRegister(4);
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User Programs&User Memory
Process1:
char buffer[10]
Create(buffer);
Process2:
char buffer[10]
Open(buffer);
0000
0004
0008 0000
FFFF
Kernel
Process 1
AAA0 0000
AAA4
FFFF
Process 2
FFFF
Memory
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Important !
„ User space vs. Kernel Space
„ User address vs. Kernel Address
„ Pointer arguments passed to system calls must be
translated to/from user space
„ Example:
„ Create(char* filename) -> UserToKernel
„ Read(char* buf, int size, OpenFileId id) -> KernelToUser
Virtual memory vs. Phisycal memory
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UserToKernel
void UserToKernel(void) {
//buffer allocated in kernel space; we copy here
//the content of buffer from the user program
char* filename = new char[100];
//address
int address = machine->ReadRegister(4);
i=0;
for (;;) {
int data;
machine->ReadMem(address + i, 1, &data);
filename[i] = data;
if (filename[i]==’\0’) break;
i++;
}
}
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machine->ReadMem
ƒmachine->ReadMem(address ,n_of_bytes, &data)
ƒTranslates internally the virtual address address to a
physical address, reads n_of_bytes from that address
and stores them in the variable data
ƒmachine->WriteMem also available in Nachos
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OpenFile
FileSystem
class FileSystem {
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// code/filesys/filesys.h
class OpenFile {
public:
FileSystem(bool format) {}
// code/filesys/openfile.h
public:
OpenFile(int f) { file = f; currentOffset = 0; }
bool Create(char *name, int initialSize) {
~OpenFile() { Close(file); }
int fileDescriptor = OpenForWrite(name);
if (fileDescriptor == -1) return FALSE;
close(fileDescriptor);
return TRUE;
int Read(char *into, int numBytes) {
…
}
}
OpenFile* Open(char *name) {
int Write(char *from, int numBytes) {
int fileDescriptor = OpenForReadWrite(name, FALSE);
…
}
……
if (fileDescriptor == -1) return NULL;
return new OpenFile(fileDescriptor);
}
}
bool Remove(char *name) { return Unlink(name) == 0; }
};
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Exception Handling: System Calls
TTIT61 Lab lesson
Assignment 2
void ExceptionHandler(ExceptionType which) {
…
switch (type) {
case SC_Create:
char k_filename[20];
UserToKernel(k_filename);
fileSystem->Create(k_filename);
break;
}
…
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„ Implement and test the system calls related to file &
console operations
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Why OpenFileID ?
System Calls for File Operations
„ File operations (lab 2)
„ void Create(char* filename)
‰Create an empty file
„ OpenFileId Open(char *filename)
‰open file for read/write
„ int Read(char* buf, int size, OpenFileId id)
‰read from an open file
„ void Write(char* buf, int size, OpenFileId id)
‰write to an open file
„ void Close(int fileid)
‰Close an open file
„ See code/userprog/syscall.h for the list
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#include ”syscall.h”
void main(void) {
int file_id;
char a[10];
for (i=0;i<=8;i++) a[i]=’a’;
a[9]=’\0’;
Create(a);
//a is the name of the file
file_id = Open(a); //a is the name of the file
Write(”some text”, 10, file_id); //file_id is the OpenFileId
Close(file_id);
//file_id is the OpenFileId
Halt();
}
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File Related System Calls
„ OpenFileId (an integer)
„ Global unique id for each open file
„ ConsoleInput (o_id==0) and ConsoleOutput (o_id==1)
„ The operating system keeps a list of open files
„ Read and Write on an open file or on the Console must
be choosen based on the OpenFileId
‰if (o_id>=2) then file; else console;
„ The actual file operations are handled by the FileSystem
and OpenFile
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List of Open Files
„ Implement in the kernel a list of files that are open
„ For each open file, store in the list:
„ OpenFileId (this could be the index of your list)
„ OpenFile pointer
„ Pointer to the thread that opened the file
(currentThread)
„ ...
„ Open and Close system calls append and remove files
from this list
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Console
Console Internals
ƒ Console class constructor:
„ The Console class handles reading/writing from/to the
console at the char level
„ One object with type Console must be used throughout
Nachos
„ void PutChar(char ch)
„ char GetChar()
„ TODO: introduce the possibility to read/write strings
(char* or char[])
„ ie. synchronize the console operations
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Console(
char *readFile, char *writeFile,
VoidFunctionPtr readAvail,
VoidFunctionPtr writeDone,
int callArg
);
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Console Internals
„ void ReadAvail(int arg) {…}
„ void WriteDone(int arg) {…}
„ console = new
Console(NULL,NULL,ReadAvail,WriteDone,0);
„ An example of a console implementation is given in:
code/userprog/progtest.cc
„ Hint: use two semaphores to signal the availability of
an input character (for Read) and the finishing of a
char writing (for Write)
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TODO: System Calls for File Operations (Lab2)
„ File operations (lab 2)
„ void Create(char* filename)
„ Create an empty file
„ OpenFileId Open(char *filename)
„ open file for read/write
„ int Read(char* buf, int size, OpenFileId id)
„ void Write(char* buf, int size, OpenFileId id)
„ void Close(int fileid)
„ See …/code/userprog/syscall.h for details
„ Compile and run nachos always in code/userprog
directory
„ Test programs implemented and compiled in code/test
directory
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Nachos Syscalls Info
„ Read ”Road Map through Nachos”
„ Machine: section 2.1 – 2.4
„ Examine exception handling in
‰machine/mipssim.cc (RaiseException)
‰machine.cc (Machine::RaiseException)
‰Userprog/exception.cc (ExceptionHandler)
„ System calls ”prototypes” in syscall.h
„ FileSystem and OpenFile: 5.2 – 5.3
„ User Level Processes: Section 4
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Nachos Syscalls Info (cont)
„ Source files:
„ Working dir. is ../code/userprog
„ Central files for assignment
‰”Exception.cc” contains ExceptionHandler function
‰syscall.h - definitions and constants for system calls
„ Files necessary for understanding how Nachos works
‰Progtest.cc – test routines, how to load & execute
user prog.
‰Example of running Nachos with a user program:
¾nachos –x <user_program_name>
¾E.g.:
../userprog/nachos –x ../test/halt
‰/machine/machine.* - how MIPS emulator works
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„ In code/userprog: addrspace.h, addrspace.cc
‰data structures to keep track of executing user programs
(address spaces)
„ In code/machine:
‰translate.cc
¾data structures for managing translation from virtual
page # -> physical page #
¾used for managing memory on behalf of user programs
‰mipssim.cc (ReadMem, Writemem, Translate, and
OneInstruction)
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Lab 3
Nachos Syscalls Info (cont.)
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Memory Management
Linear Page Table
TLB
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Lab 3 - Memory Management
User Programs
„ Multiprogramming
„ Several processes (user programs) reside in memory at
the same time
„ Exec
„ Exit
„ Join
„ Reside in user address space
„ Communication with the kernel through system calls
(open file, read, write, start a new process, etc.)
„ Only one user program in the lab #2
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Example 1
/*test2.c*/
#include ”syscall.h”
void main(void) {
Write(”test2”,5,1);
Exit(1);
}
/*test1.c*/
#include ”syscall.h”
void main(void) {
Write(”test1”,5,1);
Exec(”test2”);
Exit(1);
}
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Example 2
/*test1.c*/
#include ”syscall.h”
void main(void) {
int x,s;
Write(”test1”,5,1);
x=Exec(”test2”);
s=Join(x;)
Write(”test2 exited with
status”,s,1);
Exit(1);
}
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/*test2.c*/
#include ”syscall.h”
void main(void) {
Write(”test2”,5,1);
Exit(2);
}
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Lab 3
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TTIT61 Lab lesson
User Level Processes
„ When starting a user program
„ Create an address space (memory for the process)
„ Copy the content of the instructions’ and initialized
variable segments into the address space.
„ Associate a new kernel thread to the process
„ Uninitialized variable section are not read from the file (as
it contains all 0’s).
Memory Management
Linear Page Table
TLB
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Memory Organization
Lab 3 - Page tables in lab 3
machine.h:
#define PageSize
SectorSize
#define NumPhysPages 32 //CHANGE IT !
#define MemorySize
(NumPhysPages * PageSize)
Memory
0
Process1
2
Pages
•Page number
•Same size
•Allocation unit for
a process
3
4
5
Physical
Memory
0
1
2
3
1
Virtual Memory
(virtual pages)
?
Process2
0
1
2
0
1
2
3
4
5
6
7
Physical
pages
ƒmachine.h contains the details regarding the memory size,
number of pages, page size, etc.
