Screen Buffer

Function Name

BuildScreenBuffer

Description

This function uses heap memory allocation which enables the creation of a screen buffer. This screen buffer can be assimilated to a virtual video memory copy. (The heap is a part of memory out of code and data segment which is on top of the stack).

This is used as a pre-version of the video memory, we write all the data in the screen buffer. Once all the data has been included in the screen buffer, we copy its content to the video memory. This content copy is done once for each frame.

The data opbjects contained in the screen buffer are the following:

• Background buffer content
  • Maze layout
  • Pellets
• Ghosts and Pac-Man
• Score, Lives & Non-related gameplay elements

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. This function needs to be executed at the very beginning. After its execution, the new destination to display something is no more at the adress segment 0xA000, but instead is now at the '[ScreenBufferSegment]'.

Return Type

This function doesn't return anything. Even if this function overwrites 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.

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 filled by the provided color, you used well the screen buffer and there are no bugs.

Author:

Benoît DE KEYN