Add vga_print_string.

Makefile

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

kernel.c

@@ -1,7 +1,9 @@
 asm("jmp main");
 
+#include "vga.h"
+
 int main() {
-    char* video_memory = (char*) 0xb8000;
-    *video_memory = 'X';
+    vga_clear_screen();
+    vga_print_string("hello world");
     return 0;
 }

port.h

@@ -0,0 +1,11 @@
+#pragma once
+
+static unsigned char port_byte_in(unsigned short port) {
+    unsigned char result;
+    __asm__("in %%dx, %%al" : "=a" (result) : "d" (port));
+    return result;
+}
+
+static void port_byte_out(unsigned short port, unsigned char data) {
+    __asm__("out %%al, %%dx" : : "a" (data), "d" (port));
+}

vga.c

@@ -0,0 +1,51 @@
+#include "port.h"
+
+char* const video_memory = (char*) 0xb8000;
+
+enum {
+    LINES = 25,
+    COLS = 80,
+    WHITE_ON_BLACK = 0x0f,
+
+    VGA_CTRL_REGISTER = 0x3d4,
+    VGA_DATA_REGISTER = 0x3d5,
+    VGA_OFFSET_LOW = 0x0f,
+    VGA_OFFSET_HIGH = 0x0e,
+};
+
+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));
+    port_byte_out(VGA_CTRL_REGISTER, VGA_OFFSET_LOW);
+    port_byte_out(VGA_DATA_REGISTER, (unsigned char) (offset & 0xff));
+}
+
+unsigned vga_get_cursor() {
+    port_byte_out(VGA_CTRL_REGISTER, VGA_OFFSET_HIGH);
+    unsigned offset = port_byte_in(VGA_DATA_REGISTER) << 8;
+    port_byte_out(VGA_CTRL_REGISTER, VGA_OFFSET_LOW);
+    offset += port_byte_in(VGA_DATA_REGISTER);
+    return offset;
+}
+
+void vga_set_char(unsigned offset, char c) {
+    video_memory[2 * offset] = c;
+    video_memory[2 * offset + 1] = WHITE_ON_BLACK;
+}
+
+void vga_clear_screen() {
+    for (unsigned i = 0; i < LINES * COLS; ++i) {
+        vga_set_char(i, ' ');
+    }
+    vga_set_cursor(0);
+}
+
+void vga_print_string(const char* s) {
+    int offset = vga_get_cursor();
+    while (*s != 0) {
+        vga_set_char(offset, *s);
+        s++;
+        offset++;
+    }
+    vga_set_cursor(offset);
+}

vga.h

@@ -0,0 +1,5 @@
+#pragma once
+
+void vga_clear_screen();
+
+void vga_print_string(const char* s);