source: rtems/cpukit/score/cpu/mips/cpu_asm.S @ 9165349d

/*
 *  This file contains the basic algorithms for all assembly code used
 *  in an specific CPU port of RTEMS.  These algorithms must be implemented
 *  in assembly language
 *
 *  History:
 *    Baseline: no_cpu
 *    1996:     Ported to MIPS64ORION by Craig Lebakken <craigl@transition.com>
 *          COPYRIGHT (c) 1996 by Transition Networks Inc.
 *          To anyone who acknowledges that the modifications to this file to
 *          port it to the MIPS64ORION are provided "AS IS" without any
 *          express or implied warranty:
 *             permission to use, copy, modify, and distribute this file
 *             for any purpose is hereby granted without fee, provided that
 *             the above copyright notice and this notice appears in all
 *             copies, and that the name of Transition Networks not be used in
 *             advertising or publicity pertaining to distribution of the
 *             software without specific, written prior permission. Transition
 *             Networks makes no representations about the suitability
 *             of this software for any purpose.
 *    2000: Reworked by Alan Cudmore <alanc@linuxstart.com> to become
 *          the baseline of the more general MIPS port.
 *    2001: Joel Sherrill <joel@OARcorp.com> continued this rework,
 *          rewriting as much as possible in C and added the JMR3904 BSP
 *          so testing could be performed on a simulator.
 *    2001: Greg Menke <gregory.menke@gsfc.nasa.gov>, bench tested ISR
 *          performance, tweaking this code and the isr vectoring routines
 *          to reduce overhead & latencies.  Added optional
 *          instrumentation as well.
 *    2002: Greg Menke <gregory.menke@gsfc.nasa.gov>, overhauled cpu_asm.S,
 *          cpu.c and cpu.h to manage FP vs int only tasks, interrupt levels
 *          and deferred FP contexts.
 *    2002: Joel Sherrill <joel@OARcorp.com> enhanced the exception processing
 *          by increasing the amount of context saved/restored.
 *    2004: 24March, Art Ferrer, NASA/GSFC, added save of FP status/control
 *          register to fix intermittent FP error encountered on ST5 mission
 *          implementation on Mongoose V processor.
 *    2004: April 7, Greg Menke <gregory.menke@gsfc.nasa.gov> Added __mips==32
 *          support for R4000 processors running 32 bit code.  Fixed #define
 *          problems that caused fpu code to always be included even when no
 *          fpu is present.
 *
 *  COPYRIGHT (c) 1989-2002.
 *  On-Line Applications Research Corporation (OAR).
 *
 *  The license and distribution terms for this file may be
 *  found in the file LICENSE in this distribution or at
 *  http://www.rtems.org/license/LICENSE.
 */

#ifdef HAVE_CONFIG_H
#include "config.h"
#endif

#include <rtems/asm.h>
#include <rtems/mips/iregdef.h>
#include <rtems/mips/idtcpu.h>
#include <rtems/score/percpu.h>

#define ASSEMBLY_ONLY
#include <rtems/score/cpu.h>

#if TRUE
#else
#error TRUE is not true
#endif
#if FALSE
#error FALSE is not false
#else
#endif

/*
#if ( CPU_HARDWARE_FP == TRUE )
#warning CPU_HARDWARE_FP == TRUE
#else
#warning CPU_HARDWARE_FP != TRUE
#endif
*/


/* enable debugging shadow writes to misc ram, this is a vestigal
* Mongoose-ism debug tool- but may be handy in the future so we
* left it in...
*/

/* #define INSTRUMENT_ISR_VECTORING */
/* #define INSTRUMENT_EXECUTING_THREAD */



/*  Ifdefs prevent the duplication of code for MIPS ISA Level 3 ( R4xxx )
 *  and MIPS ISA Level 1 (R3xxx).
 */

#if __mips == 3
/* 64 bit register operations */
#define NOP     nop
#define ADD     dadd
#define STREG   sd
#define LDREG   ld
#define MFCO    dmfc0           /* Only use this op for coprocessor registers that are 64 bit in R4000 architecture */
#define MTCO    dmtc0           /* Only use this op for coprocessor registers that are 64 bit in R4000 architecture */
#define ADDU    addu
#define ADDIU   addiu
#if (__mips_fpr==32)
#define STREGC1 swc1
#define LDREGC1 lwc1
#elif (__mips_fpr==64)          /* Use these instructions if there are 64 bit floating point registers. This requires FR bit to be set in C0_SR */
#define STREGC1 sdc1
#define LDREGC1 ldc1
#endif
#define R_SZ    8
#define F_SZ    8
#define SZ_INT  8
#define SZ_INT_POW2 3

/* XXX if we don't always want 64 bit register ops, then another ifdef */

#elif (__mips == 1 ) || (__mips == 32)
/* 32 bit register operations*/
#define NOP     nop
#define ADD     add
#define STREG   sw
#define LDREG   lw
#define MFCO    mfc0
#define MTCO    mtc0
#define ADDU    add
#define ADDIU   addi
#define STREGC1 swc1
#define LDREGC1 lwc1
#define R_SZ    4
#define F_SZ    4
#define SZ_INT  4
#define SZ_INT_POW2 2
#else
#error "mips assembly: what size registers do I deal with?"
#endif


