vga.h

#ifndef VGA_H
#define VGA_H

#include "math.h"

#define VGA_PWIDTH  320
#define VGA_PHEIGHT 200

enum VGA_COLOR {
    BLACK, BLUE, GREEN, CYAN, RED, MAGENTA, BROWN, LIGHT_GRAY, DARK_GRAY,
    LIGHT_BLUE, LIGHT_GREEN, LIGHT_CYAN, LIGHT_RED, LIGHT_MAGENTA,
    YELLOW, WHITE
};

struct point {
    short x, y;
};

struct rect {
    struct point tl, br;
};

/* Switch to VGA mode 13 and set ES. */
static void vga_on()
{
    asm volatile ("mov   $0x0013, %%ax\n"
                  "int   $0x10\n"
                  "mov   $0xA000, %%ax\n"
                  "mov   %%ax, %%es\n"
                  : /* no outputs */
                  : /* no inputs */
                  : "ax");
}

static void vga_off()
{
    asm volatile ("mov   $0x0003, %%ax\n"
                  "int   $0x10\n"
                  "mov   $0xA000, %%dx\n"
                  "mov   %%dx, %%es\n"
                  : /* no outputs */
                  : /* no inputs */
                  : "ax", "dx");
}

static void vga_pixel(volatile struct point p, uint8_t color)
{
    if (p.x >= 0 && p.x < VGA_PWIDTH && p.y >= 0 && p.y < VGA_PHEIGHT)
        asm volatile ("imul  $320, %%bx\n"
                      "add   %%ax, %%bx\n"
                      "mov   %%cl, %%es:(%%bx)\n"
                      : /* no outputs */
                      : "a"(p.x), "b"(p.y), "c"(color)
                      : "dx");
}

static void vga_clear(char color)
{
    asm volatile ("mov   %%al, %%ah\n"
                  "mov   $0, %%di\n"
                  "push  %%ax\n"
                  "shl   $16, %%eax\n"
                  "pop   %%ax\n"
                  "mov   $16000, %%cx\n"
                  "rep\n"
                  "stosl\n"
                  : /* no outputs */
                  : "a"(color)
                  : "cx", "di");
}

static void vga_line(struct point a, struct point b, uint8_t color)
{
    int dx = abs(b.x - a.x), sx = a.x < b.x ? 1 : -1;
    int dy = abs(b.y - a.y), sy = a.y < b.y ? 1 : -1;
    int err = (dx > dy ? dx : -dy) / 2, e2;
    for(;;) {
        vga_pixel(a, color);
        if (a.x == b.x && a.y == b.y)
            break;
        e2 = err;
        if (e2 > -dx) {
            err -= dy;
            a.x += sx;
        }
        if (e2 < dy) {
            err += dx;
            a.y += sy;
        }
    }
}

static void vga_rect(struct rect r, uint8_t color)
{
    vga_line(r.tl, (struct point){r.br.x, r.tl.y}, color);
    vga_line(r.tl, (struct point){r.tl.x, r.br.y}, color);
    vga_line(r.br, (struct point){r.br.x, r.tl.y}, color);
    vga_line(r.br, (struct point){r.tl.x, r.br.y}, color);
}

static void vga_vsync()
{
    asm volatile ("mov   $0x03DA, %%dx\n"
                  "current%=:"
                  "in    %%dx, %%al\n"
                  "and   $0x8, %%al\n"
                  "jnz   current%=\n"
                  "restart%=:"
                  "in    %%dx, %%al\n"
                  "and   $0x8, %%al\n"
                  "jz    restart%=\n"
                  : /* no outputs */
                  : /* no inputs */
                  : "al", "dx");
}

#endif