nachos instructional os: part 2 cs 270, tao yang, spring 2011
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Nachos Instructional OS: Part 2
CS 270, Tao Yang, Spring 2011
04/21/23 2
Topics Program execution in Nachos
Given a C program, produce binary MIPS code. Execute this MIPS code.
Machine object How to load a program How to execute a program: process-level flow.
Demo of single-process execution flow. Programming Assignment 2.
Support multiprogramming. System calls for process execution. System calls for a simple file system interface.
System Layers
Base Operating System (Linux for our class)
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)
User processUser process
04/21/23 4
Machine Source code: under machine subdirectory. Roughly approximates the MIPS
architecture Machine has registers, memory, a CPU
and a clock. Simulated clock used for event scheduling
and execution (interrupts). Can execute an arbitrary program with a
sequence of MIPS instructions.
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Machine: Code execution Steps Load instructions into the machine's memory. Initialize registers (including the program counter
PCReg). Tell the machine to start executing instructions. The machine then fetches the instruction PCReg
points at, decodes it, and executes it. The process is repeated until an illegal operation
is performed or an interrupt is generated. When a trap or interrupt takes place, an interrupt
service routine deals with the condition.
04/21/23 6
Two modes of executions User mode (executing MIPS instructions of a
user program) Example: Halt code in test directory User-programs can only access the memory
associated with the simulated machine.
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Two modes of executions Kernel mode
kernel executes when Nachos first starts up or when a user-program executes an
instruction that causes a ``hardware’’ trap illegal instruction, page fault system call
04/21/23 8
Machine Object: Implement a MIPS machine
an instance created when Nachos starts up. Supported public variables:
Registers: 40 registers. Memory: Byte-addressable. Virtual memory: use a single linear page
table or a software-managed TLB.
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Machine Object: Supported operations
Machine(bool debug). Translate(int virtAddr, int* physAddr, int
size, bool writing). OneInstruction(). Run() ReadRegister(int num) WriteRegister(int num, int value) ReadMem(int addr, int size, int* value) WriteMem(int addr, int size, int value)
04/21/23 10
Machine Object: Supported operations
Machine(bool debug). Translate(int virtAddr, int* physAddr, int
size, bool writing). OneInstruction(). Run() ReadRegister(int num) WriteRegister(int num, int value) ReadMem(int addr, int size, int* value) WriteMem(int addr, int size, int value)
04/21/23 11
Interrupt Object Maintain an event queue together with a simulated clock. Supported operations:
Schedule(VoidFunctionPtr handler, int arg, int when, IntType type) Schedule a future event to take place at time ``when''.
Usage: schedule a yield at random interval. SetLevel(IntStatus level). Used to temporarily disable and re-
enable interrupts. Two levels are supported: IntOn and IntOff. OneTick()-- CheckIfDue(bool advanceClock). Examines if some event
should be serviced. Idle(). ``advances'' to the clock to the time of the next
scheduled event
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Interrupt::OneTick()
Software managed clock. The clock advances 1 tick for user mode, 10
for system mode after every restored interrupt
(disable/enable Interrupt) or after the MIPS simulator executes one
instruction. When the ready list is empty, fast-advance
ticks until the next scheduled event happens.
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Timer object
Generate interrupts at regular or random intervals
Then Nachos invokes the predefined clock event handling procedure.
Supported operation: Timer(VoidFunctionPtr timerHandler, int callArg, bool
doRandom). Create a real-time clock that interrupts
every TimerTicks (100) time units Or set this a random number for random mode
04/21/23 14
Console Object
Simulates the behavior of a character-oriented CRT device Data can be written to the device one character at a time
through the PutChar() routine. Input characters arrive one-at-a-time. They can be
retrieved by GetChar(). Supported operations:
Console(char *readFile, char *writeFile, VoidFunctionPtr readAvail,VoidFunctionPtr writeDone, int callArg).
Create a console instance.``readFile'' is the Unix file of where the data is to be read from; if NULL, standard input is assumed.
PutChar(char ch) GetChar()
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Disk Object Simulates the behavior of a real disk.
The disk has only a single platter, with multiple tracks (32). Each track contains the same number of sectors (32). Allow only one pending operation at a time. Contain a ``track buffer'' cache. Immediately after seeking
to a new track, the disk starts reading sectors, placing them in the track buffer.