#define ISR_VEC_SIZE    4
#define EXCP_STACK_SIZE (NREGS*R_SZ)


#ifdef __GNUC__
#define ASM_EXTERN(x,size) .extern x,size
#else
#define ASM_EXTERN(x,size)
#endif

/* NOTE: these constants must match the Context_Control structure in cpu.h */
#define S0_OFFSET 0
#define S1_OFFSET 1
#define S2_OFFSET 2
#define S3_OFFSET 3
#define S4_OFFSET 4
#define S5_OFFSET 5
#define S6_OFFSET 6
#define S7_OFFSET 7
#define SP_OFFSET 8
#define FP_OFFSET 9
#define RA_OFFSET 10
#define C0_SR_OFFSET 11
#define C0_EPC_OFFSET 12

/* NOTE: these constants must match the Context_Control_fp structure in cpu.h */
#define FP0_OFFSET  0
#define FP1_OFFSET  1
#define FP2_OFFSET  2
#define FP3_OFFSET  3
#define FP4_OFFSET  4
#define FP5_OFFSET  5
#define FP6_OFFSET  6
#define FP7_OFFSET  7
#define FP8_OFFSET  8
#define FP9_OFFSET  9
#define FP10_OFFSET 10
#define FP11_OFFSET 11
#define FP12_OFFSET 12
#define FP13_OFFSET 13
#define FP14_OFFSET 14
#define FP15_OFFSET 15
#define FP16_OFFSET 16
#define FP17_OFFSET 17
#define FP18_OFFSET 18
#define FP19_OFFSET 19
#define FP20_OFFSET 20
#define FP21_OFFSET 21
#define FP22_OFFSET 22
#define FP23_OFFSET 23
#define FP24_OFFSET 24
#define FP25_OFFSET 25
#define FP26_OFFSET 26
#define FP27_OFFSET 27
#define FP28_OFFSET 28
#define FP29_OFFSET 29
#define FP30_OFFSET 30
#define FP31_OFFSET 31
#define FPCS_OFFSET 32


ASM_EXTERN(__exceptionStackFrame, SZ_INT)

/*
 *  _CPU_Context_save_fp_context
 *
 *  This routine is responsible for saving the FP context
 *  at *fp_context_ptr.  If the point to load the FP context
 *  from is changed then the pointer is modified by this routine.
 *
 *  Sometimes a macro implementation of this is in cpu.h which dereferences
 *  the ** and a similarly named routine in this file is passed something
 *  like a (Context_Control_fp *).  The general rule on making this decision
 *  is to avoid writing assembly language.
 */

/* void _CPU_Context_save_fp(
 *   void **fp_context_ptr
 * );
 */

#if ( CPU_HARDWARE_FP == TRUE )
FRAME(_CPU_Context_save_fp,sp,0,ra)
        .set noreorder
        .set noat

        /*
        ** Make sure the FPU is on before we save state.  This code
        ** is here because the FPU context switch might occur when an
        ** integer task is switching out with a FP task switching in.
        */
        mfc0    t0,C0_SR
        li      t2,SR_CU1
        move    t1,t0
        or      t0,t2           /* turn on the fpu */
#if (__mips == 3) || (__mips == 32)
        li      t2,SR_IE
#elif __mips == 1
        li      t2,SR_IEC
#endif
        not     t2
        and     t0,t2           /* turn off interrupts */
        mtc0    t0,C0_SR

        lw      a1,(a0)         /* get address of context storage area */
        move    t0,ra
        jal     _CPU_Context_save_fp_from_exception
        NOP

        /*
        ** Reassert the task's state because we've not saved it yet.
        */
        mtc0    t1,C0_SR
        j       t0
        NOP

