rkya / sbunix

Preemptive multi-tasking operating system

Introduction

This is a preemptive multi-tasking operating system called SBUnix developed as a part of operating systems course in Stony Brook University. I have implemented paging, free list, kmalloc, interrupts, scheduling, task switching, terminal, syscalls, kernel-threads, VFS and tarfs file access. SBUnix also contains binaries like echo, sleep, cat, ls, kill and ps.

Running The OS

To boot the system in QEMU, run:

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:9961
  

Commands supported

• Print the present working directory

  pwd
  

• Print the contents of the current directory

  ls
  

• Change the current working directory

  cd <Path>
  e.g.
  cd /rootfs/etc/
  cd /rootfs/
  

  Note that you have to enter absolute path right from /rootfs/. Also, note that every directory ends with a '/'. So if you enter cd /rootfs instead of cd /rootfs/, it won't work.
• Echo

  echo <Word/Variable>
  e.g.
  echo hello
  echo $HOME
  

• Print the information of the running processes

  ps
  

• Kill a process

  kill -9 <pid>
  e.g.
  kill -9 1
  

  If you kill a process with pid > 1, the process will be killed.
  But if you try to kill process with pid 1, SBUnix will ask you if you want to shut down the system as this is the special sbush process which runs as soon as you start the SBUnix.
• Print the contents of the file on the terminal

  cat <Path/To/The/File>
  e.g.
  cat /rootfs/etc/test.txt
  

• Add/change value of an environment variable

  export Name=Value
  e.g.
  export hello=world
  

  You can later check the value of the variable using the echo command as follows:

  echo $hello
  

• Sleep for 's' seconds

  sleep <Time in seconds>
  e.g.
  sleep 5
  

• Shutdown SBUnix

  shutdown
  

References

• http://www.ibiblio.org/gferg/ldp/GCC-Inline-Assembly-HOWTO.html
• https://eli.thegreenplace.net/2011/09/06/stack-frame-layout-on-x86-64/
• https://wiki.osdev.org/USTAR
• http://www.osdever.net/bkerndev/Docs/intro.htm
• http://www.jamesmolloy.co.uk/tutorial_html/4.-The%20GDT%20and%20IDT.html
• https://www.win.tue.nl/~aeb/linux/kbd/scancodes-1.html
• https://notes.shichao.io/lkd/ch15/