Computer Science 454

1
Computer Science 454

• Processes In Xinu

2
Process vs. Program

• process
• dynamic
• part of a program in execution.
• a live entity, it can be created, executed, and
  terminated.
• it goes trough different states
• wait
• running
• ready, etc.

• Requires resources to be allocates by the OS.
• zero or more processes may executing the same
  code.
• program
• static
• no states

3
Process creation, system calls and Context
Switching

• getpid() /gets the current pid/
• getppid() /gets the parent pid/
• exec, execl, execv /See the man page!/
• execl(/bin/ls, ls, -l, (char 0))
  /overlay original program
• use fork then exec() - how shell commands work
• orphaned/zombie processes (later)
• XINU implements fork() as create() -(see man fork)

• Context Switch
• Saves current machine state
• loads the next scheduled process
• via interrupt, multiprogramming, etc.

4
Parent child processes

• main()
• int pid /hold process-id in parent/
• printf(Just one process so far\n)
• printf(Calling fork \n)
• pid fork() / create new process /
• if ( pid 0 )
• printf( I am the child\n)
• else if ( pid gt 0 )
• printf( Im the parent, the child has pid
  d\n, pid)
• else
• printf( fork returned error code, no
  child\n)

5
fork() example
6
List and Queue manipulation

• Important Constants
• NSEM of semaphores
• NQENT of entries in q array (see q.h on page
  58)
• NPROC Max of processes a system can have.

• List and Queue Processing
• Doubly linked lists
• head
• tail
• node
• prevPtr
• data
• nextPtr

7
Queue Structure
Highest priority items at link next to tail.
(see page 62 in text) This is an array
implementation of a doubly linked list enqueue,
dequeue (see page 60 in text)
8
Implementation of Queue Array

• Used to keep track of
• Ready List
• 2 entries
• List for each Semaphore
• 2NSEM
• Sleep List
• 2 entries
• See page 57 in text
• 42NSEM NPROC NQENT