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ARMv7M: Improve exception handler routine and add comments on SP selection

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This patch adds a brief description of how context state is saved into the
SP on exception entry, and makes a few changes to _ARMV7M_Exception_default
in order to make it a bit more efficient. I also removed the unused 'v7mfsz'
input parameter.

This should apply over Sudarshan's patch.
This commit is contained in:
Martin Galvan
2015-09-18 18:53:31 -03:00
committed by Sebastian Huber
parent 7263a50d6c
commit f52885b6bc
1 changed files with 21 additions and 11 deletions
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32
cpukit/score/cpu/arm/armv7m-exception-default.c
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@@ -22,20 +22,31 @@
void __attribute__((naked)) _ARMV7M_Exception_default( void )
{
/* On exception entry, ARMv7M saves context state onto a stack pointed to
* by either MSP or PSP. The value stored in LR indicates whether we were
* in Thread or Handler mode, whether we were using the FPU (if any),
* and which stack pointer we were using.
* In particular, bit 2 of LR will be 0 if we were using MSP.
*
* For a more detailed explanation, see the Exception Entry Behavior
* section of the ARMv7M Architecture Reference Manual.
*/
/* As we're in Handler mode here, we'll always operate on MSP.
* However, we need to store the right SP in our CPU_Exception_frame.
*/
__asm__ volatile (
"sub sp, %[cpufsz]\n"
"sub sp, %[cpufsz]\n" /* Allocate space for a CPU_Exception_frame. */
"stm sp, {r0-r12}\n"
"mov r2, lr\n"
"mrs r1, msp\n"
"mrs r0, psp\n"
"tst lr, #4\n"
"itt eq\n"
"moveq r0, r1\n"
"addeq r0, %[cpufsz]\n"
"tst lr, #4\n" /* Check if bit 2 of LR is zero. If so, PSR.Z = 1 */
"itte eq\n" /* IF bit 2 of LR is zero... (PSR.Z == 1) */
"mrseq r0, msp\n" /* THEN we were using MSP. */
"addeq r0, %[cpufsz]\n" /* THEN, set r0 = old MSP value. */
"mrsne r0, psp\n" /* ELSE it's not zero; we were using PSP. */
"add r2, r0, %[v7mlroff]\n"
"add r1, sp, %[cpulroff]\n"
"ldm r2, {r3-r5}\n"
"stm r1, {r3-r5}\n"
"ldm r2, {r3-r5}\n" /* Grab LR, PC and PSR from the stack.. */
"stm r1, {r3-r5}\n" /* ..and store them in our CPU_Exception_frame. */
"mrs r1, ipsr\n"
"str r1, [sp, %[cpuvecoff]]\n"
@@ -74,7 +85,6 @@ void __attribute__((naked)) _ARMV7M_Exception_default( void )
"b _ARM_Exception_default\n"
:
: [cpufsz] "i" (sizeof(CPU_Exception_frame)),
[v7mfsz] "i" (sizeof(ARMV7M_Exception_frame)),
[cpulroff] "i" (offsetof(CPU_Exception_frame, register_lr)),
[v7mlroff] "i" (offsetof(ARMV7M_Exception_frame, register_lr)),
[cpuvecoff] "J" (offsetof(CPU_Exception_frame, vector)),