        .globl _CPU_Context_save_fp_from_exception
_CPU_Context_save_fp_from_exception:
        STREGC1 $f0,FP0_OFFSET*F_SZ(a1)
        STREGC1 $f1,FP1_OFFSET*F_SZ(a1)
        STREGC1 $f2,FP2_OFFSET*F_SZ(a1)
        STREGC1 $f3,FP3_OFFSET*F_SZ(a1)
        STREGC1 $f4,FP4_OFFSET*F_SZ(a1)
        STREGC1 $f5,FP5_OFFSET*F_SZ(a1)
        STREGC1 $f6,FP6_OFFSET*F_SZ(a1)
        STREGC1 $f7,FP7_OFFSET*F_SZ(a1)
        STREGC1 $f8,FP8_OFFSET*F_SZ(a1)
        STREGC1 $f9,FP9_OFFSET*F_SZ(a1)
        STREGC1 $f10,FP10_OFFSET*F_SZ(a1)
        STREGC1 $f11,FP11_OFFSET*F_SZ(a1)
        STREGC1 $f12,FP12_OFFSET*F_SZ(a1)
        STREGC1 $f13,FP13_OFFSET*F_SZ(a1)
        STREGC1 $f14,FP14_OFFSET*F_SZ(a1)
        STREGC1 $f15,FP15_OFFSET*F_SZ(a1)
        STREGC1 $f16,FP16_OFFSET*F_SZ(a1)
        STREGC1 $f17,FP17_OFFSET*F_SZ(a1)
        STREGC1 $f18,FP18_OFFSET*F_SZ(a1)
        STREGC1 $f19,FP19_OFFSET*F_SZ(a1)
        STREGC1 $f20,FP20_OFFSET*F_SZ(a1)
        STREGC1 $f21,FP21_OFFSET*F_SZ(a1)
        STREGC1 $f22,FP22_OFFSET*F_SZ(a1)
        STREGC1 $f23,FP23_OFFSET*F_SZ(a1)
        STREGC1 $f24,FP24_OFFSET*F_SZ(a1)
        STREGC1 $f25,FP25_OFFSET*F_SZ(a1)
        STREGC1 $f26,FP26_OFFSET*F_SZ(a1)
        STREGC1 $f27,FP27_OFFSET*F_SZ(a1)
        STREGC1 $f28,FP28_OFFSET*F_SZ(a1)
        STREGC1 $f29,FP29_OFFSET*F_SZ(a1)
        STREGC1 $f30,FP30_OFFSET*F_SZ(a1)
        STREGC1 $f31,FP31_OFFSET*F_SZ(a1)
        cfc1 a0,$31                    /* Read FP status/conrol reg */
        cfc1 a0,$31                    /* Two reads clear pipeline */
        NOP
        NOP
        sw a0, FPCS_OFFSET*F_SZ(a1)    /* Store value to FPCS location */
        NOP
        j ra
        NOP
        .set at
ENDFRAME(_CPU_Context_save_fp)
#endif

/*
 *  _CPU_Context_restore_fp_context
 *
 *  This routine is responsible for restoring the FP context
 *  at *fp_context_ptr.  If the point to load the FP context
 *  from is changed then the pointer is modified by this routine.
 *
 *  Sometimes a macro implementation of this is in cpu.h which dereferences
 *  the ** and a similarly named routine in this file is passed something
 *  like a (Context_Control_fp *).  The general rule on making this decision
 *  is to avoid writing assembly language.
 */

/* void _CPU_Context_restore_fp(
 *   void **fp_context_ptr
 * )
 */

#if ( CPU_HARDWARE_FP == TRUE )
FRAME(_CPU_Context_restore_fp,sp,0,ra)
        .set noat
        .set noreorder

        /*
        ** Make sure the FPU is on before we retrieve state.  This code
        ** is here because the FPU context switch might occur when an
        ** integer task is switching out with a FP task switching in.
        */
        mfc0    t0,C0_SR
        li      t2,SR_CU1
        move    t1,t0
        or      t0,t2           /* turn on the fpu */
#if (__mips == 3) || (__mips == 32)
        li      t2,SR_IE
#elif __mips == 1
        li      t2,SR_IEC
#endif
        not     t2
        and     t0,t2           /* turn off interrupts */
        mtc0    t0,C0_SR

        lw      a1,(a0)         /* get address of context storage area */
        move    t0,ra
        jal     _CPU_Context_restore_fp_from_exception
        NOP

        /*
        ** Reassert the old task's state because we've not restored the
        ** new one yet.
        */
        mtc0    t1,C0_SR
        j       t0
        NOP

        .globl _CPU_Context_restore_fp_from_exception
_CPU_Context_restore_fp_from_exception:
        LDREGC1 $f0,FP0_OFFSET*F_SZ(a1)
        LDREGC1 $f1,FP1_OFFSET*F_SZ(a1)
        LDREGC1 $f2,FP2_OFFSET*F_SZ(a1)
        LDREGC1 $f3,FP3_OFFSET*F_SZ(a1)
        LDREGC1 $f4,FP4_OFFSET*F_SZ(a1)
        LDREGC1 $f5,FP5_OFFSET*F_SZ(a1)
        LDREGC1 $f6,FP6_OFFSET*F_SZ(a1)
        LDREGC1 $f7,FP7_OFFSET*F_SZ(a1)
        LDREGC1 $f8,FP8_OFFSET*F_SZ(a1)
        LDREGC1 $f9,FP9_OFFSET*F_SZ(a1)
        LDREGC1 $f10,FP10_OFFSET*F_SZ(a1)
        LDREGC1 $f11,FP11_OFFSET*F_SZ(a1)
        LDREGC1 $f12,FP12_OFFSET*F_SZ(a1)
        LDREGC1 $f13,FP13_OFFSET*F_SZ(a1)
        LDREGC1 $f14,FP14_OFFSET*F_SZ(a1)
        LDREGC1 $f15,FP15_OFFSET*F_SZ(a1)
        LDREGC1 $f16,FP16_OFFSET*F_SZ(a1)
        LDREGC1 $f17,FP17_OFFSET*F_SZ(a1)
        LDREGC1 $f18,FP18_OFFSET*F_SZ(a1)
        LDREGC1 $f19,FP19_OFFSET*F_SZ(a1)
        LDREGC1 $f20,FP20_OFFSET*F_SZ(a1)
        LDREGC1 $f21,FP21_OFFSET*F_SZ(a1)
        LDREGC1 $f22,FP22_OFFSET*F_SZ(a1)
        LDREGC1 $f23,FP23_OFFSET*F_SZ(a1)
        LDREGC1 $f24,FP24_OFFSET*F_SZ(a1)
        LDREGC1 $f25,FP25_OFFSET*F_SZ(a1)
        LDREGC1 $f26,FP26_OFFSET*F_SZ(a1)
        LDREGC1 $f27,FP27_OFFSET*F_SZ(a1)
        LDREGC1 $f28,FP28_OFFSET*F_SZ(a1)
        LDREGC1 $f29,FP29_OFFSET*F_SZ(a1)
        LDREGC1 $f30,FP30_OFFSET*F_SZ(a1)
        LDREGC1 $f31,FP31_OFFSET*F_SZ(a1)
        cfc1 a0,$31                  /* Read from FP status/control reg */
        cfc1 a0,$31                  /* Two reads clear pipeline */
        NOP                          /* NOPs ensure execution */
        NOP
        lw a0,FPCS_OFFSET*F_SZ(a1)   /* Load saved FPCS value */
        NOP
        ctc1 a0,$31                  /* Restore FPCS register */
        NOP
        j ra
        NOP
        .set at
ENDFRAME(_CPU_Context_restore_fp)
#endif

