# HG changeset patch
# User Paul Boddie <paul@boddie.org.uk>
# Date 1496523186 -7200
# Node ID 466e83cd82b7b2234be2006fd13eb5b1d85f7234
# Parent  2d185cb3e4373abfe8f6b002e43635b10158b6dd
Disable the timer interrupt in order to reduce memory contention with the line
channel DMA, itself initiated by the timer wraparound event.

diff -r 2d185cb3e437 -r 466e83cd82b7 vga.S
--- a/vga.S	Sat Jun 03 22:50:28 2017 +0200
+++ b/vga.S	Sat Jun 03 22:53:06 2017 +0200
@@ -717,11 +717,26 @@
 	li $v1, (1 << 9)	/* IFS0<9> = T2IF = 0 */
 	sw $v1, CLR($v0)
 
-	/* Increment the line counter. */
+	/*
+	The timer interrupt will only occur active outside the visible region,
+	but the interrupt condition will still occur as the timer wraps around.
+	Therefore, the handling of other interrupts may find the timer interrupt
+	condition set.
+	
+	For the visible region, the event handler is invoked when handling the
+	DMA interrupt. Otherwise, the event handler is invoked in response to
+	the timer interrupt.
+	*/
+
+	la $t8, visible_active
+	beq $s1, $t8, irq_dma
+	nop
+
+	/* Increment the line counter (only outside the visible region). */
 
 	addiu $s0, $s0, 1
 
-	/* Jump to the event handler. */
+	/* Jump to the event handler (only outside the visible region). */
 
 	jalr $s1
 	nop
@@ -754,6 +769,21 @@
 	li $v1, (1 << 3)	/* CHBCIF = 0 */
 	sw $v1, CLR($v0)
 
+	/*
+	The DMA interrupt should only be active within the visible region.
+	The event handler is invoked here instead of in response to a timer
+	interrupt within that region.
+	*/
+
+	/* Increment the line counter (only within the visible region). */
+
+	addiu $s0, $s0, 1
+
+	/* Jump to the event handler (only within the visible region). */
+
+	jalr $s1
+	nop
+
 	/* Jump to the DMA update routine. */
 
 	j visible_update_address
@@ -857,6 +887,12 @@
 	li $v1, (1 << 4)
 	sw $v1, SET($v0)
 
+	/* Disable the timer interrupt during the visible period. */
+
+	la $v0, IEC0
+	li $v1, (1 << 9)
+	sw $v1, CLR($v0)	/* T2IE = 0 */
+
 _vbp_active_ret:
 	jr $ra
 	nop
@@ -876,6 +912,18 @@
 
 	la $s1, vfp_active
 
+	/* Clear the timer interrupt condition. */
+
+	la $v0, IFS0
+	li $v1, (1 << 9)	/* IFS0<9> = T2IF = 0 */
+	sw $v1, CLR($v0)
+
+	/* Re-enable the timer interrupt after the visible period. */
+
+	la $v0, IEC0
+	li $v1, (1 << 9)
+	sw $v1, SET($v0)	/* T2IE = 1 */
+
 _visible_active_ret:
 	jr $ra
 	nop