From f52885b6bc246e090cc66fe9ef92d26db2bca646 Mon Sep 17 00:00:00 2001 From: Martin Galvan Date: Fri, 18 Sep 2015 18:53:31 -0300 Subject: ARMv7M: Improve exception handler routine and add comments on SP selection 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. --- cpukit/score/cpu/arm/armv7m-exception-default.c | 32 ++++++++++++++++--------- 1 file changed, 21 insertions(+), 11 deletions(-) diff --git a/cpukit/score/cpu/arm/armv7m-exception-default.c b/cpukit/score/cpu/arm/armv7m-exception-default.c index 2ddc6fc772..0e4efad263 100644 --- a/cpukit/score/cpu/arm/armv7m-exception-default.c +++ b/cpukit/score/cpu/arm/armv7m-exception-default.c @@ -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)), -- cgit v1.2.3