/*  _CPU_Context_switch
 *
 *  This routine performs a normal non-FP context switch.
 */

/* void _CPU_Context_switch(
 *   Context_Control  *run,
 *   Context_Control  *heir
 * )
 */

FRAME(_CPU_Context_switch,sp,0,ra)
        .set noreorder

        mfc0    t0,C0_SR
#if (__mips == 3) || (__mips == 32)
        li      t1,SR_IE
#elif __mips == 1
        li      t1,SR_IEC
#endif
        STREG   t0,C0_SR_OFFSET*R_SZ(a0)        /* save the task's SR */
        not     t1
        and     t0,t1                           /* mask off interrupts while we context switch */
        mtc0    t0,C0_SR
        NOP

        STREG ra,RA_OFFSET*R_SZ(a0)             /* save current context */
        STREG sp,SP_OFFSET*R_SZ(a0)
        STREG fp,FP_OFFSET*R_SZ(a0)
        STREG s0,S0_OFFSET*R_SZ(a0)
        STREG s1,S1_OFFSET*R_SZ(a0)
        STREG s2,S2_OFFSET*R_SZ(a0)
        STREG s3,S3_OFFSET*R_SZ(a0)
        STREG s4,S4_OFFSET*R_SZ(a0)
        STREG s5,S5_OFFSET*R_SZ(a0)
        STREG s6,S6_OFFSET*R_SZ(a0)
        STREG s7,S7_OFFSET*R_SZ(a0)


        /*
        ** this code grabs the userspace EPC if we're dispatching from
        ** an interrupt frame or supplies the address of the dispatch
        ** routines if not.  This is entirely for the gdbstub's benefit so
        ** it can know where each task is running.
        **
        ** Its value is only set when calling threadDispatch from
        ** the interrupt handler and is cleared immediately when this
        ** routine gets it.
        */

        la      t0,__exceptionStackFrame        /* see if we're coming in from an exception */
        LDREG   t1, (t0)
        NOP
        beqz    t1,1f

        STREG   zero, (t0)                      /* and clear it */
        NOP
        LDREG   t0,R_EPC*R_SZ(t1)               /* get the userspace EPC from the frame */
        b       2f
        NOP

1:      la      t0,_Thread_Dispatch             /* if ==0, we're switched out */

2:      STREG   t0,C0_EPC_OFFSET*R_SZ(a0)


_CPU_Context_switch_restore:
        LDREG ra,RA_OFFSET*R_SZ(a1)             /* restore context */
        LDREG sp,SP_OFFSET*R_SZ(a1)
        LDREG fp,FP_OFFSET*R_SZ(a1)
        LDREG s0,S0_OFFSET*R_SZ(a1)
        LDREG s1,S1_OFFSET*R_SZ(a1)
        LDREG s2,S2_OFFSET*R_SZ(a1)
        LDREG s3,S3_OFFSET*R_SZ(a1)
        LDREG s4,S4_OFFSET*R_SZ(a1)
        LDREG s5,S5_OFFSET*R_SZ(a1)
        LDREG s6,S6_OFFSET*R_SZ(a1)
        LDREG s7,S7_OFFSET*R_SZ(a1)

        LDREG t0, C0_SR_OFFSET*R_SZ(a1)

