kernel-hello-world.c

	/* Surely you will remove the processor conditionals and this comment
	   appropriately depending on whether or not you use C++. */
	#if !defined(__cplusplus)
	#include <stdbool.h> /* C doesn't have booleans by default. */
	#endif
	#include <stddef.h>
	#include <stdint.h>
	 
	/* Check if the compiler thinks we are targeting the wrong operating system. */
	#if defined(__linux__)
	#error "You are not using a cross-compiler, you will most certainly run into trouble"
	#endif
	 
	/* This tutorial will only work for the 32-bit ix86 targets. */
	#if !defined(__i386__)
	#error "This tutorial needs to be compiled with a ix86-elf compiler"
	#endif
	 
	/* Hardware text mode color constants. */
	enum vga_color {
		VGA_COLOR_BLACK = 0,
		VGA_COLOR_BLUE = 1,
		VGA_COLOR_GREEN = 2,
		VGA_COLOR_CYAN = 3,
		VGA_COLOR_RED = 4,
		VGA_COLOR_MAGENTA = 5,
		VGA_COLOR_BROWN = 6,
		VGA_COLOR_LIGHT_GREY = 7,
		VGA_COLOR_DARK_GREY = 8,
		VGA_COLOR_LIGHT_BLUE = 9,
		VGA_COLOR_LIGHT_GREEN = 10,
		VGA_COLOR_LIGHT_CYAN = 11,
		VGA_COLOR_LIGHT_RED = 12,
		VGA_COLOR_LIGHT_MAGENTA = 13,
		VGA_COLOR_LIGHT_BROWN = 14,
		VGA_COLOR_WHITE = 15,
	};
	 
	static inline uint8_t vga_entry_color(enum vga_color fg, enum vga_color bg) {
		return fg | bg << 4;
	}
	 
	static inline uint16_t vga_entry(unsigned char uc, uint8_t color) {
		return (uint16_t) uc | (uint16_t) color << 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_color;
	uint16_t* terminal_buffer;
	 
	void terminal_initialize(void) {
		terminal_row = 0;
		terminal_column = 0;
		terminal_color = vga_entry_color(VGA_COLOR_LIGHT_GREY, VGA_COLOR_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_color);
			}
		}
	}
	 
	void terminal_setcolor(uint8_t color) {
		terminal_color = color;
	}
	 
	void terminal_putentryat(char c, uint8_t color, size_t x, size_t y) {
		const size_t index = y * VGA_WIDTH + x;
		terminal_buffer[index] = vga_entry(c, color);
	}

	void terminal_scroll() {
		for (size_t y = 0; y < VGA_HEIGHT-1; y++) {
			for (size_t x = 0; x < VGA_WIDTH; x++) {
				const size_t index = y * VGA_WIDTH + x;
				terminal_buffer[index] = terminal_buffer[index + VGA_WIDTH];
			}
		}
		for (size_t x = 0; x < VGA_WIDTH; x++) {
			const size_t index = VGA_HEIGHT * VGA_WIDTH - VGA_HEIGHT + x;
			terminal_buffer[index] = vga_entry(' ', terminal_color);
		}
		
	}
	 
	void terminal_putchar(char c) {
		if (c != '\n') {
			terminal_putentryat(c, terminal_color, terminal_column, terminal_row);
			if (++terminal_column == VGA_WIDTH) {
				terminal_column = 0;
				terminal_row++;
			}
		}
		else {
			terminal_column = 0;
			terminal_row++;
		}
		if (terminal_row == VGA_HEIGHT) {
			terminal_scroll();
			terminal_column = 0;
			terminal_row--;
		}
		
	}
	 
	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));
	}
	 
	#if defined(__cplusplus)
	extern "C" /* Use C linkage for kernel_main. */
	#endif
	void kernel_main(void) {
		/* Initialize terminal interface */
		terminal_initialize();
	 
		/* Newline support is left as an exercise. */
		terminal_writestring("Goodbye world");
		
	}