# HG changeset patch
# User Paul Boddie <paul@boddie.org.uk>
# Date 1510873196 -3600
# Node ID 7edceda19310c46d591e96bec2eb73131b90079e
# Parent  0c6e88eb049f2a1f53ee953f65dd66fd923b904a
Reorganised interrupt handling to only test either timer or DMA interrupt
conditions, not both, also merging the execution of the visible/active region
and address update routines within a single call.

diff -r 0c6e88eb049f -r 7edceda19310 vga.S
--- a/vga.S	Tue Nov 07 14:38:33 2017 +0100
+++ b/vga.S	Thu Nov 16 23:59:56 2017 +0100
@@ -709,40 +709,29 @@
 
 	li $sp, IRQ_STACK_TOP
 
-	/* Check for a timer interrupt condition. */
-
-	la $v0, IFS0
-	lw $v1, 0($v0)
-	andi $v1, $v1, (1 << 9)	/* T2IF */
-	beqz $v1, irq_dma
-	nop
-
-	/* Clear the timer interrupt condition. */
-
-	sw $v1, CLR($v0)
-
 	/*
 	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.
-	
-	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.
+
+	Here, we deliberately ignore the timer condition during the visible/
+	active region.
+
+	The DMA interrupt should only be active within the visible region.
 	*/
 
 	la $t8, visible_active
 	beq $s1, $t8, irq_dma
 	nop
 
-	/* Increment the line counter (only outside the visible region). */
-
-	addiu $s0, $s0, 1
+	/* Check for a timer interrupt condition. */
 
-	/* Jump to the event handler (only outside the visible region). */
+	la $v0, IFS0
+	lw $v1, 0($v0)
+	andi $v1, $v1, (1 << 9)	/* T2IF */
+	beqz $v1, irq_exit
+	nop
 
-	jalr $s1
+	j irq_handle
 	nop
 
 irq_dma:
@@ -767,30 +756,20 @@
 	beqz $v1, irq_exit
 	nop
 
-	/* Clear the block transfer completion interrupt flag. */
+irq_handle:
+	/* Clear the interrupt condition. */
 
 	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). */
+	/* Increment the line counter. */
 
 	addiu $s0, $s0, 1
 
-	/* Jump to the event handler (only within the visible region). */
+	/* Jump to the event handler. */
 
 	jalr $s1
 	nop
 
-	/* Jump to the DMA update routine. */
-
-	j visible_update_address
-	nop
-
 irq_exit:
 	/* Save state. */
 
@@ -920,7 +899,7 @@
 	/* Test for front porch. */
 
 	sltiu $v0, $s0, VFP_START
-	bnez $v0, _visible_active_ret
+	bnez $v0, visible_update_address
 	nop
 
 	/* Start the front porch region. */
@@ -943,6 +922,12 @@
 	li $v1, (1 << 9)
 	sw $v1, SET($v0)	/* T2IE = 1 */
 
+	/* Disable the line channel. */
+
+	la $v0, DCH0ECON
+	li $v1, (1 << 4)	/* DCH0ECON<4> = SIRQEN = 0 */
+	sw $v1, CLR($v0)
+
 _visible_active_ret:
 	jr $ra
 	nop
@@ -953,23 +938,6 @@
 
 visible_update_address:
 
-	/* Test for the last visible line. */
-
-	la $v0, vfp_active
-	bne $s1, $v0, _visible_update_address
-	nop
-
-	/* Disable the line channel. */
-
-	la $v0, DCH0ECON
-	li $v1, (1 << 4)	/* DCH0ECON<4> = SIRQEN = 0 */
-	sw $v1, CLR($v0)
-
-	j _visible_update_ret
-	nop
-
-_visible_update_address:
-
 	/*
 	Update the line data address if the line counter (referring to the
 	next line) is even.
@@ -1002,7 +970,7 @@
 	sw $s2, 0($v0)
 
 _visible_update_ret:
-	j irq_exit
+	jr $ra
 	nop