Nachos Instructional OS
CS 270, Tao Yang, Spring 2011
What is Nachos OS?
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Allow students to examine, modify and execute
operating system software.
A skeletal OS that supports
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kernel threads
user-level processes
Simulates MIPS instruction execution.
Running on a virtual machine, executed as a single
Unix process in a hosting OS.
Over 9K lines of C++ code.
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Can understand its basic mechanisms by reading about 12K lines of code.
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System Layers
User process
Thread 1
User process
Nachos kernel threads
Thread 2
Thread N
Nachos OS modules
(Threads mgm, File System, Code execution/memory mapping,
System calls/Interrupt)
Simulated MIPS Machine
(CPU, Memory, Disk, Console)
Base Operating System
(Linux for our class)
Steps to Install Nachos
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Obtain and install Nachos source code.
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Compile the source code using gmake
Run threads demo under the threads
subdirectory (just run kernel test threads).
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Copy the source code from
~cs270t/nachosSept20.tar.gz
Run user program demo under the userprog
subdirectory.
http://www.cs.ucsb.edu/~cs270t/HW/warmu
p.html
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Nachos code directory
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machine --- Basic machine specification (MIPS
simulator).
threads --- threads management (HW1).
userprog -- binary code execution and system calls
(HW2).
vm -- virtual memory (HW3).
filesys -- file system (HW3)
test -- binary test code
network -- networking protocol
bin -- utilities/tools (binary format conversion)
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Source code reading and HW1
Objectives of next 2 weeks
 Scan through ~1,000-2,000 lines of code
under threads directory
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Learn how context switch is accomplished among
threads.
Learn how thread scheduling is done.
Learn how locks/synchronization are implemented
and used.
Complete HW1
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Few hundred lines of code
Sample solution for Task 1, 2, &3 are available
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Single-threaded vs multithreaded
Process
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Sample Example of Nacho Threads
main ()
{
Thread *t1 = new Thread("forked thread1");
Thread *t2 = new Thread("forked thread2");
t1->Fork(SimpleThread, 1);
t2->Fork(SimpleThread, 2);
Create 2 new threads.
Start to fork and execute a
function in each child thread.
Parent also
executes the same
function
SimpleThread(3);
}
SimpleThread(int i)
{
printf(“Hello %d\n”, i);
currentThread->Yield();
}
Function executed by
threads
Nachos Threads
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Nachos threads execute and share the same code, share same global
variables.
Nachos scheduler maintains a ready list, containing all threads that
are ready to execute.
Each thread is in one of four states: READY, RUNNING, BLOCKED,
JUST_CREATED.
Each thread object maintains a context block.
Thread object supports the following operations:
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Thread(char *debugName). Create a thread.
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Fork(VoidFunctionPtr func, int arg). Let a thread execute a
function.
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Yield(). Suspend the calling thread and select a new one for
execution.
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Sleep(). Suspend the current thread, change its state to
BLOCKED, and remove it from the ready list
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Finish()
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Nachos Thread States and
Transitions
running
(user)
In HW 1 we are only
concerned with the
states in this box.
Machine::Run,
interrupt or ExceptionHandler
exception
When running in user
mode, the thread
executes within the
machine simulator. HW
2 covers this.
Thread::Yield
running
Thread::Sleep
blocked
(kernel)
Scheduler::Run
Scheduler::ReadyToRun
ready
Thread Switching
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Switching involves suspending current thread, saving
its state, and restoring the state of new thread.
Following code involved in execution: the old code,
the new code, and the code that performs switching.
Switch(oldThread, newThread):
 Save all registers in oldThread's context block.
 Save the program address to be used when the
old thread is resumed.
 Load new values into the registers from the
context block of the new thread.
 Once the saved PC of the new thread is loaded,
Switch() is no longer executing.
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Scheduler object for thread
scheduling
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A scheduler decides which thread to run next by scanning the
ready list.
The scheduler is invoked whenever the current thread gives up
the CPU.
The current Nachos scheduling policy is round-robin: new
threads are appended to the end of the ready list, and the
scheduler selects the front of the list.
The Scheduler object has the following operations:
 ReadyToRun(Thread *thread).