Supported operations: Disk(char *name, VoidFunctionPtr callWhenDone, int callArg) ReadRequest(int sectorNumber, char *data) WriteRequest(int sectorNumber, char *data) ComputeLatency(int newSector, bool writing)
Executing a user program
data datauser space
MIPS instructions executed by the emulator
Nachos
kernelMIPS emulator
halt shell
Machineobject
fetch/executeexamine/deposit
SaveState/RestoreStateexamine/deposit
Machine::Run()
ExceptionHandler()
SP
Rn
PC
registers memory
page table
process page tables
From C program to MIPS binary
int j;char* s = “hello\n”;
int p() { j = write(1, s, 6); return(j);}
myprogram.c
gcccompiler
…..p: store this store that push jsr _write ret etc.
myprogram.s
assembler data
myprogram.o
linker
object file
data program
(executable file)myprogram
datadatadata
libraries and
other objects
04/21/23 18
Binary code format (Noff)Source code: under userprog subdirectory. The current Nachos can run a single MIPS binary
(Noff format) e.g. type ``nachos -x ../test/halt''. A user program must be compiled using a cross-
platform gcc compiler that generates MIPS code. A Noff-format file contains (bin/noff.h)
the Noff header, describes the contents of the rest of the file
executable code segment (TEXT) initialized data segment (DATA) uninitialized data segment (BSS).
Space usage during execution of a C program
STACK
HEAP
BSS
DATA
TEXT
Stack grows from top-down.
Heap grows bottom-up
Uninitialized data
Initialized data
Code
04/21/23 20
TEXT, DATA, BSS, HEAP, STACK in C
Int f3=3; /* DATA segment */Int f1; /*BSS segment*/def[] = "1"; /* DATA segment */int main(void)
{static char abc[12], /* BSS segment */static float pi = 3.14159; /* DATA segment */int i = 3; /* Stack*/char *cp; /*stack*/cp= malloc(10); /*malloc allocates space from HEAP*/f1= i+f3; /* code is in TEXT*/
strcpy(abc , "Test" ); /* “Test” is located in DATA segment */
}
04/21/23 21
Noff format. Each segment has the following
information: virtualAddr: virtual address that
segment begins at. inFileAddr: Pointer within the Noff file
where that section actually begins. The size (in bytes) of that segment.
04/21/23
size = noffH.code.size + noffH.initData.size +
noffH.uninitData.size+ UserStackSize
User process for executing a program
A Nachos thread is extended as a process Each process has its own address space containing
Executable code (Code segment) Initialized data (Data segment) Uninitialized data (BSS) Stack space for function calls/local variables
how big is address space?
A process owns some other objects, such as open file descriptors.
04/21/23 23
Steps in User process creationCurrently only execute a single user program. Create an address space. Zero out all of physical memory (machine-
>mainMemory) Read the binary into physical memory and initialize
data segment. Initialize the translation tables to do a one-to-one
mapping between virtual and physical addresses. Zero all registers, setting PCReg and NextPCReg to 0
and 4 respectively. Set the stackpointer to the largest virtual address of
the process (stack grows downward).
04/21/23 24
Key Calling graph when Nachos executes under userprog directory
main() inmain.cc
Initialize()in system.cc
StartProcess ()in progtest.cc
Space=New AddrSpace()in addrspace.cc
Space->InitRegisters()
Executable fileReadAt()
Space->RestoreState()
Machine->Run ()in mipssim.cc
Machine->WriteRegister()
Machine->OneInstruction()
Interupt->OneTick()In Interupt.cc
void StartProcess(char *filename) { OpenFile *executable; AddrSpace *space;
executable = fileSystem->Open(filename); if (executable == NULL) { printf("Unable to open file %s\n", filename); return; } space = new AddrSpace(executable); currentThread->space = space; delete executable; // close file space->InitRegisters(); space->RestoreState(); machine->Run(); ASSERT(FALSE);}
Creating a Nachos Process (userprog/progtest.cc)
Create an AddrSpace object, allocating physicalmemory and setting up the process page table.
Set address space of current thread/process.
Initialize registers, load pagetable, and begin execution in usermode.
Create a handle for reading text and initial data out of the executable file.
AddrSpace::AddrSpace(OpenFile *executable) { NoffHeader noffH; unsigned int i, size; executable->ReadAt((char *)&noffH, sizeof(noffH), 0);
// 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;
pageTable = new TranslationEntry[numPages]; for (i = 0; i < numPages; i++) { pageTable[i].virtualPage = i; // for now, virtual page # = phys page # pageTable[i].physicalPage = i; pageTable[i].valid = TRUE; } ....
