# HG changeset patch
# User Paul Boddie <paul@boddie.org.uk>
# Date 1506715811 -7200
# Node ID 70fc39a067073463413eca7de481a871c9193009
# Parent  466e83cd82b7b2234be2006fd13eb5b1d85f7234
Added some comments.

diff -r 466e83cd82b7 -r 70fc39a06707 vga.S
--- a/vga.S	Sat Jun 03 22:53:06 2017 +0200
+++ b/vga.S	Fri Sep 29 22:10:11 2017 +0200
@@ -69,8 +69,14 @@
 
 	la $v0, BMXCON
 	lw $v1, 0($v0)
+
+	/* Set zero wait states for address setup. */
+
 	li $t8, ~(1 << 6)	/* BMXCON<6> = BMXWSDRM = 0 */
 	and $v1, $v1, $t8
+
+	/* Set bus arbitration mode. */
+
 	li $t8, ~0b111		/* BMXCON<2:0> = BMXARB<2:0> = 0 */
 	ori $t8, $t8, 0b010	/* BMXCON<2:0> = BMXARB<2:0> = 2 */
 	and $v1, $v1, $t8
@@ -307,7 +313,8 @@
 suitable start and end values.
 
 Using OC2, Timer 2 triggers a level shifting event and OC2 is reconfigured to
-reverse the level at a later point.
+reverse the level at a later point. In this way, the vsync pulse is generated
+and is synchronised to the display lines.
 */
 
 init_oc:
@@ -369,7 +376,6 @@
 	jr $ra
 	nop
 
-
 init_oc_pins:
 	/* Unlock the configuration register bits. */
 
@@ -718,8 +724,8 @@
 	sw $v1, CLR($v0)
 
 	/*
-	The timer interrupt will only occur active outside the visible region,
-	but the interrupt condition will still occur as the timer wraps around.
+	The timer interrupt will only occur 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.
 	
@@ -881,10 +887,10 @@
 	la $v0, DCH0SSA
 	sw $s2, 0($v0)
 
-	/* Enable the line channel. */
+	/* Enable the line channel for timer event transfer initiation. */
 
 	la $v0, DCH0ECON
-	li $v1, (1 << 4)
+	li $v1, (1 << 4)	/* DCH0ECON<4> = SIRQEN = 1 */
 	sw $v1, SET($v0)
 
 	/* Disable the timer interrupt during the visible period. */
@@ -943,7 +949,7 @@
 	/* Disable the line channel. */
 
 	la $v0, DCH0ECON
-	li $v1, (1 << 4)
+	li $v1, (1 << 4)	/* DCH0ECON<4> = SIRQEN = 0 */
 	sw $v1, CLR($v0)
 
 	j _visible_update_ret