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Lab 3 - Page tables in lab 3
Process1
0
1
2
3
pgTable1
0
1
2
3
0
1
2
3
4
5
6
7
pgTable2
Process2
0
1
2
0
1
2
Physical
Memory
4
1
0
3
2
5
6
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Lab 3 - Page tables in lab 3
ƒProcess1 is running now
ƒThe machine page table points to the page table of Process1
Process1
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ƒProcess2 is running now
ƒThe machine page table points to the page table of Process2
Process1
pgTable1
0
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2
3
0
1
2
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Physical
Memory
pgTable
0
1
2
pgTable2
Process2
0
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2
4
1
0
3
2
5
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2
5
6
0
1
2
3
4
5
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7
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4
1
0
3
pgTable2
Process2
Physical
Memory
pgTable
0
1
2
3
4
1
0
3
2
5
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Lab 3 - Memory Management
Lab 3 - Page tables in lab 3
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pgTable1
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3
0
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„ Multiprogramming
„ Several processes (user programs) reside in memory at the same
time
„ machine->pageTable contains the pageTable of the current
thread’s (process) address space
„ One address space (class AddrSpace) object for each process
„ Different page table for each process
„ currentThread->space points to the current AddrSpace
„ A pageTable inside the AddrSpace object: translation of
virtual address to physical address
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AddrSpace::RestoreState()
void AddrSpace::RestoreState()
{
machine->pageTable = pageTable;
machine->pageTableSize = numPages;
}
„ Called by Nachos at every context switch
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AddrSpace
AddrSpace::AddrSpace(OpenFile *executable, SpaceId id) {
NoffHeader noffH;
unsigned int size;
//reading the noffH.code.size,
//noffH.initData.size//noffH.uninitData.size from the Nachos
executable file
// how big is address space?
size = noffH.code.size + noffH.initData.size + noffH.uninitData.size
+ UserStackSize;
// we need to increase the size
// to leave room for the stack
numPages = divRoundUp(size, PageSize);
size = numPages * PageSize;
ASSERT(numPages <= NumPhysPages);
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AddrSpace
AddrSpace::AddrSpace(OpenFile *executable, SpaceId id) {
...
pageTable = new TranslationEntry[numPages];
for (i = 0; i < numPages; i++) {
// for now, virtual page = phys page
pageTable[i].virtualPage = i;
pageTable[i].physicalPage = i;
pageTable[i].valid = TRUE;
pageTable[i].use = FALSE;
pageTable[i].dirty = FALSE;
pageTable[i].readOnly = FALSE;
}
....
(continues on the next slide)
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Allocating the Address Space
„ Address spaces (AddrSpace object)
„ AddrSpace constructor allocates memory pages
„ In lab 2 all memory is assigned to one address space
„ In lab 3 several address spaces will exist
„ Use a BitMap object to keep track of free pages
‰BitMap(int nitems)// total number of items
‰Mark
// mark (allocate) a position
‰Clear
// clear (free) a position
‰Test
// check if a position is marked
‰Find
// find a free position and mark it
„ bitmap.h, bitmap.cc
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AddrSpace
AddrSpace::AddrSpace(OpenFile *executable, SpaceId id) {
...
pageTable = new TranslationEntry[numPages];
for (i = 0; i < numPages; i++) {
//now, virtual page != phys page !!!
pageTable[i].virtualPage = i;
pageTable[i].physicalPage = xxx //USE THE BITMAP
pageTable[i].valid = TRUE;
pageTable[i].use = FALSE;
pageTable[i].dirty = FALSE;
pageTable[i].readOnly = FALSE;
}
....
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At This Point ...
...you have reserved the required memory space for each process
Process1
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3
pgTable1
0
1
2
3
0
1
2
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7
pgTable2
Process2
0
1
2
Physical
Memory
4
1
0
3
2
5
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Now, Load the Binary Code
AddrSpace
AddrSpace::AddrSpace(OpenFile *executable, SpaceId id) {
...
pageTable = new TranslationEntry[numPages];
...
//CHANGE HERE
if (noffH.code.size > 0) {
executable->ReadAt(&(machine->mainMemory[noffH.code.virtualAddr]),
noffH.code.size, noffH.code.inFileAddr);
}
if (noffH.initData.size > 0) {
executable->ReadAt(&(machine>mainMemory[noffH.initData.virtualAddr]),
noffH.initData.size, noffH.initData.inFileAddr);
}
}//done AddrSpace
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Lab 3 - Address spaces
Process Creation (Exec)
„ Nachos process are formed by:
„ Create a new thread
„ Creating an address space associated to the thread
‰Allocating physical memory for the AddrSpace
‰Load content of executable into the physical
memory
„ Initialize registers and address translation tables
„ Invoke machine::Run() to start executing.
„ machine::Run() turns on the simulated MIPS machine
that enters into an infinite loop, executing one
instruction at a time.
