Working with the Unix OS

Processes I
! System Call Interface
algorithm syscall
/* algorithm for invocation of system call */
input: system call number
output: result of system call
{
find entry in system call table corresponding to system
call number;
determine number of parameters to system call;
copy parameters from user address space to u area;
save current context for abortive return;
invoke system call code in kernel;
if (error during execution of system call)
{
set register 0 in user saved register context to
error number;
turn on carry bit in PS register in user saved
register context;
}
else
set registers 0, 1 in user saved register context
to return values from system call;
}
Figure 14. Example of Interrupts
Figure 15. Algorithm for System Calls Invocations
! Context Switch
Figure 16. Stack configuration for creat system call
1. Decide whether to do a context switch, and
whether a context switch is permissible now.
2. Save the context of the "old" process.
3. Find the "best" process to schedule for execution,
using process scheduling algorithm of Figure 46.
4. Restore its context.
Figure 17. Steps for a Context Switch
103
if (save context()) /* save context of executing process * /
{
/* pick another process to run */
…
resume_context (new...process); /* never gets here! */
}
/* resuming process executes from here */
Figure 18. Pseudo-Code for Context Switch