xv6.htm

Notes on xv6 OS

Features of xv6

• Processes
• Virtual address spaces, page tables
• Files, directories
• Pipes
• Multi-tasking, time slicing
• 21 sys calls

It contains the main Unix sys calls: grep, ls, grep, echo, kill, ln...

What is missing?

• All of the complexity of a "real" OS
• User IDs, login, file protection, mountable filesystems
• Paging to disk
• Sockets, support for networks
• IPC (interprocess comms)
• Device drivers (only 2)
• User code / apps

Anatomy os a system call

User space

• user.h
• usys.S
  • Short sub-routine for all the sys calls
  • Calls syscall.h -> looks for the number of the syscall there
• syscall.C

Process:

• User mode calls TRAP
• Trap operates in Kernel Mode
• Then we call sret to go back into user mode