From a9ac2db02f009e086df85d25dc9b32452f259538 Mon Sep 17 00:00:00 2001
From: John Tsiombikas <nuclear@member.fsf.org>
Date: Tue, 8 Jun 2021 23:02:03 +0300
Subject: [PATCH] infinite hardware scrolling

---
 sys1/kern/src/main.c |   26 +++++++++++++++++++++++---
 sys1/kern/src/vga.c  |   41 ++++++++++++++++++++++++++++++++++-------
 sys1/kern/src/vga.h  |    3 ++-
 3 files changed, 59 insertions(+), 11 deletions(-)

diff --git a/sys1/kern/src/main.c b/sys1/kern/src/main.c
index f0441e5..00d3882 100644
--- a/sys1/kern/src/main.c
+++ b/sys1/kern/src/main.c
@@ -12,11 +12,31 @@ void drawtext(int x, int y, const char *s)
 
 void kmain(void)
 {
+	int i, row;
+	int line, scroll;
 	char buf[64];
+	char twirl[] = "-\\|/";
 
 	vga_reset();
-	vga_setcolor(VGA_YELLOW | VGA_BRIGHT, VGA_BLACK);
 
-	sprintf(buf, "kmain addr: %p", (void*)kmain);
-	drawtext(10, 5, buf);
+	line = 0;
+	for(;;) {
+		scroll = line <= 24 ? 0 : line - 24;
+		row = line <= 24 ? line : 24;
+		sprintf(buf, "line %d  (scroll %d [%d])", line, scroll, scroll % 25);
+		vga_scroll(scroll);
+		if(scroll) {
+			vga_clearline(24);
+		}
+		vga_setcolor(VGA_YELLOW | VGA_BRIGHT, VGA_BLACK);
+		drawtext(line & 7, row, buf);
+
+		for(i=0; i<65536 * 4; i++) {
+			buf[0] = twirl[(i >> 18) & 3];
+			buf[1] = 0;
+			vga_setcolor(VGA_WHITE, VGA_BLACK);
+			drawtext(50, row, buf);
+		}
+		line++;
+	}
 }
diff --git a/sys1/kern/src/vga.c b/sys1/kern/src/vga.c
index 2967299..ae7c049 100644
--- a/sys1/kern/src/vga.c
+++ b/sys1/kern/src/vga.c
@@ -28,25 +28,52 @@ void vga_setcursor(int x, int y)
 	crtc_write(CRTC_CURPOS_L, loc);
 }
 
-void vga_setstart(int start)
+void vga_scroll(int s)
 {
-	yoffs = start;
-	crtc_write(CRTC_START_H, start >> 8);
-	crtc_write(CRTC_START_L, start);
+	yoffs = s % 25;
+	s = yoffs * 80;
+	crtc_write(CRTC_START_H, s >> 8);
+	crtc_write(CRTC_START_L, s);
 }
 
 void vga_reset(void)
 {
 	vga_setcolor(VGA_WHITE, VGA_BLACK);
-	vga_setstart(0);
+	vga_scroll(0);
 	vga_setcursor(0, 0);
 	memset((void*)0xb8000, 0, 80 * 25 * 2);
 }
 
+void vga_clearline(int row)
+{
+	uint16_t *ptr;
+
+	row += yoffs;
+
+	ptr = (uint16_t*)0xb8000 + row * 80;
+	memset16(ptr, attr, 80);
+
+	if(row - 25 >= 0) {
+		/* write a copy to wrap-around future scrolling */
+		ptr -= 80 * 25;
+		memset16(ptr, attr, 80);
+	}
+}
+
 void vga_drawchar(int x, int y, int c)
 {
-	uint16_t *ptr = (uint16_t*)0xb8000 + (y + yoffs) * 80 + x;
-	*ptr = (c & 0xff) | attr;
+	uint16_t *ptr, val = (c & 0xff) | attr;
+
+	y += yoffs;
+
+	ptr = (uint16_t*)0xb8000 + y * 80 + x;
+	*ptr = val;
+
+	if(y - 25 >= 0) {
+		/* write a copy to wrap-around future scrolling */
+		ptr -= 80 * 25;
+		*ptr = val;
+	}
 }
 
 static void crtc_write(int reg, unsigned char val)
diff --git a/sys1/kern/src/vga.h b/sys1/kern/src/vga.h
index e8be27a..14a91ff 100644
--- a/sys1/kern/src/vga.h
+++ b/sys1/kern/src/vga.h
@@ -15,10 +15,11 @@ enum {
 
 void vga_setcolor(int fg, int bg);
 void vga_setcursor(int x, int y);
-void vga_setstart(int start);
+void vga_scroll(int s);
 
 void vga_reset(void);
 
+void vga_clearline(int row);
 void vga_drawchar(int x, int y, int c);
 
 #endif	/* VGA_H_ */
-- 
1.7.10.4