/*      NOP */
/*#if (__mips == 3) || (__mips == 32) */
/*        andi  t0,SR_EXL */
/*        bnez  t0,_CPU_Context_1 */   /* set exception level from restore context */
/*        li    t0,~SR_EXL */
/*        MFC0  t1,C0_SR */
/*        NOP */
/*        and   t1,t0 */
/*        MTC0  t1,C0_SR */
/* */
/*#elif __mips == 1 */
/* */
/*        andi  t0,(SR_INTERRUPT_ENABLE_BITS) */ /* we know 0 disabled */
/*        beq   t0,$0,_CPU_Context_1  */         /* set level from restore context */
/*        MFC0  t0,C0_SR */
/*        NOP */
/*        or    t0,(SR_INTERRUPT_ENABLE_BITS) */ /* new_sr = old sr with enabled  */
/*        MTC0  t0,C0_SR */                     /* set with enabled */
/*        NOP */


/*
** Incorporate the incoming task's FP coprocessor state and interrupt mask/enable
** into the status register.  We jump thru the requisite hoops to ensure we
** maintain all other SR bits as global values.
**
** Get the task's FPU enable, int mask & int enable bits.  Although we keep the
** software int enables on a per-task basis, the rtems_task_create
** Interrupt Level & int level manipulation functions cannot enable/disable them,
** so they are automatically enabled for all tasks.  To turn them off, a task
** must itself manipulate the SR register.
**
** Although something of a hack on this processor, we treat the SR register
** int enables as the RTEMS interrupt level.  We use the int level
** value as a bitmask, not as any sort of greater than/less than metric.
** Manipulation of a task's interrupt level corresponds directly to manipulation
** of that task's SR bits, as seen in cpu.c
**
** Note, interrupts are disabled before context is saved, though the task's
** interrupt enable state is recorded.  The task swapping in will apply its
** specific SR bits, including interrupt enable.  If further task-specific
** SR bits are arranged, it is this code, the cpu.c interrupt level stuff and
** cpu.h task initialization code that will be affected.
*/

        li      t2,SR_CU1
        or      t2,SR_IMASK

        /* int enable bits */
#if (__mips == 3) || (__mips == 32)
        /*
        ** Save IE
        */
        or      t2,SR_IE
#elif __mips == 1
        /*
        ** Save current, previous & old int enables.  This is key because
        ** we can dispatch from within the stack frame used by an
        ** interrupt service.  The int enables nest, but not beyond
        ** previous and old because of the dispatch interlock seen
        ** in the interrupt processing code.
        */
        or      t2,SR_IEC + SR_IEP + SR_IEO
#endif
        and     t0,t2           /* keep only the per-task bits */

        mfc0    t1,C0_SR        /* grab the current SR */
        not     t2
        and     t1,t2           /* mask off the old task's per-task bits */
        or      t1,t0           /* or in the new task's bits */
        mtc0    t1,C0_SR        /* and load the new SR */
        NOP

/* _CPU_Context_1: */
        j       ra
        NOP
ENDFRAME(_CPU_Context_switch)


/*
 *  _CPU_Context_restore
 *
 *  This routine is generally used only to restart self in an
 *  efficient manner.  It may simply be a label in _CPU_Context_switch.
 *
 *  NOTE: May be unnecessary to reload some registers.
 *
 *  void _CPU_Context_restore(
 *    Context_Control *new_context
 *  );
 */

FRAME(_CPU_Context_restore,sp,0,ra)
        .set noreorder
        move    a1,a0
        j       _CPU_Context_switch_restore
        NOP

ENDFRAME(_CPU_Context_restore)

.extern _Thread_Dispatch

/*  void _DBG_Handler()
 *
 *  This routine services the (at least) MIPS1 debug vector,
 *  only used the the hardware debugging features.  This code,
 *  while optional, is best located here because its intrinsically
 *  associated with exceptions in general & thus tied pretty
 *  closely to _ISR_Handler.
 */
FRAME(_DBG_Handler,sp,0,ra)
        .set noreorder
        la      k0,_ISR_Handler
        j       k0
        NOP
        .set reorder
ENDFRAME(_DBG_Handler)

/*  void __ISR_Handler()
 *
 *  This routine provides the RTEMS interrupt management.
 *
 *  void _ISR_Handler()
 *
 *
 *  This discussion ignores a lot of the ugly details in a real
 *  implementation such as saving enough registers/state to be
 *  able to do something real.  Keep in mind that the goal is
 *  to invoke a user's ISR handler which is written in C and
 *  uses a certain set of registers.
 *
 *  Also note that the exact order is to a large extent flexible.
 *  Hardware will dictate a sequence for a certain subset of
 *  _ISR_Handler while requirements for setting
 *
 *  At entry to "common" _ISR_Handler, the vector number must be
 *  available.  On some CPUs the hardware puts either the vector
 *  number or the offset into the vector table for this ISR in a
 *  known place.  If the hardware does not give us this information,
 *  then the assembly portion of RTEMS for this port will contain
 *  a set of distinct interrupt entry points which somehow place
 *  the vector number in a known place (which is safe if another
 *  interrupt nests this one) and branches to _ISR_Handler.
 *
 */

FRAME(_ISR_Handler,sp,0,ra)
        .set noreorder

        /* Q: _ISR_Handler, not using IDT/SIM ...save extra regs? */

        /* wastes a lot of stack space for context?? */
        ADDIU    sp,sp,-EXCP_STACK_SIZE

