kernel.c

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

/* Check if the compiler thinks you 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;
}

#define VGA_WIDTH   80
#define VGA_HEIGHT  25
#define VGA_MEMORY  0xB8000 

size_t terminal_row;
size_t terminal_column;
uint8_t terminal_color;
uint16_t* terminal_buffer = (uint16_t*)VGA_MEMORY;

void terminal_initialize(void) 
{
	terminal_row = 0;
	terminal_column = 0;
	terminal_color = vga_entry_color(VGA_COLOR_LIGHT_GREY, VGA_COLOR_BLACK);
	
	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(void) 
{
    // Shift all lines up by one
    for (size_t y = 1; y < VGA_HEIGHT; y++) {
        for (size_t x = 0; x < VGA_WIDTH; x++) {
            const size_t index = (y - 1) * VGA_WIDTH + x;
            terminal_buffer[index] = terminal_buffer[y * VGA_WIDTH + x];
        }
    }

    // Clear the last line
    for (size_t x = 0; x < VGA_WIDTH; x++) {
        const size_t index = (VGA_HEIGHT - 1) * VGA_WIDTH + x;
        // Fill the last line with spaces
        terminal_buffer[index] = vga_entry(' ', terminal_color);
    }

    // Reset the cursor position
    terminal_row = VGA_HEIGHT - 1;
    terminal_column = 0;
}

void terminal_putchar(char c) 
{
    if (c == '\n') {
        terminal_column = 0;
        if (++terminal_row == VGA_HEIGHT) {
            terminal_scroll();
        }
        return;
    }

    terminal_putentryat(c, terminal_color, terminal_column, terminal_row);

    if (++terminal_column == VGA_WIDTH) {
        terminal_column = 0;
        if (++terminal_row == VGA_HEIGHT) {
            terminal_scroll();
        }
    }
}

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) 
{
	/* Initialize terminal interface */
	terminal_initialize();

	/* Newline support is left as an exercise. */
	terminal_writestring("Hello, kernel World!\n");
    terminal_writestring("Welcome to the Operating System Kernel.\n");
    terminal_writestring("This is a simple kernel written in C.\n");

	/*
	for(int i = 0; i < 100; i++) {
        terminal_writestring("A\n");
    } 
	*/
}