timer-preempted kernel thread resumes execution in trap() after yield().
otherwise the kernel could get an arbitrary number of nested timer intrs.

also thanks to greg price.

thanks to greg price.

accessible to user from the hidden CPU segment registers.

add slides for shell, x86 intro, x86 virtual memory  (deleted JOS from slides)

I understand this one.

bda[0xE] is a 16-bit segment number,
not a real address.  So shift 4.

Reported by Silas.

Jim McKie says this code only matters
on ancient EISA MP systems.

Model verifying that wakeup really
can be called after release without
causing deadlock.

Incorporate new understanding of/with Intel SMP spec.

Dropped cmpxchg in favor of xchg, to match lecture notes.

Use xchg to release lock, for future protection and to
keep gcc from acting clever.