# HG changeset patch
# User Paul Boddie <paul@boddie.org.uk>
# Date 1540919978 -3600
# Node ID f9acc9c732f176764387f35589a77b4df1dc4105
# Parent  32cb6b232444bb6a0b045a6dbcfe94bb07e481b3
Fixed invocation of the interrupt handler function, avoiding $gp usage. The
absence of any computation of $gp in the handler routine meant that where such
computations (in function prologues, for instance) were being interrupted, a
corrupt $gp was being used, causing an exception.
Changed the exception handler routine to switch to the IRQ stack and to use the
appropriate calling convention.

diff -r 32cb6b232444 -r f9acc9c732f1 lib/cpu.S
--- a/lib/cpu.S	Mon Oct 29 01:46:31 2018 +0100
+++ b/lib/cpu.S	Tue Oct 30 18:19:38 2018 +0100
@@ -21,6 +21,9 @@
 #include "pic32.h"
 #include "cpu.h"
 
+#define IRQ_STACK_LIMIT	(KSEG0_BASE + IRQ_STACK_SIZE)
+#define IRQ_STACK_TOP	(IRQ_STACK_LIMIT - 34 * 4)
+
 .globl enable_interrupts
 .globl handle_error_level
 .globl init_interrupts
@@ -126,7 +129,19 @@
 .org 0x180
 
 exc_handler:
-        j exception_handler
+	move $k0, $sp
+	move $k1, $t9
+
+	/* Switch to the IRQ stack. */
+
+	lui $sp, %hi(IRQ_STACK_TOP)
+	ori $sp, $sp, %lo(IRQ_STACK_TOP)
+
+	/* Obtain the address directly to avoid initialising $gp here. */
+
+	lui $t9, %hi(exception_handler)
+	ori $t9, $t9, %lo(exception_handler)
+	jr $t9
         nop
 
 
@@ -136,9 +151,6 @@
 .org 0x200
 .set noat
 
-#define IRQ_STACK_LIMIT	(KSEG0_BASE + IRQ_STACK_SIZE)
-#define IRQ_STACK_TOP	(IRQ_STACK_LIMIT - 34 * 4)
-
 int_handler:
 
         /* Store affected registers from IRQ_STACK_LIMIT - 4 downwards. */
@@ -163,7 +175,11 @@
         lui $sp, %hi(IRQ_STACK_TOP)
         ori $sp, $sp, %lo(IRQ_STACK_TOP)
 
-	jal interrupt_handler
+	/* Obtain the address directly to avoid initialising $gp here. */
+
+	lui $t9, %hi(interrupt_handler)
+	ori $t9, $t9, %lo(interrupt_handler)
+	jalr $t9
 	nop
 
         /* Restore affected registers. */