Make thread ready to run and place it on the ready list.
 Thread *FindNextToRun()
 Run(Thread *nextThread)
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Semaphore object for thread
synchronization
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Disable and re-enable interrupts to achieve mutual
exclusion (e.g., by calling Interrupt::SetLevel()).
Operations for a Semaphore object:
 Semaphore(char* debugName, int initialValue)
 P(): Decrement the semaphore's count, blocking
the caller if the count is zero.
 V() :Increment the semaphore's count, releasing
one thread if any are blocked waiting on the
count.
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Key steps when Nachos executes
After you type ``nachos'' under threads subdirectory:
 It is executing as a single Unix process.
 The main() calls
 Initialize() to start up interrupt handling, create a
scheduler for managing the ready queue.
 ThreadTest() (to be explained for HW 1).
 currentThread->Finish() to let other threads
continue to run.
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Key Calling graph when Nachos
executes under thread directory
All files are in threads
directory.
Thread:Fork ()
in thread.cc
Initialize()
in system.cc
main() in
main.cc
Thread:Yield ()
in thread.cc
ThreadTest ()
in threadtest.cc
currentThread->Finish ()
in threadtest.cc
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StackAllocate()
in thread.cc
FindNextToRun ()
in scheduler.cc
SWITCH ()
in switch.s
ReadyToRun ()
in scheduler.cc
ThreadRoot ()
in switch.s
Run ()
in scheduler.cc
func() such as
SimpleThread()
in ThreadTest.cc
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ThreadRoot()
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Executed by SWITCH(oldThread, nextThread)
jal
StartupPC
# call startup procedure. For a
new thread, it is InterruptEnable().
move
jal
jal
procedure.
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a0, InitialArg
InitialPC
WhenDonePC
# call main procedure
# when were done, call clean up
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QA
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When will thread:Finish() be called?
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At the end of the forked thread. Check ThreadRoot assembly
code.
At the end of the main() thread.
Thread:Finish() calls scheduler->run() to run a new
thread. Will the old thread (that calls Thread:Finish())
be returned and continue to be executed?
No. Because the following is called first.
threadToBeDestroyed = currentThread;
 Scheduler->Run() will delete such TCB
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QA
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In Sleep() or Yield(), the interrupt is turned off before calling
scheduler->() to execute another thread.
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When will interrupt be turned on?
In executing the switched thread, ThreadRoot() assembly code
first executes StartupPC function which is
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machineState[StartupPCState] = (int) InterruptEnable;
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HW 1: threads & synchronization
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Work under threads subdirectory.
Modify ThreadTest() to do simple threads
programming (spawning multiple threads).
Implement locks and condition variables (missing
from the file synch.cc).
Workload:
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Read Nachos code and add few hundred lines of code.
Undocumented sample solution is provided.
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HW 1: Files involved
Key files
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main.cc, threadtest.cc -- a simple test of our thread routines.
thread.h thread.cc -- Nachos thread data structure and operations.
scheduler.h scheduler.cc -- The thread ready list.
synch.h synch.cc -- synchronization routines.
Other related files.
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synchlist.h, synchlist.cc -- synchronized access to lists using
locks/conditions (useful examples for your programming).
list.h list.cc -- generic list management.
system.h, system.cc -- Nachos startup/shutdown routines.
utility.h utility.cc -- some useful definitions and debugging routines.
interrupt.h interrupt.cc -- manage interrupts.
time.h timer.cc -- clock emulation.
switch.h, switch.s -- assembly code for thread switching.
stats.h stats.cc -- collect interesting statistics.
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HW Sample solution
~cs240t/sampleSolutionCode.tar.gz
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Has an old solution for HW1, HW2, HW3
 HW1  threads subdirectory. ~400 lines of new code.
~50% are for Tasks 1/2/3. Code for task 4 is not useful.
 HW2 -> userprog subdirectory. ~1300 lines of new code.
 HW3 -> vm and filesys. ~1200 lines of new code. 800 may
be enough.
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Caveat:
Mixed benefits/problems in using other students’ code.
e.g. Not well documented, not fully tested. Possibly awkward
design.
 Still your responsibility to produce good solutions
(correctness, performance, style).
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