Lecture 3
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Questions?

 - where's good assembly documentation?
 - local variables?  environment variables?
 - gcc arguments


What is a function call?
 - save arguments (to registers or the stack)
 - call function
   - saves (pushes) current instruction pointer onto
     call stack
   - changes instruction pointer to location of start
     of function
   - ...function runs...
   - on function return, pops instruction pointer
     from stack, restoring old function pointer

 - everything is based on pointers to code

Pointers in a process are *local*
 - addresses only have meaning in the context of the process

address space - name space for pointers

System calls are invocations of kernel code
 - calling code that isn't necessarily in your address space
 - calling code with higher privileges
    - running in supervisor mode, not user mode

How can I (user space process) call privileged code safely?
(code that runs in supervisor mode)
 - restrict entry points to privileged code
   - can't call arbitrary routines
 - entry point should check whether operation is allowed

In the kernel, this is what the system call dispatcher does

System call dispatcher
 - process requests system call (write)
 - CPU switches to supervisor mode, runs system call dispatcher
 - dispatcher decides if system call is allowed
 - system call code (write) is invoked

CPU switches into supervisor mode using special instructions
 - "software interrupts"
 - "upcall"