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.