kernel.c

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

enum vga_color {
  COLOR_BLACK         = 0,
  COLOR_BLUE          = 1,
  COLOR_GREEN         = 2,
  COLOR_CYAN          = 3,
  COLOR_RED           = 4,
  COLOR_MAGENTA       = 5,
  COLOR_BROWN         = 6,
  COLOR_LIGHT_GREY    = 7,
  COLOR_DARK_GREY     = 8,
  COLOR_LIGHT_BLUE    = 9,
  COLOR_LIGHT_GREEN   = 10,
  COLOR_LIGHT_CYAN    = 11,
  COLOR_LIGHT_RED     = 12,
  COLOR_LIGHT_MAGENTA = 13,
  COLOR_LIGHT_BROWN   = 14,
  COLOR_WHITE         = 15,
};

uint8_t make_color(enum vga_color fg, enum vga_color bg) {
  return fg | bg << 4;
}

uint16_t make_vgaentry(char c, uint8_t color) {
  uint16_t c16     = c;
  uint16_t color16 = color;

  return c16 | color16 << 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  = make_color(COLOR_LIGHT_GREY, 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] = make_vgaentry(' ', 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] = make_vgaentry(c, color);
}

void terminal_scroll() {
  for (size_t height_c = 0; height_c < VGA_HEIGHT - 1; height_c++) {
    for (size_t width_c = 0; width_c < VGA_WIDTH; width_c++) {
      size_t to   = height_c * VGA_WIDTH + width_c;
      size_t from = (height_c + 1) * VGA_WIDTH + width_c;
      terminal_buffer[to] = terminal_buffer[from];
    }
  }
}

void terminal_putchar(char c) {
  if (c == '\n')
  {
    terminal_column = 0;

    if (++terminal_row == VGA_HEIGHT)
    {
      terminal_scroll();
    }
    return;
  }
  else {
    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_writestring(const char *data) {
  size_t datalen = strlen(data);

  for (size_t i = 0; i < datalen; i++) terminal_putchar(data[i]);
}

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

  for (int i = 0; i <= 2; i++)
  {
    for (char z = 'A'; z <= 'z'; z++)
    {
      char foo[] = { z, '\n', '\0' };
      terminal_writestring(foo);
    }
  }
}