Screen 12 drawing graphics

[KenshinHimura]

How would you do that? All I get is white pixels put randomly when I use the method of drawing pixels in screen 13. I know why that happens and all, but then how would you draw graphics using the memory at A000:0000. Are the colors stored somewhere else in memory? thx

 

[Thazager]

I believe mode 12 is either CGA or EGA graphics. If thats the case, the 16kb buffer would be at b000:8000h. Writing screen code for CGA or EGA differs from VGA. They have levels of screens rather then 1 pixel per color.

Thaz

 

[lionelhill]

On VGA cards and upwards, text modes start at b800, graphics modes at A000.

If you want to write pixels, you need a good intro to the VGA card. The colours are stored in the same place as the rest, but in 16-colour modes the VGA card addresses 4 memory planes on top of each other! When you write to a single byte, you actually write to any/all of 4 bytes of VGA memory, each containing 8 pixels. Which planes (which bytes) are affected depends on what else you've set up in the VGA's own registers, which you do by IN and OUT instructions. The 4 bytes/planes deal with colour; between them they make for the 16 combinations of colour. In the original colour scheme you could look on the planes as R/G/B/brightness, but now they just create a look-up into a colour table, so they can correspond to anything you want.