increase PHYSTOP

put an invalid page below the stack
have fork() handle invalid pages

find out the hard way why user and kernel must have separate segment descriptors

fix corner cases with alignment when mapping kernel ELF file

replace jstack with asm()s

(maybe this never worked, but it works now)

passes all tests on two-processor smp

Includes code for TLB shootdown (which actually seems unnecessary for xv6)

locks were moved around.

memcpy directly.