/* switch.s 
 *   	Machine dependent context switch routines.  DO NOT MODIFY THESE! 
 *
 *	Context switching is inherently machine dependent, since
 *	the registers to be saved, how to set up an initial
 *	call frame, etc, are all specific to a processor architecture.
 *
 * 	This file currently supports the following architectures:
 *	    DEC MIPS
 *	    SUN SPARC
 *	    HP PA-RISC
 *	    Intel 386
 *
 * We define two routines for each architecture:
 *
 * ThreadRoot(InitialPC, InitialArg, WhenDonePC, StartupPC)
 *	InitialPC  - The program counter of the procedure to run
 *			in this thread.
 *      InitialArg - The single argument to the thread.
 *	WhenDonePC - The routine to call when the thread returns.
 *	StartupPC  - Routine to call when the thread is started.
 *
 *	ThreadRoot is called from the SWITCH() routine to start
 *	a thread for the first time. 
 *
 * SWITCH(oldThread, newThread)
 * 	oldThread  - The current thread that was running, where the
 *		CPU register state is to be saved.
 * 	newThread  - The new thread to be run, where the CPU register
 *		state is to be loaded from.
 */

/*
 Copyright (c) 1992-1993 The Regents of the University of California.
 All rights reserved.  See copyright.h for copyright notice and limitation 
 of liability and disclaimer of warranty provisions.
 */

#include "copyright.h"
#include "switch.h"

        .text
        .align  2

        .globl  ThreadRoot

/* void ThreadRoot( void )
**
** expects the following registers to be initialized:
**      eax     points to startup function (interrupt enable)
**      edx     contains inital argument to thread function
**      esi     points to thread function
**      edi     point to Thread::Finish()
*/
ThreadRoot:
        pushl   %ebp
        movl    %esp,%ebp
        pushl   InitialArg
        call    *StartupPC
        call    *InitialPC
        call    *WhenDonePC

        # NOT REACHED
        movl    %ebp,%esp
        popl    %ebp
        ret



/* void SWITCH( thread *t1, thread *t2 )
**
** on entry, stack looks like this:
**      8(esp)  ->              thread *t2
**      4(esp)  ->              thread *t1
**       (esp)  ->              return address
**
** we push the current eax on the stack so that we can use it as
** a pointer to t1, this decrements esp by 4, so when we use it
** to reference stuff on the stack, we add 4 to the offset.
*/
        .comm   _eax_save,4

        .globl  SWITCH
SWITCH:
        movl    %eax,_eax_save          # save the value of eax
        movl    4(%esp),%eax            # move pointer to t1 into eax
        movl    %ebx,_EBX(%eax)         # save registers
        movl    %ecx,_ECX(%eax)
        movl    %edx,_EDX(%eax)
        movl    %esi,_ESI(%eax)
        movl    %edi,_EDI(%eax)
        movl    %ebp,_EBP(%eax)
        movl    %esp,_ESP(%eax)         # save stack pointer
        movl    _eax_save,%ebx          # get the saved value of eax
        movl    %ebx,_EAX(%eax)         # store it
        movl    0(%esp),%ebx            # get return address from stack into ebx
        movl    %ebx,_PC(%eax)          # save it into the pc storage

        movl    8(%esp),%eax            # move pointer to t2 into eax

        movl    _EAX(%eax),%ebx         # get new value for eax into ebx
        movl    %ebx,_eax_save          # save it
        movl    _EBX(%eax),%ebx         # retore old registers
        movl    _ECX(%eax),%ecx
        movl    _EDX(%eax),%edx
        movl    _ESI(%eax),%esi
        movl    _EDI(%eax),%edi
        movl    _EBP(%eax),%ebp
        movl    _ESP(%eax),%esp         # restore stack pointer
        movl    _PC(%eax),%eax          # restore return address into eax
        movl    %eax,4(%esp)            # copy over the ret address on the stack
        movl    _eax_save,%eax

        ret