First you need to change the screen resolution to Mode 13h.

void init_mode13h() { asm { mov ax, 0x13 // We set ax to our mode number (13h) int 0x10 // We tell the BIOS to switch mode } }

The basic of drawing anything on the screen is drawing a PIXEL.
To draw a pixel by X,Y coordinates you need to calculate its position (offset) on the Video Memory:
You see the screen as a 2D surface, but in the Video Memory it is stored as a 1D array, like a line (linear).
For example: The first row starts at offset 0, all the pixels of the first row are stored from 0 to 319, the second row starts at offset 320 and ends at offset 639 and so on...
The Monitor passes a row every 320 pixels.
Therefore, the formula of getting to the offset of a certain row is: Y*320 and to get the pixel's offset we just add the column number: Y*320+X
We just treat the screen as a 2D array for comfort.

Here is an example of a coordinate on the screen.
Its coordinates: X = 3, Y = 4, Color = RED:

unsigned char *video_memory = (unsigned char *) 0xA0000000L; void put_pixel(int x, int y, unsigned char color) { video_memory[y * 320 + x] = color; }

In the end of your program you better return to text-mode:

void init_textmode() // 80x25 Chars with 16 colors { asm { mov ax, 0x3 // We set ax to textmode's number (3) int 0x10 } }