# HG changeset patch
# User Paul Boddie <paul@boddie.org.uk>
# Date 1496001722 -7200
# Node ID 647180a1278b44b2a5f0ce1cb000f49496db5521
# Parent  124757d0843fb23c8a93c30daa854238ed8aad2e
Added register and display state saving and retrieval in the interrupt handler.

diff -r 124757d0843f -r 647180a1278b display.c
--- a/display.c	Sun May 28 19:11:01 2017 +0200
+++ b/display.c	Sun May 28 22:02:02 2017 +0200
@@ -23,7 +23,7 @@
 
 void init_framebuffer(uint32_t *data)
 {
-	uint32_t *addr = (uint32_t *) KSEG0_BASE;
+	uint32_t *addr = (uint32_t *) SCREEN_BASE_KSEG0;
 	uint16_t x, y;
 
 	for (y = 0; y < LINE_COUNT; y++)
@@ -39,7 +39,7 @@
 
 void init_framebuffer_with_pattern()
 {
-	uint32_t *addr = (uint32_t *) KSEG0_BASE;
+	uint32_t *addr = (uint32_t *) SCREEN_BASE_KSEG0;
 	uint32_t base, value;
 	uint16_t x, y;
 	uint8_t row, offset;
diff -r 124757d0843f -r 647180a1278b vga.S
--- a/vga.S	Sun May 28 19:11:01 2017 +0200
+++ b/vga.S	Sun May 28 22:02:02 2017 +0200
@@ -140,8 +140,16 @@
 
 	li $s0, 0		/* line counter */
 	la $s1, vbp_active	/* current event */
-	move $s2, $zero		/* line address */
-	move $s3, $zero		/* screen address */
+	li $s2, SCREEN_BASE	/* line address */
+	li $s3, SCREEN_BASE	/* screen address */
+
+	/* Save the state for retrieval in the interrupt handler. */
+
+	li $k0, IRQ_STACK_LIMIT
+	sw $s0, -44($k0)
+	sw $s1, -48($k0)
+	sw $s2, -52($k0)
+	sw $s3, -56($k0)
 
 	/* Enable interrupts and loop. */
 
@@ -482,7 +490,8 @@
 	*/
 
 	la $v0, DCH0SSA
-	sw $zero, 0($v0)
+	li $v1, SCREEN_BASE
+	sw $v1, 0($v0)
 
 	/*
 	For the reset channel, a single byte of zero is transferred:
@@ -631,6 +640,28 @@
 
 interrupt_handler:
 
+	/* Store affected registers. */
+
+	li $k0, IRQ_STACK_LIMIT
+	sw $v0, -4($k0)
+	sw $v1, -8($k0)
+	sw $s0, -12($k0)
+	sw $s1, -16($k0)
+	sw $s2, -20($k0)
+	sw $s3, -24($k0)
+	sw $t8, -28($k0)
+	sw $ra, -32($k0)
+	sw $sp, -36($k0)
+
+	/* Load state. */
+
+	lw $s0, -44($k0)
+	lw $s1, -48($k0)
+	lw $s2, -52($k0)
+	lw $s3, -56($k0)
+
+	li $sp, IRQ_STACK_TOP
+
 	/* Check for a timer interrupt condition. */
 
 	la $v0, IFS0
@@ -695,14 +726,14 @@
 
 	/* Test for wraparound. */
 
-	li $t8, SCREEN_SIZE
+	li $t8, (SCREEN_BASE + SCREEN_SIZE)
 	sltu $t8, $s2, $t8
 	bnez $t8, irq_dma_update
 	nop
 
 	/* Reset the source address. */
 
-	move $s2, $zero
+	li $s2, SCREEN_BASE
 
 irq_dma_update:
 
@@ -718,6 +749,26 @@
 	sw $v1, CLR($v0)
 
 irq_exit:
+	/* Save state. */
+
+	li $k0, IRQ_STACK_LIMIT
+	sw $s0, -44($k0)
+	sw $s1, -48($k0)
+	sw $s2, -52($k0)
+	sw $s3, -56($k0)
+
+	/* Restore affected registers. */
+
+	lw $v0, -4($k0)
+	lw $v1, -8($k0)
+	lw $s0, -12($k0)
+	lw $s1, -16($k0)
+	lw $s2, -20($k0)
+	lw $s3, -24($k0)
+	lw $t8, -28($k0)
+	lw $ra, -32($k0)
+	lw $sp, -36($k0)
+
 	eret
 	nop
 
diff -r 124757d0843f -r 647180a1278b vga.h
--- a/vga.h	Sun May 28 19:11:01 2017 +0200
+++ b/vga.h	Sun May 28 22:02:02 2017 +0200
@@ -1,19 +1,25 @@
 #ifndef __VGA_H__
 #define __VGA_H__
 
-#define LINE_LENGTH	160	/* pixels */
-#define LINE_COUNT	256	/* distinct display lines */
+#define LINE_LENGTH	160				/* pixels */
+#define LINE_COUNT	256				/* distinct display lines */
 
-#define HFREQ_LIMIT	643	/* 24MHz cycles */
-#define HSYNC_START	460	/* 24MHz cycles */
-#define HSYNC_LIMIT	64	/* 24MHz cycles */
+#define HFREQ_LIMIT	643				/* 24MHz cycles */
+#define HSYNC_START	460				/* 24MHz cycles */
+#define HSYNC_LIMIT	64				/* 24MHz cycles */
 #define HSYNC_END	(HSYNC_START + HSYNC_LIMIT)
 
-#define VISIBLE_START	70	/* horizontal lines, back porch end */
+#define VISIBLE_START	70				/* horizontal lines, back porch end */
 #define VFP_START	(VISIBLE_START + 2 * LINE_COUNT)
-#define VSYNC_START	620	/* horizontal lines, front porch end */
-#define VSYNC_END	622	/* horizontal lines, back porch start */
+#define VSYNC_START	620				/* horizontal lines, front porch end */
+#define VSYNC_END	622				/* horizontal lines, back porch start */
 
+#define SCREEN_BASE	256
 #define SCREEN_SIZE	(40 * 1024)
 
+#define SCREEN_BASE_KSEG0	(KSEG0_BASE + SCREEN_BASE)
+
+#define IRQ_STACK_LIMIT		SCREEN_BASE_KSEG0
+#define IRQ_STACK_TOP		(IRQ_STACK_LIMIT - 56)
+
 #endif /* __VGA_H__ */