Scrolling support.

Makefile

@@ -16,7 +16,7 @@ debug: image.bin
 image.bin: mbr.bin kernel.bin
 	cat $^ >$@
 
-kernel.bin: kernel.o vga.o
+kernel.bin: kernel.o vga.o string.o
 	$(LD) -m elf_i386 -o $@ -Ttext 0x1000 $^ --oformat binary
 
 %.o: %.c

kernel.c

@@ -4,6 +4,8 @@ asm("jmp main");
 
 int main() {
     vga_clear_screen();
-    vga_print_string("hello world");
+    for (int i = 0; i < 24; i++) {
+        vga_print_string("hello world\n");
+    }
     return 0;
 }

string.c

@@ -0,0 +1,16 @@
+#include "string.h"
+
+void kmemmove(char* dst, char* src, size_t size) {
+	if (dst == src) return;
+	if (dst > src && dst < src + size) {  //   s d
+		// copy right-to-left
+		for (; size != 0; size--) {
+			dst[size - 1] = src[size - 1];
+		}
+	} else {
+		// copy left-to-right
+		for (size_t i = 0; i < size; ++i) {
+			dst[i] = src[i];
+		}
+	}
+}

string.h

@@ -0,0 +1,5 @@
+#pragma once
+
+typedef unsigned size_t;
+
+void kmemmove(char* dst, char* src, size_t size);

vga.c

@@ -1,11 +1,34 @@
 #include "port.h"
+#include "string.h"
 
 char* const video_memory = (char*) 0xb8000;
 
+enum colors16 {
+    black = 0,
+    blue,
+    green,
+    cyan,
+    red,
+    magenta,
+    brown,
+    light_gray,
+    dark_gray,
+    light_blue,
+    light_green,
+    light_cyan,
+    light_red,
+    light_magenta,
+    yellow,
+    white,
+};
+
+static unsigned char get_color(unsigned char fg, unsigned char bg) {
+    return (bg << 4) + fg;
+}
+
 enum {
-    LINES = 25,
+    ROWS = 25,
     COLS = 80,
-    WHITE_ON_BLACK = 0x0f,
 
     VGA_CTRL_REGISTER = 0x3d4,
     VGA_DATA_REGISTER = 0x3d5,
@@ -13,6 +36,14 @@ enum {
     VGA_OFFSET_HIGH = 0x0e,
 };
 
+static unsigned get_offset(unsigned col, unsigned row) {
+    return row * COLS + col;
+}
+
+static unsigned get_row_from_offset(unsigned offset) {
+    return offset / COLS;
+}
+
 void vga_set_cursor(unsigned offset) {
     port_byte_out(VGA_CTRL_REGISTER, VGA_OFFSET_HIGH);
     port_byte_out(VGA_DATA_REGISTER, (unsigned char) (offset >> 8));
@@ -30,22 +61,37 @@ unsigned vga_get_cursor() {
 
 void vga_set_char(unsigned offset, char c) {
     video_memory[2 * offset] = c;
-    video_memory[2 * offset + 1] = WHITE_ON_BLACK;
+    video_memory[2 * offset + 1] = get_color(light_gray, black);
 }
 
 void vga_clear_screen() {
-    for (unsigned i = 0; i < LINES * COLS; ++i) {
+    for (unsigned i = 0; i < ROWS * COLS; ++i) {
         vga_set_char(i, ' ');
     }
     vga_set_cursor(0);
 }
 
+static unsigned scroll() {
+    kmemmove(video_memory, video_memory + COLS, 2 * COLS * (ROWS-1));
+    for (int col = 0; col < COLS; col++) {
+        vga_set_char(get_offset(col, ROWS - 1), ' ');
+    }
+    return get_offset(0, ROWS - 1);
+}
+
 void vga_print_string(const char* s) {
-    int offset = vga_get_cursor();
+    unsigned offset = vga_get_cursor();
     while (*s != 0) {
-        vga_set_char(offset, *s);
+        if (*s == '\n') {
+            offset = get_offset(0, get_row_from_offset(offset) + 1);
+        } else {
+            vga_set_char(offset, *s);
+            offset++;
+        }
         s++;
-        offset++;
+        if (offset > COLS * ROWS) {
+            offset = scroll();
+        }
     }
     vga_set_cursor(offset);
 }