        STREG ra, R_RA*R_SZ(sp)  /* store ra on the stack */
        STREG v0, R_V0*R_SZ(sp)
        STREG v1, R_V1*R_SZ(sp)
        STREG a0, R_A0*R_SZ(sp)
        STREG a1, R_A1*R_SZ(sp)
        STREG a2, R_A2*R_SZ(sp)
        STREG a3, R_A3*R_SZ(sp)
        STREG t0, R_T0*R_SZ(sp)
        STREG t1, R_T1*R_SZ(sp)
        STREG t2, R_T2*R_SZ(sp)
        STREG t3, R_T3*R_SZ(sp)
        STREG t4, R_T4*R_SZ(sp)
        STREG t5, R_T5*R_SZ(sp)
        STREG t6, R_T6*R_SZ(sp)
        STREG t7, R_T7*R_SZ(sp)
        mflo  t0
        STREG t8, R_T8*R_SZ(sp)
        STREG t0, R_MDLO*R_SZ(sp)
        STREG t9, R_T9*R_SZ(sp)
        mfhi  t0
        STREG gp, R_GP*R_SZ(sp)
        STREG t0, R_MDHI*R_SZ(sp)
        STREG fp, R_FP*R_SZ(sp)

        .set noat
        STREG AT, R_AT*R_SZ(sp)
        .set at

        mfc0     t0,C0_SR
        MFCO     t1,C0_EPC
        STREG    t0,R_SR*R_SZ(sp)
        STREG    t1,R_EPC*R_SZ(sp)


#ifdef INSTRUMENT_EXECUTING_THREAD
        lw t2, THREAD_EXECUTING
        NOP
        sw t2, 0x8001FFF0
#endif

        /* determine if an interrupt generated this exception */

        mfc0     t0,C0_CAUSE
        NOP

        and      t1,t0,CAUSE_EXCMASK
        beq      t1, 0, _ISR_Handler_1

_ISR_Handler_Exception:

        /*  If we return from the exception, it is assumed nothing
         *  bad is going on and we can continue to run normally.
         *  But we want to save the entire CPU context so exception
         *  handlers can look at it and change it.
         *
         *  NOTE: This is the path the debugger stub will take.
         */

        /* already got t0 = cause in the interrupt test above */
        STREG    t0,R_CAUSE*R_SZ(sp)

        STREG    sp, R_SP*R_SZ(sp)

        STREG    s0,R_S0*R_SZ(sp)     /* save s0 - s7 */
        STREG    s1,R_S1*R_SZ(sp)
        STREG    s2,R_S2*R_SZ(sp)
        STREG    s3,R_S3*R_SZ(sp)
        STREG    s4,R_S4*R_SZ(sp)
        STREG    s5,R_S5*R_SZ(sp)
        STREG    s6,R_S6*R_SZ(sp)
        STREG    s7,R_S7*R_SZ(sp)

        /* CP0 special registers */

#if __mips == 1
        mfc0     t0,C0_TAR
#endif
        MFCO     t1,C0_BADVADDR

#if __mips == 1
        STREG    t0,R_TAR*R_SZ(sp)
#else
        NOP
#endif
        STREG    t1,R_BADVADDR*R_SZ(sp)

#if ( CPU_HARDWARE_FP == TRUE )
        mfc0     t0,C0_SR                 /* FPU is enabled, save state */
        NOP
        srl      t0,t0,16
        andi     t0,t0,(SR_CU1 >> 16)
        beqz     t0, 1f
        NOP

        la       a1,R_F0*R_SZ(sp)
        jal      _CPU_Context_save_fp_from_exception
        NOP
        mfc1     t0,C1_REVISION
        mfc1     t1,C1_STATUS
        STREG    t0,R_FEIR*R_SZ(sp)
        STREG    t1,R_FCSR*R_SZ(sp)

1:
#endif

        move     a0,sp
        jal      mips_vector_exceptions
        NOP


        /*
        ** Note, if the exception vector returns, rely on it to have
        ** adjusted EPC so we will return to some correct address.  If
        ** this is not done, we might get stuck in an infinite loop because
        ** we'll return to the instruction where the exception occured and
        ** it could throw again.
        **
        ** It is expected the only code using the exception processing is
        ** either the gdb stub or some user code which is either going to
        ** panic or do something useful.  Regardless, it is up to each
        ** exception routine to properly adjust EPC, so the code below
        ** may be helpful for doing just that.
        */

/* *********************************************************************
** this code follows the R3000's exception return logic, but is not
** needed because the gdb stub does it for us.  It might be useful
** for something else at some point...
**
        * compute the address of the instruction we'll return to *

        LDREG   t1, R_CAUSE*R_SZ(sp)
        LDREG   t0, R_EPC*R_SZ(sp)

        * first see if the exception happened in the delay slot *
        li      t3,CAUSE_BD
        AND     t4,t1,t3
        beqz    t4,excnodelay
        NOP

