2001-05-14 Till Straumann <strauman@slac.stanford.edu>

	* rtems/new-exceptions/cpu.h: Per PR211 fix
	saving/restoring floating point context.  The fpsave and fprestore
	routines are only used in a executing context which _is_ fp and hence
	has the FPU enabled. The current behavior required the FPU always to
	be on which is very dangerous if lazy context switching is used.
	[Joel Note: Some ports explicitly enabled the FPU in the FP save and
	restore routines to avoid this.]

	The patch also makes sure (on powerpc only) that the FPU is disabled
	for integer tasks. Note that this is crucial if deferred fp context
	switching is used. Otherwise, fp context corruption may go undetected!
	Also note that even tasks which merely push/pop FP registers to/from
	the stack without modifying them still MUST be FP tasks - otherwise
	(if lazy FP context switching is used), FP register corruption (of
	other, FP, tasks may occur)!

	Furthermore, (on PPC) by default, lazy FP context save/restore
	is _disabled_.

1 files changed, 8 insertions, 1 deletions

diff --git a/c/src/exec/score/cpu/powerpc/rtems/new-exceptions/cpu.h b/c/src/exec/score/cpu/powerpc/rtems/new-exceptions/cpu.h
index 371626cd4b..6b30c0ed0b 100644
--- a/c/src/exec/score/cpu/powerpc/rtems/new-exceptions/cpu.h
+++ b/c/src/exec/score/cpu/powerpc/rtems/new-exceptions/cpu.h
@@ -228,12 +228,19 @@ extern "C" {
  *  until a context switch is made to another, different FP task.
  *  Thus in a system with only one FP task, the FP context will never
  *  be saved or restored.
+ *
+ *  Note, however that compilers may use floating point registers/
+ *  instructions for optimization or they may save/restore FP registers
+ *  on the stack. You must not use deferred switching in these cases
+ *  and on the PowerPC attempting to do so will raise a "FP unavailable"
+ *  exception.
  */
 /*
  *  ACB Note:  This could make debugging tricky..
  */
 
-#define CPU_USE_DEFERRED_FP_SWITCH       TRUE
+/* conservative setting (FALSE); probably doesn't affect performance too much */
+#define CPU_USE_DEFERRED_FP_SWITCH       FALSE
 
 /*
  *  Does this port provide a CPU dependent IDLE task implementation?