Creating a Nachos Address Space (userprog/addrspace.cc)
Read the header of binary file
Compute address space need
Setup a page table for address translation
bzero(machine->mainMemory, size);
// copy in the code and data segments into memory 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); }
Initializing a Nachos Address Space
Zero out memory allocated
Copy code segment to memory
Copy initialized data segment to memory
04/21/23 28
System Calls & Exception Handling
User programs invoke system calls by executing the MIPS ``syscall'' instruction
``syscall'' generates a hardware trap into the Nachos kernel. A trap is implemented by invoking RaiseException() with arguments
indicating the trap cause. RaiseException() calls ExceptionHandler() to take care of the specific problem. The system call number is stored in Register 2 and the return
address is in Register 31.Assembly code for Halt():Halt: addiu $2,$0,SC_Halt syscall j $31 .end Halt
04/21/23 29
When typing “nachos –x halt” The main thread starts by running function StartProcess()
in file progtest.cc. This thread is used to run halt binary. StartProcess() allocates a new address space and loads
the halt binary. It also initializes registers and sets up the page table.
Call Machine::Run() to execute the halt binary using the MPIS emulator.
The halt binary invokes the system call Halt(), which causes a trap back to the Nachos kernel via functions RaiseException() and ExceptionHandler().
The exception handler determines that a Halt() system call was requested from user mode, and it halts Nachos.
04/21/23 30
Nachos –x halt using halt.c in test directory
Machine->RaiseException(SyscallException)
Machine->Run ()in mipssim.cc
ExceptionHandler(SyscallException)
Machine->OneInstruction()
Interrupt->Halt()In Interupt.cc
04/21/23 31
Assignment 2: Multiprogramming&System Calls
Modify source code under userprog subdirectory. ~1200 lines of code.
The crossplatform compiler is under ~cs270t/gcc. This compiler on x86 machines produces a.out with the coff
format. Use utility coff2noff (under nachos’ bin directory) to
convert it as Noff. Check the makefile under test subdirectory on how to use
gcc and coff2noff. System calls to be implemented:
Multiprogramming: Fork(), Yield(), Exit(), Exec() and Join(). File and console I/O: Creat(), Open(), Read(), Write(), and
Close().
Multi-Processes and the Kernel
text
data
BSS
user stack
args/envkernel area
0
data
2n-1
Nachos kernel2n-1
2n-1
text
data
BSS
user stack
args/envkernel area
0
data
text
data
BSS
user stack
args/envkernel area
0
data
04/21/23 33
To run multiple processesNachos should Provide the physical memory management; Set up an address translation table with linear
page tables; Save/restore address-space related state
during process switching (AddrSpace::SaveUserState() and AddrSpace:RestoreUserState() are called).
04/21/23 34
Address translation with linear page tables
A virtual address is split into page number and page offset components
Machine->pageTable points to the page table to be used.
Machine->pageTableSize -- the actual size of the page table.
Process switching requires to set the above pointer properly for each user.
Physical page 0 starts at machine-> mainMemory. Each page has 128-bytes. The actual number of
physical pages is NumPhysPages (32).
A Simple Page Table
PFN 0PFN 1
PFN i
page #i offset
user virtual address
PFN i+
offset
process page table
physical memorypage frames
In this example, each VPN j maps to
PFN j, but in practice any
physical frame may be used for any
virtual page.
Each process/VAS has its own page
table. Virtual addresses are
translated relative to the current page
table.
The page tables are themselves stored in
memory; a protected register holds a pointer to the current page
table.
04/21/23 36
Assignment 2: Files involved Key files.
progtest.cc -- test routines to run user code. addrspace.h addrspace.cc -- create an address space and load the
program from disk. syscall.h -- the system call interface. exception.cc -- the handler for system calls and other user-level
exceptions such as page faults. filesys.h, openfile.h console.h -- interface to the Nachos file system and
console. Other related files:
bitmap.h bitmap.cc -- manipulate bitmpas (useful for keeping track of physical page frames).
translate.h, translate.cc -- translation tables. machine.h, machine.cc -- emulates main memory, processor, etc. mipsim.cc -- emulates MPIS R2/3000 instructions. console.cc -- emulates a terminal using UNIX files.
04/21/23 37
Questions Process management
How to let two processes run in parallel? What are fundamental mechanisms to enable this functionality?
Another Nachos Process? Thread? How to execute Machine->Run?
How to set PC (program counter) correctly in the new process?
How to invoke Machine->Run() from the current execution process?
Assignment 2: Implementation Notes
Tao Yang
Part I: Multiprogramming
Fork(func) creates a new user-level (child) process, whose address space starts out as an exact copy of that of the caller (the parent),
Yield(): temporarily relinquish the CPU to another process.
Exit(int) call takes a single argument, which is an integer status value as in Unix. The currently executing process is terminated..
Exec(filename) spawns a new user-level thread (process), but creates a new address space. It should return to the parent a SpaceId.
Join(ID) call waits and returns only after a process with the specified ID has finished.
Getting Started Review start.s (under test directory) which
includes all system call stubs, following the style of Halt.