        * it did, now see if the branch occured or not *
        li      t3,CAUSE_BT
        AND     t4,t1,t3
        beqz    t4,excnobranch
        NOP

        * branch was taken, we resume at the branch target *
        LDREG   t0, R_TAR*R_SZ(sp)
        j       excreturn
        NOP

excnobranch:
        ADDU    t0,R_SZ

excnodelay:
        ADDU    t0,R_SZ

excreturn:
        STREG   t0, R_EPC*R_SZ(sp)
        NOP
********************************************************************* */


 /* if we're returning into mips_break, move to the next instruction */

        LDREG   t0,R_EPC*R_SZ(sp)
        la      t1,mips_break
        xor     t2,t0,t1
        bnez    t2,3f

        addu    t0,R_SZ
        STREG   t0,R_EPC*R_SZ(sp)
        NOP
3:




#if ( CPU_HARDWARE_FP == TRUE )
        mfc0     t0,C0_SR               /* FPU is enabled, restore state */
        NOP
        srl      t0,t0,16
        andi     t0,t0,(SR_CU1 >> 16)
        beqz     t0, 2f
        NOP

        la       a1,R_F0*R_SZ(sp)
        jal      _CPU_Context_restore_fp_from_exception
        NOP
        LDREG    t0,R_FEIR*R_SZ(sp)
        LDREG    t1,R_FCSR*R_SZ(sp)
        mtc1     t0,C1_REVISION
        mtc1     t1,C1_STATUS
2:
#endif
        LDREG    s0,R_S0*R_SZ(sp)    /* restore s0 - s7 */
        LDREG    s1,R_S1*R_SZ(sp)
        LDREG    s2,R_S2*R_SZ(sp)
        LDREG    s3,R_S3*R_SZ(sp)
        LDREG    s4,R_S4*R_SZ(sp)
        LDREG    s5,R_S5*R_SZ(sp)
        LDREG    s6,R_S6*R_SZ(sp)
        LDREG    s7,R_S7*R_SZ(sp)

        /* do NOT restore the sp as this could mess up the world */
        /* do NOT restore the cause as this could mess up the world */

        /*
        ** Jump all the way out.  If theres a pending interrupt, just
        ** let it be serviced later.  Since we're probably using the
        ** gdb stub, we've already disrupted the ISR service timing
        ** anyhow.  We oughtn't mix exception and interrupt processing
        ** in the same exception call in case the exception stuff
        ** might interfere with the dispatching & timer ticks.
        */
        j        _ISR_Handler_exit
        NOP

_ISR_Handler_1:

        mfc0     t1,C0_SR
        and      t0,CAUSE_IPMASK
        and      t0,t1

        /* external interrupt not enabled, ignore */
        /* but if it's not an exception or an interrupt, */
        /* Then where did it come from??? */

        beq      t0,zero,_ISR_Handler_exit
        NOP


  /*
   *  save some or all context on stack
   *  may need to save some special interrupt information for exit
   *
   *  if ( _ISR_Nest_level == 0 )
   *    switch to software interrupt stack
   */


  /*
   *  _ISR_Nest_level++;
   */
        lw      t0,ISR_NEST_LEVEL
        NOP
        add     t0,t0,1
        sw      t0,ISR_NEST_LEVEL
  /*
   *  _Thread_Dispatch_disable_level++;
   */
        lw      t1,THREAD_DISPATCH_DISABLE_LEVEL
        NOP
        add     t1,t1,1
        sw      t1,THREAD_DISPATCH_DISABLE_LEVEL

  /*
   *  Call the CPU model or BSP specific routine to decode the
   *  interrupt source and actually vector to device ISR handlers.
   */

#ifdef INSTRUMENT_ISR_VECTORING
        NOP
        li      t1, 1
        sw      t1, 0x8001e000
#endif

        move     a0,sp
        jal      mips_vector_isr_handlers
        NOP

#ifdef INSTRUMENT_ISR_VECTORING
        li      t1, 0
        sw      t1, 0x8001e000
        NOP
#endif

  /*
   *  --_ISR_Nest_level;
   */
        lw      t2,ISR_NEST_LEVEL
        NOP
        add     t2,t2,-1
        sw      t2,ISR_NEST_LEVEL
  /*
   *  --_Thread_Dispatch_disable_level;
   */
        lw      t1,THREAD_DISPATCH_DISABLE_LEVEL
        NOP
        add     t1,t1,-1
        sw      t1,THREAD_DISPATCH_DISABLE_LEVEL
  /*
   *  if ( _Thread_Dispatch_disable_level || _ISR_Nest_level )
   *    goto the label "exit interrupt (simple case)"
   */
        or  t0,t2,t1
        bne t0,zero,_ISR_Handler_exit
        NOP


  /*
   *  restore stack
   *
   *  if !_Thread_Dispatch_necessary 
   *    goto the label "exit interrupt (simple case)"
   */
        lbu     t0,DISPATCH_NEEDED
        NOP
        or      t0,t0,t0
        beq     t0,zero,_ISR_Handler_exit
        NOP



