SNOW OS: 64-bit cooperative multi-tasking OS

Snow OS is a 64-bit cooperative multi-tasking operating system implemented in C. In short, it supports memory management, paging, context switching, terminal, shell and file management for tarfs.

The project was completed as a part of the graduate course CSE 506 Operating Systems by Prof. Mike Ferdman at Stony Brook University in Fall '17.

OS Functionality

• Memory Subsystem (page descriptors, free list, virtual memory, page tables, kmalloc)
• Process Subsystem (kernel threads, context switch)
• User-level Subsystem (VMAs/vm_map_entrys, switch to ring 3, page faults)
• COW fork(), auto-growing stack, dynamic heap allocation, graceful SEGV
• I/O subsystem (syscalls, terminals, VFS, tarfs file access, elf loading)
• tarfs: open, read, close, opendir, readdir, closedir
• Terminal: read(stdin), write(stdout), write(stderr)
• Virtual File System
• kprintf, IDT, keyboard, timer interrupts
• Shell with PATH, PS1, cd, and &
• Script handling with shebang (#!)
• bin/init user process to launch shell using /etc/rc
• Reaper/Idle process to cleanup zombie processes
• Reused PIDs
• Binaries implemented: init, echo, sleep, cat, ls, kill -9, ps
• Background processes
• Runs its own Shell

Shell support

cd: change current working directory  
echo <input>: print the given input  
export PATH=<_path>: update path variable with _path  
export PS1=<_ps1>: update PS1 variable with _ps1  
ls: list the current directory contents  
ps: list all process in the OS  
sleep: sleep for given input time in seconds  
kill -9: kills the process with given pid  
cat <file>: prints the given file content  
./<binary_file>: execution of binary file with name binary_file  
<command> &: execution of background process  

Build

make clean  
make  
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  

Examples

ls  
cat /etc/test.txt  
.//etc/test.sbush  
sbush /etc/test.sbush  
ls /etc  
ps  
echo hello  
export PS1=SHELL  

Example files

Under the rootfs/etc/ folder, the following test files can be found:

• test: test directory for cd
• test.txt: simple text file for cat
• test.sbush: simple sbush script
• stress_test.sbush: stress test sbush script with 60+ commands to spawn 60+ processes, system remains stable

Syscalls Implemented

chdir  
close  
closedir  
execvpe  
exit  
fork  
free  
getcwd  
getpid  
getppid  
kill  
malloc  
open  
opendir  
ps  
read  
readdir  
sleep  
wait  
waitpid  
write  
yield  

License

MIT License

Authors

Ankit Aggarwal
Saraj Munjal