Screen Buffer

Function Name

BuildScreenBuffer

Description

This function serves to allocate in the heap (part of memory out of code and data segment), a virtual video memory, the screen buffer. Each time we need to display something, we write in this screen buffer, as if it were the video memory, and once all the modification of the loop are done , we update the real display by copying quickly the screen buffer into the video memory, for each frame.

Parameters:

This function needs to define a bss variable ('ScreenBufferSegment resw 1') to store the adress' segment of the screen buffer. This function needs to include the Heap Library. After this function had beeen executed, at the very beginning, the segment new destination to display something is not 0xA000 anymore, but '[ScreenBufferSegment]'.

Return Type

This function doesn't return anything. Even if this function overwrite ax and bx, as it is called at the very beginning, it doesn't matter at all.

Usage Example:

An example were we display something (black color on all the screen)

ClearScreen:
      ;clear the screen by filling it with a unique color (stored in al)

      ;set the source 'al'
      mov al, 0x00 ; color to fill the screen 
      
      ;set the destination 'es:di' :
      push word [ScreenBufferSegment] ; segment destination
      pop es 
      mov di, 0 ; begin the offset (pixel position) at 0

      mov cx, SCREEN_WIDTH*SCREEN_HEIGHT 
      rep stosb 
      
      ret

And then, to update the screen, you just call the function 'UpdateScreen'

Important Notes

Call this function once , before dispalying anything.

Modifications History

None

Test Cases

call SetVideoMode call BuildScreenBuffer call ClearScreen -> modify the value of 'al' to have a special color call UpdateScreen

if you see the screen filed by this special color : you used well the screen buffer, which had been well built.

Additional Documentation:

None

Author:

@benoitdekeyn , developper