# HG changeset patch
# User Paul Boddie <paul@boddie.org.uk>
# Date 1496520276 -7200
# Node ID b106107349679a1bf2805d3592138a89157a7573
# Parent  e3ec1c594d8e4a7325729a0dd996b007b7541e8f
Created a separate DMA address update routine. Note that it does not use $ra
since this appears to cause an exception somehow.

diff -r e3ec1c594d8e -r b10610734967 vga.S
--- a/vga.S	Sat Jun 03 19:13:09 2017 +0200
+++ b/vga.S	Sat Jun 03 22:04:36 2017 +0200
@@ -675,6 +675,7 @@
 	nop
 
 
+
 /* Interrupt servicing. */
 
 .org 0x200
@@ -753,47 +754,10 @@
 	li $v1, (1 << 3)	/* CHBCIF = 0 */
 	sw $v1, CLR($v0)
 
-	/*
-	Update the line data address if the line counter (referring to the
-	next line) is even.
-	*/
-
-	andi $t8, $s0, 1
-	bnez $t8, irq_exit
-	nop
-
-	/* Reference the next line and update the DMA source address. */
-
-	addiu $s2, $s2, LINE_LENGTH
-
-	/* Test for wraparound. */
-
-	li $t8, (SCREEN_BASE + SCREEN_SIZE)
-	sltu $t8, $s2, $t8
-	bnez $t8, irq_dma_update
-	nop
+	/* Jump to the DMA update routine. */
 
-	/* Reset the source address. */
-
-	li $s2, SCREEN_BASE
-
-irq_dma_update:
-
-	/* Disable line channel. */
-
-	la $v0, DCH0CON
-	li $v1, 0b10000000
-	sw $v1, CLR($v0)
-
-	/* Update the source address. */
-
-	la $v0, DCH0SSA
-	sw $s2, 0($v0)
-
-	/* Enable line channel. */
-
-	la $v0, DCH0CON
-	sw $v1, SET($v0)
+	j visible_update_address
+	nop
 
 irq_exit:
 	/* Save state. */
@@ -823,7 +787,7 @@
 
 exc_handler:
 	li $t9, 0x80000000
-	mfc0 $t6, CP0_CAUSE
+	mfc0 $t6, CP0_ERROREPC
 	nop
 exc_loop:
 	and $t7, $t9, $t6
@@ -924,6 +888,58 @@
 
 
 
+/* DMA update routine. */
+
+visible_update_address:
+
+	/*
+	Update the line data address if the line counter (referring to the
+	next line) is even.
+	*/
+
+	andi $t8, $s0, 1
+	bnez $t8, _visible_update_ret
+	nop
+
+	/* Reference the next line and update the DMA source address. */
+
+	addiu $s2, $s2, LINE_LENGTH
+
+	/* Test for wraparound. */
+
+	li $t8, (SCREEN_BASE + SCREEN_SIZE)
+	sltu $t8, $s2, $t8
+	bnez $t8, _visible_dma_update
+	nop
+
+	/* Reset the source address. */
+
+	li $s2, SCREEN_BASE
+
+_visible_dma_update:
+
+	/* Disable line channel. */
+
+	la $v0, DCH0CON
+	li $v1, 0b10000000
+	sw $v1, CLR($v0)
+
+	/* Update the source address. */
+
+	la $v0, DCH0SSA
+	sw $s2, 0($v0)
+
+	/* Enable line channel. */
+
+	la $v0, DCH0CON
+	sw $v1, SET($v0)
+
+_visible_update_ret:
+	j irq_exit
+	nop
+
+
+
 /* Within the vertical front porch. */
 
 vfp_active: