thanks wxdao

improve swtch comment

move jkstack to main.c
replace jstack with asm()s

Initial version of single-cpu xv6 with page tables

initproc, usegment, swtch tweaks

cleaner swtch.S

yank out stack overflow checking ugliness

overkill: use segments to catch stack overflow (delete before next year)

replace setjmp/longjmp with swtch

Changes to allow use of native x86 ELF compilers, which on my
Linux 2.4 box using gcc 3.4.6 don't seem to follow the same
conventions as the i386-jos-elf-gcc compilers.
Can run make 'TOOLPREFIX=' or

Changes to allow use of native x86 ELF compilers, which on my
Linux 2.4 box using gcc 3.4.6 don't seem to follow the same
conventions as the i386-jos-elf-gcc compilers.
Can run make 'TOOLPREFIX=' or edit the Makefile.

curproc[cpu()] can now be NULL, indicating that no proc is running.
This seemed safer to me than having curproc[0] and curproc[1]
both pointing at proc[0] potentially.

The old implementation of swtch depended on the stack frame layout
used inside swtch being okay to return from on the other stack
(exactly the V6 you are not expected to understand this).
It also could be called in two contexts: at boot time, to schedule
the very first process, and later, on behalf of a process, to sleep
or schedule some other process.

I split this into two functions: scheduler and swtch.

The scheduler is now a separate never-returning function, invoked
by each cpu once set up. The scheduler looks like:

scheduler() {
setjmp(cpu.context);

pick proc to schedule
blah blah blah

longjmp(proc.context)
}

The new swtch is intended to be called only when curproc[cpu()] is not NULL,
that is, only on behalf of a user proc. It does:

swtch() {
if(setjmp(proc.context) == 0)
longjmp(cpu.context)
}

to save the current proc context and then jump over to the scheduler,
running on the cpu stack.

Similarly the system call stubs are now in assembly in usys.S to avoid
needing to know the details of stack frame layout used by the compiler.

Also various changes in the debugging prints.

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