#ifdef INSTRUMENT_EXECUTING_THREAD
        lw      t0,THREAD_EXECUTING
        NOP
        sw      t0,0x8001FFF4
#endif

/*
** Turn on interrupts before entering Thread_Dispatch which
** will run for a while, thus allowing new interrupts to
** be serviced.  Observe the Thread_Dispatch_disable_level interlock
** that prevents recursive entry into Thread_Dispatch.
*/

        mfc0    t0, C0_SR
#if __mips == 1

        li      t1,SR_IEC
        or      t0, t1

#elif (__mips == 3) || (__mips == 32)

        /*
        ** clear XL and set IE so we can get interrupts.
        */
        li      t1, SR_EXL
        not     t1
        and     t0,t1
        or      t0, SR_IE

#endif
        mtc0    t0, C0_SR
        NOP

        /* save off our stack frame so the context switcher can get to it */
        la      t0,__exceptionStackFrame
        STREG   sp,(t0)

        jal     _Thread_Dispatch
        NOP

        /*
        ** And make sure its clear in case we didn't dispatch.  if we did, its
        ** already cleared
        */
        la      t0,__exceptionStackFrame
        STREG   zero,(t0)
        NOP

/*
** turn interrupts back off while we restore context so
** a badly timed interrupt won't mess things up
*/
        mfc0    t0, C0_SR

#if __mips == 1

        /* ints off, current & prev kernel mode on (kernel mode enabled is bit clear..argh!) */
        li      t1,SR_IEC | SR_KUP | SR_KUC
        not     t1
        and     t0, t1
        mtc0    t0, C0_SR
        NOP

#elif (__mips == 3) || (__mips == 32)

        /* make sure EXL and IE are set so ints are disabled & we can update EPC for the return */
        li   t1,SR_IE           /* Clear IE first (recommended) */
        not  t1
        and  t0,t1
        mtc0 t0,C0_SR
        NOP
  
        /* apply task's SR with EXL set so the eret will return properly */
        or      t0, SR_EXL | SR_IE
        mtc0    t0, C0_SR
        NOP

        /* store new EPC value, which we can do since EXL=0 */
        LDREG   t0, R_EPC*R_SZ(sp)
        NOP
        MTCO    t0, C0_EPC
        NOP
 
#endif






#ifdef INSTRUMENT_EXECUTING_THREAD
        lw      t0,THREAD_EXECUTING
        NOP
        sw      t0,0x8001FFF8
#endif


  /*
   *  prepare to get out of interrupt
   *  return from interrupt
   *
   *  LABEL "exit interrupt (simple case):"
   *  prepare to get out of interrupt
   *  return from interrupt
   */

_ISR_Handler_exit:
/*
** Skip the SR restore because its a global register. _CPU_Context_switch_restore
** adjusts it according to each task's configuration.  If we didn't dispatch, the
** SR value isn't changed, so all we need to do is return.
**
*/
        /* restore context from stack */

#ifdef INSTRUMENT_EXECUTING_THREAD
        lw      t0,THREAD_EXECUTING
        NOP
        sw      t0, 0x8001FFFC
#endif

        LDREG t8, R_MDLO*R_SZ(sp)
        LDREG t0, R_T0*R_SZ(sp)
        mtlo  t8
        LDREG t8, R_MDHI*R_SZ(sp)
        LDREG t1, R_T1*R_SZ(sp)
        mthi  t8
        LDREG t2, R_T2*R_SZ(sp)
        LDREG t3, R_T3*R_SZ(sp)
        LDREG t4, R_T4*R_SZ(sp)
        LDREG t5, R_T5*R_SZ(sp)
        LDREG t6, R_T6*R_SZ(sp)
        LDREG t7, R_T7*R_SZ(sp)
        LDREG t8, R_T8*R_SZ(sp)
        LDREG t9, R_T9*R_SZ(sp)
        LDREG gp, R_GP*R_SZ(sp)
        LDREG fp, R_FP*R_SZ(sp)
        LDREG ra, R_RA*R_SZ(sp)
        LDREG a0, R_A0*R_SZ(sp)
        LDREG a1, R_A1*R_SZ(sp)
        LDREG a2, R_A2*R_SZ(sp)
        LDREG a3, R_A3*R_SZ(sp)
        LDREG v1, R_V1*R_SZ(sp)
        LDREG v0, R_V0*R_SZ(sp)

#if __mips == 1
        LDREG     k1, R_EPC*R_SZ(sp)
#endif

        .set noat
        LDREG     AT, R_AT*R_SZ(sp)
        .set at

        ADDIU     sp,sp,EXCP_STACK_SIZE

#if (__mips == 3) || (__mips == 32)
        eret
#elif __mips == 1
        j         k1
        rfe
#endif
        NOP

       .set    reorder
ENDFRAME(_ISR_Handler)


FRAME(mips_break,sp,0,ra)
        .set noreorder
        break   0x0     /* this statement must be first in this function, assumed so by mips-stub.c */
        NOP
        j       ra
        NOP
       .set    reorder
ENDFRAME(mips_break)