Add ehb instructions back into PIC32 port layer (upon advice).

Add configCLEAR_TICK_TIMER_INTERRUPT into PIC32 port layer to allow the timer configuration to be changed without any edits to the port layer being required.
Add prvTaskExitError() into the PIC32 port layer to trap tasks that attempt to exit from their implementing function.
Provide the ability to trap interrupt stack overflows in the PIC32 port.
Radically improve the timing in the Win32 simulator port layer.

FreeRTOS/Source/portable/MPLAB/PIC32MX/port_asm.S

@@ -1,5 +1,5 @@
 /*
-    FreeRTOS V7.5.3 - Copyright (C) 2013 Real Time Engineers Ltd. 
+    FreeRTOS V7.5.3 - Copyright (C) 2013 Real Time Engineers Ltd.
     All rights reserved
 
     VISIT http://www.FreeRTOS.org TO ENSURE YOU ARE USING THE LATEST VERSION.
@@ -116,29 +116,27 @@ vPortStartFirstTask:
 
 /*******************************************************************/
 
- 	.set		noreorder
+	.set		noreorder
 	.set 		noat
- 	.ent		vPortYieldISR
+	.ent		vPortYieldISR
 
 vPortYieldISR:
 
 	/* Make room for the context. First save the current status so it can be
 	manipulated, and the cause and EPC registers so thier original values are
 	captured. */
-	mfc0		k0, _CP0_CAUSE
-	addiu		sp,	sp, -portCONTEXT_SIZE
+	addiu		sp, sp, -portCONTEXT_SIZE
 	mfc0		k1, _CP0_STATUS
 
-	/* Also save s6 and s5 so we can use them during this interrupt.  Any
-	nesting interrupts should maintain the values of these registers
-	across the ISR. */
+	/* Also save s6 and s5 so they can be used.  Any nesting interrupts should
+	maintain the values of these registers across the ISR. */
 	sw			s6, 44(sp)
 	sw			s5, 40(sp)
 	sw			k1, portSTATUS_STACK_LOCATION(sp)
 
-	/* Interrupts above the kernel priority are going to be re-enabled. */
-	srl			k0, k0, 0xa
-	ins 		k1, k0, 10, 6
+	/* Prepare to re-enabled interrupt above the kernel priority. */
+	ins 		k1, zero, 10, 6
+	ori			k1, k1, ( configMAX_SYSCALL_INTERRUPT_PRIORITY << 10 )
 	ins			k1, zero, 1, 4
 
 	/* s5 is used as the frame pointer. */
@@ -159,16 +157,16 @@ vPortYieldISR:
 	after interrupts are enabled. */
 	mfc0 		s6, _CP0_EPC
 
-	/* Re-enable interrupts. */
+	/* Re-enable interrupts above configMAX_SYSCALL_INTERRUPT_PRIORITY. */
 	mtc0		k1, _CP0_STATUS
 
 	/* Save the context into the space just created.  s6 is saved again
 	here as it now contains the EPC value. */
-	sw			ra,	120(s5)
+	sw			ra, 120(s5)
 	sw			s8, 116(s5)
 	sw			t9, 112(s5)
-	sw			t8,	108(s5)
-	sw			t7,	104(s5)
+	sw			t8, 108(s5)
+	sw			t7, 104(s5)
 	sw			t6, 100(s5)
 	sw			t5, 96(s5)
 	sw			t4, 92(s5)
@@ -185,7 +183,7 @@ vPortYieldISR:
 	sw			s7, 48(s5)
 	sw			s6, portEPC_STACK_LOCATION(s5)
 	/* s5 and s6 has already been saved. */
-	sw			s4,	36(s5)
+	sw			s4, 36(s5)
 	sw			s3, 32(s5)
 	sw			s2, 28(s5)
 	sw			s1, 24(s5)
@@ -209,18 +207,21 @@ vPortYieldISR:
 	is below configMAX_SYSCALL_INTERRUPT_PRIORITY - so this can only ever
 	raise the IPL value and never lower it. */
 	di
+	ehb
 	mfc0		s7, _CP0_STATUS
-	ins 		s7, $0, 10, 6
+	ins 		s7, zero, 10, 6
 	ori			s6, s7, ( configMAX_SYSCALL_INTERRUPT_PRIORITY << 10 ) | 1
 
 	/* This mtc0 re-enables interrupts, but only above
 	configMAX_SYSCALL_INTERRUPT_PRIORITY. */
 	mtc0		s6, _CP0_STATUS
+	ehb
 
 	/* Clear the software interrupt in the core. */
 	mfc0		s6, _CP0_CAUSE
 	ins			s6, zero, 8, 1
 	mtc0		s6, _CP0_CAUSE
+	ehb
 
 	/* Clear the interrupt in the interrupt controller. */
 	la			s6, IFS0CLR
@@ -232,6 +233,7 @@ vPortYieldISR:
 
 	/* Clear the interrupt mask again.  The saved status value is still in s7. */
 	mtc0		s7, _CP0_STATUS
+	ehb
 
 	/* Restore the stack pointer from the TCB. */
 	la			s0, pxCurrentTCB
@@ -273,6 +275,7 @@ vPortYieldISR:
 
 	/* Protect access to the k registers, and others. */
 	di
+	ehb
 
 	/* Set nesting back to zero.  As the lowest priority interrupt this
 	interrupt cannot have nested. */
@@ -290,10 +293,11 @@ vPortYieldISR:
 	lw			k0, portEPC_STACK_LOCATION(sp)
 
 	/* Remove stack frame. */
-	addiu		sp,	sp,	portCONTEXT_SIZE
+	addiu		sp, sp, portCONTEXT_SIZE
 
 	mtc0		k1, _CP0_STATUS
 	mtc0 		k0, _CP0_EPC
+	ehb
 	eret
 	nop