„ progtest.cc to see an example
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„ AddrSpace constructor
„ Takes an executable (filename)
„ Allocates memory, sets up page table
‰Amount of memory needed is stored in noff-header
„ Loads the executable
„ Noff binary format, noff header
„ NoffHeader object stored in the beginning of the file
„ Three Segment objects in the header
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Noff Executable Format
„ Noff: Nachos Object File Format
„ Compare with coff(Unix), com, exe (DOS)
„ Noff consists of four parts
‰Noff header (information concerning segments
below)
‰Code segment – program residence
‰Initialized variables segment
‰Unitialized variables segment
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Noff Format (cont)
„ Noff: header
‰information concerning segments below
‰Describe content of the rest of the file, giving information
about programs’s instructions, initialized and uninitialized
variables
„ noffMagic: Noff format check (4 bytes)
‰Reserved number indicating that the file is in Noff format
„ For each of the remaining sections Nachos maintains
‰virtualAddr: segment’s starting address in virtual memory
‰inFileAddr: start of the segment in Noff file
‰Size: size of the segment
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Lab 3 - Address Space
Address Spaces
„ Segments:
„ code
‰the executable machine code
„ initData
‰initialized data (constant strings etc)
„ uninitData
‰Declared but unassigned data (empty arrays etc).
‰Not stored in the file.
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„ The Segment object:
„ virtualAddr
‰start address of segment in the address space
„ inFileAddr
‰offset of segment in the file
„ size
‰size of the segment
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Memory Management 1: Linear Page Tables
„ TODO:
„ Allocate/Deallocate physical pages (addrspace)
„ Setting translation tables
„ Implement Exec, Exit (and optionally Join)
„ Tests:
„ Simultaneously running user programs
„ Using pre-emptive switches between threads
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TODO: Lab 3 System Calls for Exit & Join
„ Implement at least on the following system calls
„ int Join(SpaceId id)
‰Wait for the process with id id to exit
‰Return the exit status of that process
„ void Exit(int status)
‰exits the calling user process
‰returns ”status” to the parent process
„ (Select 2 syscalls between Exec, Exit, Join)
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Lab 3
Translation Look-Aside Buffer
frame
page
offset
CPU
frame offset
TLB
Memory Management
Linear Page Table
TLB
TLB
hit
TLB
miss
page
{
Memory
Page table
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TLB
RestoreState: Invalidate the TLB
void AddrSpace::RestoreState()
{
#ifdef USE_TLB
//machine->pageTable = pageTable;
//machine->pageTableSize = numPages;
„ Is a hardware device
„ Is an associative memory, i.e. it is not addressed by
address but by data
„ Instead of “show me the house at number 10”, you say
“show me the house where the Simpson’s live”, i.e.
instead of “give the 5th entry in the TLB”, you say “give
me the entry in the TLB that corresponds to virtual
page X”
int i;
printf("using TLB\n");
for (i=0;i<TLBSize;i++) {
machine->tlb[i].valid=FALSE;
}
„ What happens to the context of the TLB upon a
context switch? Invalidate all its entries !
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#endif
}
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Exception Handling: TLB Fault
TLB
„ Upon a TLB miss, a new page table entry is copied in the
TLB
„ If all TLB entries are occupied, one of them must be
replaced
„ Replacement algorithms: Random, Least Recently Used
(LRU), Round-Robin, etc.
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// code/userprog/exception.cc
void ExceptionHandler(ExceptionType which) {
int type = machine->ReadRegister(2);
if ((which == SyscallException)&&(type == SC_Halt)){
DEBUG('a', "Shutdown, initiated by user program.\n");
interrupt->Halt();
}
if (which == PageFaultException){
//Page fault handling
}
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Handling a Page Fault
„ Bring the faulty page in the TLB
„ If there is a free entry in the TLB, bring the faulty page in
that entry (free TLB entry: machine->tlb[i].valid==FALSE)
machine->tlb[i].physicalPage=currentThread->space>pageTable[faulty_page].physicalPage;
machine->tlb[i].virtualPage=currentThread->space>pageTable[faulty_page].virtuallPage;
„ Otherwise select from the TLB an entry and replace it with
the faulty page
„ Make sure that you will not have two consecutinve TLB
misses resulting from the same page !
„ Round robin TLB page replacement ?
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ReadMem & WriteMem
„ ReadMem and WriteMem return TRUE if the operation
succeeded and FALSE otherwise
„ In case of a page fault when doing ReadMem or
WriteMem inside a system call, you need to call again
Read/WriteMem
#ifdef USE_TLB
while (machine->ReadMem(userAddr, 1, &c)==FALSE);
userAddr++;
#else
machine->ReadMem(userAddr++, 1, &c);
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code/vm directory
„ Compile and run nachos from the code/vm directory !
„ Test the TLB implementation by running the same tests
you did for the linear page table
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