Semaphores and Mailboxes

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Semaphores and Mailboxes

• B. Ramamurthy

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Critical sections and Semaphores

• When multiples tasks are executing there may be
  sections where only one task could execute at a
  given time critical region or critical section
• There may be resources which can be accessed only
  be one of the processes critical resource
• Semaphores can be used to ensure mutual exclusion
  to critical sections and critical resources

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Semaphores in exinu

• include ltkernel.hgt
• include ltqueue.hgt /lt queue.h must define
  of sem queues /
• / Semaphore state definitions /
• define SFREE 0x01 /lt this semaphore is
  free /
• define SUSED 0x02 /lt this semaphore is
  used /
• / type definition of "semaphore" /
• typedef ulong semaphore
• / Semaphore table entry /
• struct sentry
• char state /lt the state SFREE
  or SUSED /
• short count /lt count for this
  semaphore /
• queue queue /lt requires q.h.
  /

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Semaphores in exinu (contd.)

• extern struct sentry semtab
• /
• isbadsem - check validity of reqested
  semaphore id and state
• _at_param s id number to test NSEM is declared
  to be 100 in kernel.h
• A system typically has a predetermined
  limited number of semaphores
• /
• define isbadsem(s) (((ushort)(s) gt NSEM)
  (SFREE semtabs.state))
• / Semaphore function declarations /
• syscall wait(semaphore)
• syscall signal(semaphore)
• syscall signaln(semaphore, short)
• semaphore newsem(short)
• syscall freesem(semaphore)
• syscall scount(semaphore)

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Definition of Semaphores functions

• static semaphore allocsem(void)
• /
• newsem - allocate and initialize a new
  semaphore.
• _at_param count - number of resources available
  without waiting.
• example count 1 for mutual exclusion lock
• _at_return new semaphore id on success, SYSERR on
  failure
• /
• semaphore newsem(short count)
• irqmask ps
• semaphore sem
• ps disable() /
  disable interrupts /
• sem allocsem() /
  request new semaphore /
• if ( sem ! SYSERR count gt 0 ) /
  safety check /
• semtabsem.count count
  / initialize count /
• restore(ps)
  / restore interrupts /
• return sem
  / return semaphore id /

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Semaphore newsem contd.

• /
• allocsem - allocate an unused semaphore and
  return its index.
• Scan the global semaphore table for a free
  entry, mark the entry
• used, and return the new semaphore
• _at_return available semaphore id on success,
  SYSERR on failure
• /
• static semaphore allocsem(void)
• int i 0
• while(i lt NSEM) /
  loop through semaphore table /
• /
  to find SFREE semaphore /
• if( semtabi.state SFREE )
• semtabi.state SUSED
• return i
• i
• return SYSERR

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Semaphore wait()

• /
• wait - make current process wait on a
  semaphore
• _at_param sem semaphore for which to wait
• _at_return OK on success, SYSERR on failure
• /
• syscall wait(semaphore sem)
• irqmask ps
• struct sentry psem
• pcb ppcb
• ps disable() / disable
  interrupts /
• if ( isbadsem(sem) ) / safety
  check /
• restore(ps)
• return SYSERR
• ppcb proctabcurrpid / retrieve
  pcb from process table /
• psem semtabsem / retrieve
  semaphore entry /
• if( --(psem-gtcount) lt 0 ) / if
  requested resource is unavailable /

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Semaphore wait()

• ppcb-gtsem sem / record semaphore id in pcb /
• enqueue(currpid, psem-gtqueue)
• resched() / place
  in wait queue and reschedule /
• restore(ps) / restore
  interrupts /
• return OK

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Semaphore signal()

• /signal - signal a semaphore, releasing one
  waiting process, and block
• _at_param sem id of semaphore to signal
• _at_return OK on success, SYSERR on failure
• /
• syscall signal(semaphore sem)
• irqmask ps
• register struct sentry psem
• ps disable() / disable
  interrupts /
• if ( isbadsem(sem) ) / safety
  check /
• restore(ps)
• return SYSERR
• psem semtabsem /
  retrieve semaphore entry /
• if ( (psem-gtcount) lt 0 ) / release
  one process from wait queue /
• ready(dequeue(psem-gtqueue),
  RESCHED_YES)
• restore(ps) / restore
  interrupts /

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Semaphore usage

• Problem 1
• Create 3 tasks that each sleep for a random time
  and update a counter.
• Counter is the critical resources shared among
  the processes.
• Only one task can update the counter at a time so
  that counter value is correct.
• Problem 2
• Create 3 tasks task 1 updates the counter by 1
  and then signal task 2 that updates the counter
  by 2 and then signals task 3 to update the
  counter by 3.

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Problem 1

• include lt..gt
• //declare semaphore
• semaphore mutex1 newsem(1)
• int counter 0
• //declare functions proc1,proc1, proc3
• ready(create((void )proc1, INITSTK, INITPRIO,
  PROC1",, 2, 0, NULL), RESCHED_NO)
• ready(create((void )proc2, INITSTK, INITPRIO,
  PROC2",, 2, 0, NULL), RESCHED_NO)
• ready(create((void )proc3, INITSTK, INITPRIO,
  PROC3",, 2, 0, NULL), RESCHED_NO)

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Problem 1 multi-tasks

• void proc1()
• while (1)
• sleep (rand()10)
• wait(mutex1)
• counter
• signal(mutex1)
• void proc2()
• while (1)
• sleep (rand()10)
• wait(mutex1)
• counter
• signal(mutex1)
• //similarly proc3
• Simulation of this

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Problem 1
Task 1
Task 2
Counter1
Task 3
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Problem 2

• semaphore synch12 newsem(0)
• semaphore synch23 newsem(0)
• semaphore synch31 newsem(0)
• ready(create((void )proc1, INITSTK, INITPRIO,
  PROC1",, 2, 0, NULL), RESCHED_NO)
• ready(create((void )proc2, INITSTK, INITPRIO,
  PROC2",, 2, 0, NULL), RESCHED_NO)
• ready(create((void )proc3, INITSTK, INITPRIO,
  PROC3",, 2, 0, NULL), RESCHED_NO)
• signal(synch31)

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Task flow
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UART and UART Drivers
Inside the system
Outside
Actual UART Buffer (input, output) Registers (Cont
rol, status)
UART Driver
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Reverse Engineer UART read, write and intr
(interrupt)
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UART Operation and code (contd.)

• We will look at the sequence diagrams for other
  UART operations and read the code.
• Lets look at initialize.c