Background Buffer

Function Name

BackgroundBufferSegment

Description

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

The background contains:

• The maze layout
• The pellets & power pellets which can be dynamically updated and removed

This can be considered as a sub-layer of the screen buffer. Each time we need to restore the background after the passage of a ghost, we read in this buffer to overwrite the ghost with the right background.

Parameters:

This function needs to define a bss variable ('BackgroundBufferSegment resw 1') to store the adress' segment of the background buffer. This function needs to include the heap library.

Return Type

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

Usage Example:

An example where we clear a ghost , reading the background buffer

    ;need to define the y position in ax, and x position in bx
    ClearSprite:
        ; calculate the adress (offset) of the first pixel 
        mov cx, SCREEN_WIDTH
        mul cx
        add bx, ax

        ;set the destination 'es:di' as SreenBuffer:offsetOfTheFirstPixel
        push word [ScreenBufferSegment] 
        pop es 
        mov di, bx 
        
        ;set the source 'ds:si' as BackgroundBuffer:offsetOfTheFirstPixel
        push word [BackgroundBufferSegment]; load the effective adress (L.E.A.) of the backup in the destination offset
        pop ds
        mov si, di

        
        ;using source and destination, copy the data on a 8x8 bits square :
        mov dx, SPRITE_SIZE
        .eachLine:
            mov cx, SPRITE_SIZE 
            rep movsb 
            add di, SCREEN_WIDTH - SPRITE_SIZE 
            dec dx ; decrementatin de dx, when it reach 0, the flag is 0 too cause of the dec propreties
            jnz .eachLine ; while the flag != 0, it continues

        ;restore the value of 'ds' to don't disturb the rest of the code
        push cs       
        pop ds

        ret

Important Notes

You only need to call this function once at the beginning of the program. This function is also needed to clear the ghosts in the rest of the program.

Test Cases

call SetVideoMode call BuildScreenBuffer call BuildBackgroundBuffer call FirstDispalyPacMan call UpdateScreen mov word [x_PinkyPosition], 45 -> change this value to test mov word [y_PinkyPosition], 23 -> change this value to test mov word [frameOf_Pinky], PINKY_1 mov word [frameOf_Pinky_eyes], EYES_RIGHT call Display_Pinky mov ax, [y_PinkyPosition] mov bx, [x_PinkyPosition] call ClearSprite

If there is no Pinky on the screen : you used well the background buffer (so, well built) to restore the background on Pinky.

Author:

Benoît DE KEYN