SBUnix : CSE 506 Project

Professor: Mike Ferdman

Course webpage: Operating systems

Project Description

• x86_64 Preemptive multitasking operating system with following functionalities:

  • Memory Subsystem (page descriptors, free list, page tables, kmalloc)
  • Process Subsystem (kernel threads, context switch)
  • User-level Subsystem (VMAs/vm_map_entrys, switch to ring 3, page faults)
  • I/O subsystem (syscalls, terminals, VFS, tarfs file access)
  • A Shell that
    • Supports changing current directory (cd /path)
    • Executes binaries interactively
    • Executes scripts (files that start with #!sbush)
    • Sets and uses PATH and PS1 variables
    • Launches background processes (ls &)

• The provided Makefile:

  1. Builds a kernel
  2. Copies it into rootfs/boot/kernel/kernel
  3. Creates an ISO CD image with the rootfs/ contents

• Commands supported by shell:

ps                  Process List
ls                  List directory contents
ls path             List directory contents of that path
cat [file path]     Prints file contents
                    example: cat file
cd                  Change Directory
ulimits             Displays system limits
sbush {filepath}    Execute shebang files. example: sbush /etc/test/script.sh
exit                Kills the current process
shutdown            Exits all the process.
echo $VAR           Displays the current value set in the argument variable
                    example - echo $PATH
export VAR=ABC      Export used to create/set value to environment variable
                    example: export PATH=/rootfs/bin
sleep {seconds}     Puts the shell to sleep for specified seconds
                    example: sleep 5
sleep {seconds} &   Puts the shell to sleep for specified seconds in background
                    example: sleep 5 &
kill -9 pid         Kill the process with that pid [Note: Only kill -9 supported]

• To boot the system in QEMU, run:

./start.sh

OR

qemu-system-x86_64 -curses -drive id=boot,format=raw,file=$USER.img,if=none -drive id=data,format=raw,file=$USER-data.img,if=none -device ahci,id=ahci -device ide-drive,drive=boot,bus=ahci.0 -device ide-drive,drive=data,bus=ahci.1 -gdb tcp::9999

• Explanation of parameters:

  -curses         use a text console (omit this to use default SDL/VNC console)
  -drive ...      connect a CD-ROM or hard drive with corresponding image
  -device ...     configure an AHCI controller for the boot and data disks
  -gdb tcp::9999  listen for "remote" debugging connections on port NNNN
  -S              wait for GDB to connect at startup
  -no-reboot      prevent reboot when OS crashes

• When using the -curses mode, switch to the qemu> console with ESC-2.

• To connect a remote debugger to the running qemu VM, from a different window: gdb ./kernel

• At the (gdb) prompt, connect with: target remote localhost:9999

References

• http://wiki.osdev.org/Main_Page
• http://www.jamesmolloy.co.uk/tutorial_html/
• https://eli.thegreenplace.net/2011/09/06/stack-frame-layout-on-x86-64
• http://rayseyfarth.com/asm/pdf/ch04-memory-mapping.pdf
• https://wiki.cdot.senecacollege.ca/wiki/X86_64_Register_and_Instruction_Quick_Start
• https://en.wikibooks.org/wiki/X86_Assembly/X86_Architecture
• https://gist.github.com/davazp/d2fde634503b2a5bc664