switch.S

/*
 * Copyright (c) 2000, 2001, 2002, 2003, 2004, 2005, 2008, 2009
 *	The President and Fellows of Harvard College.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 3. Neither the name of the University nor the names of its contributors
 *    may be used to endorse or promote products derived from this software
 *    without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE UNIVERSITY AND CONTRIBUTORS ``AS IS'' AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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 */

/*
 * Assembly language context switch code.
 */

#include <kern/mips/regdefs.h>
  
   .text
   .set noreorder
  
   .globl switchframe_switch
   .type switchframe_switch,@function
   .ent switchframe_switch
switchframe_switch:
   /*
    * a0 contains the address of the switchframe pointer in the old thread.
    * a1 contains the address of the switchframe pointer in the new thread.
    *
    * The switchframe pointer is really the stack pointer. The other
    * registers get saved on the stack, namely:
    *
    *      s0-s6, s8
    *      gp, ra
    *
    * The order must match <mips/switchframe.h>.
    *
    * Note that while we'd ordinarily need to save s7 too, because we
    * use it to hold curthread saving it would interfere with the way
    * curthread is managed by thread.c. So we'll just let thread.c
    * manage it.
    */

   /* Allocate stack space for saving 10 registers. 10*4 = 40 */
   addi sp, sp, -40

   /* Save the registers */
   sw   ra, 36(sp)
   sw   gp, 32(sp)
   sw   s8, 28(sp)
   sw   s6, 24(sp)
   sw   s5, 20(sp)
   sw   s4, 16(sp)
   sw   s3, 12(sp)
   sw   s2, 8(sp)
   sw   s1, 4(sp)
   sw   s0, 0(sp)

   /* Store the old stack pointer in the old thread */
   sw   sp, 0(a0)

   /* Get the new stack pointer from the new thread */
   lw   sp, 0(a1)
   nop           /* delay slot for load */

   /* Now, restore the registers */
   lw   s0, 0(sp)
   lw   s1, 4(sp)
   lw   s2, 8(sp)
   lw   s3, 12(sp)
   lw   s4, 16(sp)
   lw   s5, 20(sp)
   lw   s6, 24(sp)
   lw   s8, 28(sp)
   lw   gp, 32(sp)
   lw   ra, 36(sp)
   nop                  /* delay slot for load */

   /* and return. */
   j ra
   addi sp, sp, 40      /* in delay slot */
   .end switchframe_switch