kernel.c

#include <stdbool.h>
#include <stddef.h>
#include <stdint.h>

#if defined(__linux__)
#error "You are not using a cross-compiler and will run into trouble."
#endif

#if !defined(__i386__)
#error "This tutorial needs to be compiled with a ix86-elf compiler."
#endif

enum vga_colour {
	VGA_COLOUR_BLACK = 0,
	VGA_COLOUR_BLUE = 1,
	VGA_COLOUR_GREEN = 2,
	VGA_COLOUR_CYAN = 3,
	VGA_COLOUR_RED = 4,
	VGA_COLOUR_MAGENTA = 5,
	VGA_COLOUR_BROWN = 6,
	VGA_COLOUR_LIGHT_GREY = 7,
	VGA_COLOUR_DARK_GREY = 8,
	VGA_COLOUR_LIGHT_BLUE = 9,
	VGA_COLOUR_LIGHT_GREEN = 10,
	VGA_COLOUR_LIGHT_CYAN = 11,
	VGA_COLOUR_LIGHT_RED = 12,
	VGA_COLOUR_LIGHT_MAGENTA = 13,
	VGA_COLOUR_LIGHT_BROWN = 14,
	VGA_COLOUR_WHITE = 15,
};

static inline uint8_t vga_entry_colour (enum vga_colour fg, enum vga_colour bg)
{
	return fg | bg << 4;
}

static inline uint16_t vga_entry (unsigned char uc, uint8_t colour)
{
	return (uint16_t) uc | (uint16_t) colour << 8;
}

size_t strlen (const char* str)
{
	size_t len = 0;
	while (str[len])
		len++;
	return len;
}

static const size_t VGA_WIDTH = 80;
static const size_t VGA_HEIGHT = 25;

size_t terminal_row;
size_t terminal_column;
uint8_t terminal_colour;
uint16_t* terminal_buffer;

void terminal_initialize (void)
{
	terminal_row = 0;
	terminal_column = 0;
	terminal_colour = vga_entry_colour(VGA_COLOUR_LIGHT_GREY, VGA_COLOUR_BLACK);
	terminal_buffer = (uint16_t*) 0xB8000;
	for (size_t y = 0; y < VGA_HEIGHT; y++) {
		for (size_t x = 0; x < VGA_WIDTH; x++) {
			const size_t index = y * VGA_WIDTH + x;
			terminal_buffer[index] = vga_entry(' ', terminal_colour);
		}
	}
}

void terminal_setcolour(uint8_t colour)
{
	terminal_colour = colour;
}

void terminal_putentryat (char c, uint8_t colour, size_t x, size_t y)
{
	const size_t index = y * VGA_WIDTH + x;
	terminal_buffer[index] = vga_entry(c, colour);
}

void terminal_putchar (char c)
{
	terminal_putentryat(c, terminal_colour, terminal_column, terminal_row);
	if (++terminal_column == VGA_WIDTH) {
		terminal_column = 0;
		if (++terminal_row == VGA_HEIGHT)
			terminal_row = 0;
	}
}

void terminal_write (const char* data, size_t size)
{
	for (size_t i = 0; i < size; i++)
		terminal_putchar(data[i]);
}

void terminal_writestring(const char* data)
{
	terminal_write(data, strlen(data));
}

void kernel_main(void)
{
	terminal_initialize();

	terminal_writestring("Hello, kernel world!");
}