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Nachos Instructional OS. CS 270, Tao Yang, Spring 2011. What is Nachos OS?. Allow students to examine, modify and execute operating system software. A skeletal OS that supports kernel threads user-level processes Simulates MIPS instruction execution.

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Nachos instructional os

Nachos Instructional OS

CS 270, Tao Yang, Spring 2011

What is nachos os
What is Nachos OS?

  • Allow students to examine, modify and execute operating system software.

  • A skeletal OS that supports

    • 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.

    • Can understand its basic mechanisms by reading about 1-2K lines of code.

System layers
System Layers

User process

User process

Nachos kernel threads

Thread 1

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
Steps to Install Nachos

  • Obtain and install Nachos source code.

    • Copy the source code from ~cs270t/nachosSept20.tar.gz

  • Compile the source code using gmake

  • Run threads demo under the threads subdirectory (just run kernel test threads).

    • Run user program demo under the userprog subdirectory.


Nachos code directory
Nachos code directory

  • 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)

Source code reading and hw1
Source code reading and HW1

Objectives of next 2 weeks

  • Scan through ~1,000-2,000 lines of code under threads directory

    • Learn how context switch is accomplished among threads.

    • Learn how thread scheduling is done.

    • Learn how locks/synchronization are implemented and used.

  • Complete HW1

    • Few hundred lines of code

    • Sample solution for Task 1, 2, &3 are available

Sample example of nacho threads
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);



SimpleThread(int i)


printf(“Hello %d\n”, i);



Create 2 new threads.

Start to fork and execute a function in each child thread.

Parent also executes the same function

Function executed by threads

Nachos threads
Nachos Threads

  • 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:

    • Thread(char *debugName). Create a thread.

    • Fork(VoidFunctionPtr func, int arg). Let a thread execute a function.

    • Yield(). Suspend the calling thread and select a new one for execution.

    • Sleep(). Suspend the current thread, change its state to BLOCKED, and remove it from the ready list

    • Finish()

Nachos thread states and transitions
Nachos Thread States and Transitions



When running in user mode, the thread executes within the machine simulator. HW 2 covers this.

In HW 1 we are only concerned with the states in this box.



interrupt or










Thread switching
Thread Switching

  • 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.

Scheduler object for thread scheduling
Scheduler object for thread scheduling

  • 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)

Semaphore object for thread synchronization
Semaphore object for thread synchronization

  • 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.

Key steps when nachos executes
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.

Key calling graph when nachos executes under thread directory
Key Calling graph when Nachos executes under thread directory



All files are in threads directory.

Thread:Fork ()





in switch.s

FindNextToRun ()


Thread:Yield ()


main() in

ThreadRoot ()

in switch.s

ReadyToRun ()


ThreadTest ()


func() such as



Run ()


currentThread->Finish ()


ThreadRoot() directory

  • Executed by SWITCH(oldThread, nextThread)

  • jal StartupPC # call startup procedure. For a new thread, it is InterruptEnable().

  • move a0, InitialArg

  • jal InitialPC # call main procedure

  • jal WhenDonePC # when were done, call clean up procedure.

QA directory

  • When will thread:Finish() be called?

    • 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

QA directory

  • In Sleep() or Yield(), the interrupt is turned off before calling scheduler->() to execute another thread.

    • When will interrupt be turned on?

  • In executing the switched thread, ThreadRoot() assembly code first executes StartupPC function which is

    • machineState[StartupPCState] = (int) InterruptEnable;

Hw 1 threads synchronization
HW 1: threads & synchronization directory

  • Work under threads subdirectory.

  • Modify ThreadTest() to do simple threads programming (spawning multiple threads).

  • Implement locks and condition variables (missing from the file

  • Workload:

    • Read Nachos code and add few hundred lines of code.

    • Undocumented sample solution is provided.

Hw 1 files involved
HW 1: Files involved directory

Key files

  •, -- a simple test of our thread routines.

  • thread.h -- Nachos thread data structure and operations.

  • scheduler.h -- The thread ready list.

  • synch.h -- synchronization routines.

    Other related files.

  • synchlist.h, -- synchronized access to lists using locks/conditions (useful examples for your programming).

  • list.h -- generic list management.

  • system.h, -- Nachos startup/shutdown routines.

  • utility.h -- some useful definitions and debugging routines.

  • interrupt.h -- manage interrupts.

  • time.h -- clock emulation.

  • switch.h, switch.s -- assembly code for thread switching.

  • stats.h -- collect interesting statistics.

Hw sample solution
HW Sample solution directory


  • 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.

  • 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).