在汇编语言中使用RGB颜色

Question

I am Plotting a pixel on the screen from the following code using Assembly Language of x86 processor in C++. I dont want to use any function or method from C++ as I use this code for the boot loader program. here is the code:

/**********************************
 * Mainvainsoft 2011.             *
 * Writen By: Farid-ur-Rahman     *
 * On: 24 Sep At: 1:34 AM.        *
 **********************************/
 #include <conio.h>
 void main ()
 {
 // Setting Video mode to 256 colours at 320 X 200
 _asm {
 mov ah , 0x00 // Setting Video mode or Clear Screen
 mov al , 0x13 // Setting Video mode to 256 Color Mode
 int        0x10 // Call the Registor

 mov ah , 0x0c // Plot the Pixel
 mov al , 4    // Color
 mov cx , 160  // X-Axis
 mov dx , 100  // Y-Axis
 int        0x10 // Call the Registor
}
getch();     // Wait for the key press
}

I want to use the RGB colors to display on the pixel.

Answer 1

Mode 13h uses a palette with 256 18-bit RGB (6 bits for each) entries. So you can set for example entry 4 to the RGB color you want and the plot the pixel as you are doing with color 4.

See here for an example of how to set a palette entry. After setting the video mode you can do something like:

// Set entry 4
mov dx, 0x3c8
mov al, 4
out dx, al

inc dx
mov al, Red 
out dx, al
mov al, Green
out dx, al
mov al, Blue
out dx, al

// draw pixel

Answer 2

In the video mode you're using, VGA mode 0x13 , each byte of the framebuffer points into the palette. So if you have 24 bits of RGB color information, you can't write that directly into a pixel, since the pixels just contain palette indices.

You can of course quantize your colors into the palette, but that can be quite complicated.