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 issues."
#endif

/* This only works for 32-bit ix86 targets. */
#if !defined(__i386__)
#error "This kernel will only compile with a ix86-elf compiler"
#endif

/* Hardward 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 fore, enum vga_color back) {
    return fore | back << 4;
}

static inline uint16_t vga_entry(unsigned char c, uint8_t color) {
    return (uint16_t) c | (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_BLACK, VGA_COLOR_BLUE);
    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_scroll() {
    for(size_t row = 0; row < VGA_HEIGHT; row++) {
        for (size_t column = 0; column < VGA_WIDTH; column++) {
            terminal_buffer[column * VGA_WIDTH + row] = terminal_buffer[(column+1) * VGA_WIDTH + row];
        }
    }
}

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);
}

uint16_t current_color = VGA_COLOR_BLACK;
void terminal_putchar(char c) {
    // Move down line if newline character
    if (c == '\n') {
        ++terminal_row;
        terminal_column = 0;
    } else {
        if (terminal_row == VGA_WIDTH) {
            terminal_scroll();
        }
        if (current_color == VGA_COLOR_WHITE) {
            current_color = 0;
        }
        terminal_color = vga_entry(current_color, VGA_COLOR_BLUE);
        current_color += 1;
        terminal_putentryat(c, terminal_color, 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) {
    /* Intialize terminal interface */
    terminal_initialize();
    for(size_t i = 0; i < 20; i++) {
        terminal_writestring("Something\n");
    }
    terminal_writestring("Hello, kernel World!\n");
}