xv6 Assignments

Question 1

Step by step, what happens in a system call?

Answer 1

• user.h
  
  • Definitions of all the system calls
  • Definition of other user libraries
    
    • malloc, memset,strlen, etc..
• syscall.h
  
  • Here we list the system calls with the preceding SYS_
  • Each SYS_<name> </name>refers to a specific number, so the kernel can identifie.
• sysproc.c
  
  • sys_<name> </name>, used to get the arguments given by the user and call the relevant system call implemenation.
• syscall.c
  
  • Here we implement an array, where index SYS_<s.c> </s.c>refers to sys_<s.c X>
  • In addition, we implement the function syscall(), which gets the value of curproc->trapframe>eax, and then assigned to it the value of the call to the relevant system call.
    
    • num = curproc->tf->eax;

if(num > 0 && num < NELEM(syscalls) && syscalls[num]) {

curproc->tf->eax = syscallsnum;

}

Question 2

What is the job of the kernel stack?

Answer 2

To hold necesarry data of the process for kernel sevices. That is, its trapframe, registers, program counter, etc.

Question 3

When we create a new process for a program by calling exec, what is allocated?

Answer 3

• pgdir = setupkvm()
  
  • Set up kernel part of a page table.
  • Each process has kernel mappings from virtual to physical.
• sz = allocuvm
  
  • Allocate enough pages to load the program.
• loaduvm
  
  • load the program
• again allocuvm
  
  • allocate space for user stack
• push argument string