CSE120 Discussion 5 Xinxin Jin

Where Are We?  Now we have finished some thread mechanisms to support Nachos kernel  Next, we want to enable user-level program to access these kernel routines via system calls

Prjoect 2 Introduction  You will touch two parts of the code base: The operating system running java programs as you saw in project 1 (mainly /userprog class) User programs written in C and compiled into MIPS machine code (under /test directory).

Project 2 Tasks  Implement file system calls creat,open,read,write,close,and unlink, documented in test/syscall.h  Support multiprogramming – Memory partitioning for multi-processes  Implement process control system calls exec,join,and exit, also documented in syscall.h

Task 1: Implement File System Calls  creat - Create a file and return a file descriptor  close – Close a file  unlink – Delete a file  open – Open a file and return a file descriptor, an integer specifying the position of this open file in the table of open files for the current process  read – Read data from an open file to user process  write – Write data to an open file from user process

Don’t be scared by the lengthy instructions. We will reach the end step by step !

My Suggestions  Understand the the workflow of syscall in Nachos  Read /test/syscall.h. Understand the functionality and the arguments of each syscall.  Add syscall handler at the right place  Start from the simplest prototype of each syscall leave the complex restrictions alone for the first implementation  Add the restrictions back piece by piece  Don’t forget more sanity checks ! When does this syscall fail ?

Recap: System Call

Nachos Syscall: User space program  Syscall interface (defined in syscall.h)  Syscall code (defined in syscall.h)  Match syscall name with syscall code (start.s)  The instruction that traps to nachos kernel (start.s) #define syscallHalt 0 Void halt() SYSCALLSTUB(halt, syscallHalt) syscall

Compile User progarm  Set up MIPS cross compiler on linux machine (see instructions on the website) Avoid running the compiler on Windows or Mac  Run “make” under /test Generate *.coff executable file for MIPS processor  Add your own program Write hello.c under /test Add hello to the end of TARGETS in Makefile Run “make hello”, you will see hello.coff

Nachos Syscall: The Kernel Program  Some changed fields in nachos.conf Machine.stubFileSystem = true -- You need to access the stubFileSystem returned by Machine.stubFileSystem() --The filesystem access /test directory (same as root dir on linux machine) Machine.console = true --Now we have a simulated console for standard input and standoutput Processor.numPhysPages = The total number of available physical pages of the machine Kernel.shellProgram = halt.coff --The kernel executes the user program halt.coff by default -- Run nachos –x hello.coff to overrwite the default user progarm Kernel.processClassName = nachos.userprog.UserProcess -- Manage user process. You will heavily work on this class Kernel.kernel = nachos.userprog.UserKernel -- The nachos kernel for project2

/userprog Package  UserKernel.java Nachos kernel that supports multiprogramming  UserProcess.java Manage the state of user processes, method handleSyscall is called to handle system calls made by user programs  UThead.java User level thread, a subclass of Kthread, contains a reference to a UserProcess  SynchConsole.java – Simulate a console for stdin and stdout

Sycall Handler in Kernel  The entry point of syscall handler handleSyscall(int syscall, int a0, int a1, int a2, int a3)  The syscall code (e.g. syscallHalt) defined in UserProcess.java is consistent with the one defined syscall.h

System Call Execution Flow Summary

Let’s Get Started  Read the description of creat() and open() in syscall.h  Add syscall handlers of these two Leave the method body empty for now

Pass Parameter by Virtual Address  “Pass by value” vs.“Pass by address” read (int fd, char *buffer, int size) buffer is passed as virtual address, which is a pointer to an array of bytes, so use readVirtualMemory and writeVirtualMemory for these parameters.  open(char *fileName) fileName is passed as a pointer its first character, so use readVirtualMemoryString to get this parameter.

Support Concurrently Open Files  Your implementation should support at least 16 concurrently open files per process  Each file that a process has opened should have a unique file descriptor associated with it If a program call open() on the same file twice, just return different file descriptors  A given file descriptor can be reused if the file associated with it is closed

Open File Descriptor Table  File descriptor is used by user program to refer to a file creat and open return a file descriptor to user program this descriptor will be used for the following read/write  FD table is an array associated with every process each entry corresponds to one open file creat and open add an entry to the table, return the descriptor to user program close delete an entry from the table

Support Large Data Transfer In read()/write()  You cannot allocate unlimited buffer size  What is the appropricate buffer size to pass data between the file and memory ?

Syscall Parameter Sanity Check  OS should never allow a user program to screw up the kernel or other user programs  For example, in read( int fd, char *buffer, int size ), what kinds of parameters should be rejected ? Is the file descriptor valid ? Is size valid ? Is buffer and (buffer + size) in this process’s virtual address space ?