Notes »
Color Graphics Adapter (CGA) Programming
These are some notes after making The Return of Traxtor for DOS.
The code snippets are for GCC (using the IA16 port) and stdint.h.
Setting the graphic mode
This should be done with the BIOS for compatibility.
| Int | Function | Params |
|---|---|---|
| 0x10 | ah: 0x00 |
al: mode |
Example:
void set_mode(uint8_t mode)
{
asm volatile (
"movb $0, %%ah\n\t"
"int $0x10\n\t"
: /* no output */
: "al"(mode)
: "ah"
);
}
Interesting CGA Modes
| Mode | Text | Resolution | Colors |
|---|---|---|---|
| 0x04 | 40x25 | 320x200 | 4 |
| 0x03 | 40x25 | Text mode | 16 |
| 0x06 | 80x25 | 640x200 | 2 |
Selecting the palette
This is set with the control color register on port 0x3d9 in mode 0x04 (320x200, 4 colors).
| Bit | Description |
|---|---|
| 0 | Background color blue |
| 1 | Background color green |
| 2 | Background color red |
| 3 | Background intensity |
| 4 | Palette intensity |
| 5 | Palette selection |
| 6-7 | Unused |
The palette selection bit translates as:
- 0: red, green, yellow
- 1: magenta, cyan, white
Examples:
/* palette 0, bright */
outporb(0x3d9, 16);
/* palette 1, bright, blue background */
outporb(0x3d9, 1 | 16 | 32);
These may not have effect on a VGA card.
Drawing on the screen
In mode 0x04 (320x200, 4 colors), the video memory is mapped at b800:0000.
The pixel data is encoded using 2 bit per pixel, so in one byte we have 4 pixels, with the most significant bit corresponding to the left-most pixel.
For 4 pixels (a, b, c ,d), we would have:
| Bits 7-6 | Bits 5-4 | Bits 3-2 | Bits 1-0 | |
|---|---|---|---|---|
| a1a0 | b1b0 | c1c0 | d1d0 |
Each row has 80 bytes, and the memory is split: first the even rows (0, 2, 4, 8, …, until 198), and offset by 8192 bytes, the odd rows (1, 3, 5, 7, …, 199).
Example clearing the screen:
void clear_screen()
{
asm(
"push %%es\n\t"
"movw $0xb800, %%ax\n\t"
"movw %%ax, %%es\n\t"
"movw $0, %%di\n\t"
"movw $8192, %%cx\n\t"
"xorw %%ax, %%ax\n\t"
"cld\n\t"
"rep stosw\n\t"
"pop %%es\n\t"
: /* no output */
: /* on input */
: "c", "a", "di"
);
}
A reasonably efficient “blit” function:
/* x and w in bytes */
/* y and h in pixels */
typedef struct {
uint8_t x;
uint8_t y;
uint8_t w;
uint8_t h;
} rect;
/* sprite expects 2bpp data, non-interlaced non masked */
void blit(const uint8_t *sprite, const rect *dst)
{
asm volatile (
"movw %%di, %%bx\n\t"
"cld\n\t"
"blit_loop:\n\t"
"movw %%bx, %%di\n\t"
"movw %%dx, %%cx\n\t"
"rep movsb\n\t"
"xorw $0x2000, %%bx\n\t"
"testb $1, %%al\n\t"
"je blit_skip\n\t"
"addw $80, %%bx\n\t"
"blit_skip:\n\t"
"incb %%al\n\t"
"decb %%ah\n\t"
"jne blit_loop\n\t"
: /* no output */
: "e"(0xb800),
"S"(sprite), "D"(((dst->y & 1) ? 0x2000 - 40: 0) + dst->x + dst->y * 40),
"a"((dst->h << 8)|dst->y), "d"((uint16_t)dst->w)
: "b", "c"
);
}
And a faster version with width fixed to 4 bytes (16 pixels wide):
void blit4(const uint8_t *sprite, const rect *dst)
{
asm volatile (
"movw %%di, %%bx\n\t"
"cld\n\t"
"blit4_loop:\n\t"
"movw %%bx, %%di\n\t"
"movsw\n\t"
"movsw\n\t"
"xorw $0x2000, %%bx\n\t"
"testb $1, %%al\n\t"
"je blit4_skip\n\t"
"addw $80, %%bx\n\t"
"blit4_skip:\n\t"
"incb %%al\n\t"
"decb %%ah\n\t"
"jne blit4_loop\n\t"
: /* no output */
: "e"(0xb800),
"S"(sprite), "D"(((dst->y & 1) ? 0x2000 - 40: 0) + dst->x + dst->y * 40),
"a"((dst->h << 8)|dst->y)
: "b"
);
}
Waiting for VSYNC
There is no interrupt to signal the start of vertical sync, so we have to poll the status register on port 0x3da.
| Bit | Description |
|---|---|
| 0 | Display enable |
| 1 | Light pen trigger set |
| 2 | Light pen switch status |
| 3 | Vertical retrace |
| 4-7 | Unused |
Example:
void wait_vsync()
{
while (inportb(0x3da) & 8);
while (!(inportb(0x3da) & 8));
}