README-PDX

Changes made to xv6 to facilitate projects at Portland State University.

1. Access to the ''ticks'' global has been made atomic. This was
necessary to avoid deadlock with regards to tickslock/ptable.lock,
particularly for the sleep() routine in proc.c. Since Intel has atomic
guarantees and GNU gcc has a suitable alignment syntax, we are able to
do away with the tickslock everywhere. Note that a special check had
to be put into sleep() in proc.c to handle the fact that sys_sleep()
was changed to pass a NULL lock.

The following changes were required.

1a. The definition for ticks was made 32-bit aligned in trap.c.

// set alignment to 32-bit for ticks. See Intel® 64 and IA-32 
// Architectures Software Developer’s Manual, Vol 3A, 8.1.1 Guaranteed
// Atomic Operations.  
uint ticks __attribute__ ((aligned (4)));

1b. The lcok was removed in the code for handling the timer interrupt
in trap.c

  switch(tf->trapno){
  case T_IRQ0 + IRQ_TIMER:
    if(cpu->id == 0){
      acquire(&tickslock);
      ticks++;
      release(&tickslock);
      wakeup(&ticks);
    }

changed to:

  switch(tf->trapno){
  case T_IRQ0 + IRQ_TIMER:
    if(cpu->id == 0){
      atom_inc((int *)&ticks);
      wakeup(&ticks);
    }

1c. A new routine was added to x86.h to use the Intel atomic increment
instruction.

// atom_inc() added to simplify handling of ticks global
static inline void
atom_inc(volatile int *num)
{
  asm volatile ( "lock incl %0" : "=m" (*num));
}

Wrappers for several other atomic Intel instructions were added for
completeness.

1d. Removed tickslock acquisition/free in sys_uptime (sysproc.c) since
reading is now atomic.

1e. All uses of tickslock (e.g., sys_sleep()) have been removed. This
necessitated surgery on sleep() in proc.c. Calling sleep with a NULL lock
is now legal but is limited to sys_sleep(). CAUTION

2. A new feature was added to the scheduler to turn off the CPU for
the remainder of the time slice when no work is in the ready queue.
proc.c in routine scheduler(). The hlt() routine is in x86.h.

  int idle;  // for checking if processor is idle

...

    idle = 1;  // assume idle unless we schedule a process

...

      idle = 0;  // not idle this timeslice

...

    release(&ptable.lock);
    // if idle, wait for next interrupt
    if (idle) {
      sti();
      hlt();
    }

3. A new user command, halt, was created to shutdown QEMU.  This is
distinct from the hlt() function. See sys_halt() in sysproc.c.
Function prototype added to user.h, usys.S and syscall.{hc} updated.

4. A general cleanup of the Makefile to be less confusing and more
specific to PSU.

5. The array states[] in proc.c was moved out of procdump() to be
file global. 

6. Added a define in types.h to adjust the ticks per second (TPS).  This
is useful when implementing MLFQ scheduling. See timer.c (uniprocessor)
and lapic.c (SMP) for use.

7. When TPS is other than the MIT default, we can modify trap.c to invoke
the scheduler (via yield()) at less than the TPS frequency. This makes
MLFQ more realistic.

8. Updated atoi() in ulib.c to handle negative numbers.

9. Added atoo() in ulib.c to handle ascii to octal conversion for
project 5. Function prototype added to user.h

10. Changed the procstate enum in proc.h. The purpose is to help catch
poor programming that relies on the UNUSED state just happening to be
0. This required that we remove the magic number in procdump() in proc.c.

11. Several test suites have been added; more to come. These exist
to assist students in testing assignments. See project descriptions
as appropriate

12. Changed all references to pid, nextpid to be uint as they should be.