Modify ExceptionHandler() in exception.cc to include all system call entries.
After each system call, increment PC registers so that ExceptionHandler() execution flow returns back to next instruction after user’s system call place.
counter = machine->ReadRegister(PCReg); machine->WriteRegister(PrevPCReg,counter); counter = counter + 4;machine-
>WriteRegister(PCReg,counter); counter = counter + 4; machine-
>WriteRegister(NextPCReg,counter); Arguments of a system call are in Registers 4, 5, 6 etc.
how to verify? You may review MPIS assembly code produced for a test C program using gcc -S.
If needed, return result is register 2 machine->WriteRegister(2,result);
PCB (Process Control Block)
Write the PCB and a process manager. Create a PCB class that will store the necessary information about a process. Don't worry about putting everything in it right now, you can always add more as you go along. To start, it should have a PID, parent PID, and Thread*.
The process manager- it can have getPID and clearPID methods, which return an unused process id and clear a process id respectively.
Memory Manager
Write a Memory Manager that will be used to facilitate memory allocation: Track memory page usage. Allocate a page Free a page
Modify AddrSpace:AddrSpace (addrspace.cc) to use the memory manager. Modify the page table constructors to use pages
allocated by your memory manager Create a PCB (process control block) also for
each process to include key control information.
AddSpace.cc
Write a function (e.g. AddrSpace::Translate), which converts a virtual address to a physical address. It does so by breaking the virtual address into a page table index and an offset.
Write a function( e.g. AddrSpace::ReadFile), which loads the code and data segments into the translated memory, instead of at position 0. Read data into a system buffer (diskBuffer). Copy buffered data into proper memory locations
(e.g. at machine->mainMemory[physAddr].)
Implement Fork() in ExceptionHandler()
FuncEntry = Value of register 4 ( sys call argu) Target function to be executed in the new space.
Create a new kernel thread. Create a new AddrSpace to be a duplicate of
the CurrentThread's space and get a new PCB. The current thread calls Yield() so the new
thread can run. The new thread runs a dummy function that
creates a bridge for execution of the user function). Call NewThread->Fork(ForkBridge, FuncEntry) Why? It needs to set program counter properly in a
new space.
ForkBridge() : Key parts
Set counter = FuncEntry Initialize and restore the registers. For
example, currentThread->RestoreUserState(); currentThread->space->RestoreState(); machine->WriteRegister(PCReg, counter); machine->WriteRegister(PrevPCReg,counter-4); machine->WriteRegister(NextPCReg,counter+4);
Call machine->Run() which executes the forked user process in the desired FuncEntry address.
Implement Exec() Exec is creating a new process with new code and
data segments from a file. Allocate a new address space which fits this file.
Load data/code from an OpenFile object constructed from the filename passed in by the user.
In order to get that file name you will have to write a function that copies over the string from user space.
Allocate a new kernel thread and a PCB to execute with the above space.
Fork the new thread to run a dummy bridge function that sets the machine registers straight and runs the code
Call NewThread->Fork(ExecBridge ,NULL); The calling thread should yield to give control to the newly
spawned thread. Return the process ID that executes this binary.
ExecBridge(): Key parts
Initialize registers/restore state. (currentThread->space)->InitRegisters(); (currentThread->space)->RestoreState();
Machine->run();
System Calls for File System
For the entire system, maintain a set of objects (SysOpenFile class) representing system-wide opened files. Each file may be opened by many user
processes. Call it fileOpenTable[];
For each process, maintain a set of objects in PCB (UserOpenFile class) representing files opened by this process. Each file has filename, index to the system-wide
open table entry, and offset for the current read/write pointer
Implement create()
Nachos already has a simple file system implemented filesys.cc and openfile.cc under filesys
directory. Use Nachos fileSystem to add a new file
to its directory.
Implement open()
Use fileSystem->Open() (nachos’open) to locate the file object.
Add the opened file object to the system-wide fileOpenTable. If it is already opened by some other user
process, just share the entry. Else insert to the local file open table in
PCB.
Implement read()
Allocate an internal buffer. If Inputfile ID is ConsoleInput
Then use getchar() and save data in the internal buffer.
Read until EOF or \n. Else, find the current read offset.
Use openFile:ReadAt (defined in openfile.cc) to read data from the nachos file.
Copy data from the internal buffer to the destination memory location. Need to copy one by one since the address
needs to be translated one by one.
Implement write()
Allocate an internal buffer. Copy data from the source memory
location to the internal buffer. Need to copy one by one since the address
needs to be translated one by one. If Outputfile ID is ConsoleOutput
Then use printf (assuming string type). Else, find the current write offset.
Use openFile:WriteAt (defined in openfile.cc) to write data to the nachos file.
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