Change interfaces of
some syscalls.

Fix the u-mode exception bug.

Update README.md and Makefile.

Xv6 is temporarily not working.

Merge kernel's pagetable and users';

Fix sp to align 16 in execve;

Add some conditional checks in fs.

Support copy-on-write fork.

Print args when tracing syscall;
Change envsc's type to suit execve.

Add primitive kmalloc module (not being used yet).

Only hart 0 update ticks, so tickslock isn't needed at present.