Unix foundations

Setting the interrupt priority in 4.3BSD and SVR4

Interrupt handling

Synchronization

• UNIX is re-entrant
• UNIX is non-preemptive, keeping the kernel states consistent.
• Relinquish CPU voluntarily.

Blocking Operations

• Blocks the process (make it sleep).
• Lock: a single bit flag
• wanted(flag):
• sleep():
• wake(): wake all the waiting processes.
• upon waking up: check once again to make sure.

Algorithm for resource locking

Blocking Interrupts

• Blocking interrupts while accessing critical sections.
• int x = splbio();/* raise ipl*/
• modify disk buffer cache;
• splx(x); /*restore ipl*/
• Critical region:few & brief.
• Blocked interrupts may access the critical region.
• Different interrupts may have the same ipl
• Blocking an interrupt may block others.

UNIX Process Implementation

New Processes & Programs

• fork:
• creates a new process.
• returns 0 to the child, PID to the parent
• exec:
• begins to execute a new program

Using fork & exec

• if ((result = fork()==0){
• /* child code*/
• … …
• if (execve(“new program”),…)<0)
• perror(“execve failed!”);
• } else if (result<0) {
• perror(“fork”);/*fork failed*/
• }
• /*parent continures here*/

Shell creating a process

• A highly simplified shell

The ls Command

• Steps in executing the command ls typed to the shell

Process Creation

• Almost an exact clone of the parent.
• Reserve swap space for the child
• Allocate a new PID and proc structure for the child
• Initialize proc structure
• Allocate address translation map (ATM)
• Allocate u area and copy
• Update the u area to refer to the new ATM & Swap space
• Add the child to the set of processes sharing the text region of the program
• Duplicate the parent’s data and stack regions update ATM to refer to these new pages.
• Acquire references to shared resources inherited by the child
• Initialize the hardware context
• Make the child runnable and put it on a scheduler queue
• Arrange to return with 0
• Return the PID to the parent