Double Buffering

Overview

Without double buffering, drawing directly to screen memory creates visible artifacts—half-drawn objects, flickering sprites, tearing during scrolling. Double buffering solves this by maintaining two frame buffers: one displayed, one drawn to. When drawing completes, the buffers swap instantly.

The tearing problem

When drawing to visible memory:

Frame drawing starts
User sees: partial old frame + partial new frame
Frame drawing completes

The “tear line” moves down the screen where old meets new.

The solution

Buffer	State
Front buffer	Currently displayed
Back buffer	Being drawn to

After drawing completes:

1. Wait for vertical blank
2. Swap buffer pointers
3. Display shows completed frame
4. Draw next frame to other buffer

Implementation approaches

Pointer swapping

Change where the display hardware reads from:

; C64: Point VIC-II to different screen
swap_buffers:
    lda current_buffer
    eor #$01
    sta current_buffer
    beq .use_buffer_0

.use_buffer_1:
    lda #$18          ; Screen at $0800
    jmp .set_bank

.use_buffer_0:
    lda #$10          ; Screen at $0400

.set_bank:
    sta $d018
    rts

Memory copying

If hardware doesn’t support pointer swapping, copy back buffer to front:

• Slower
• Uses more CPU time
• Still eliminates tearing if done during vblank

Platform specifics

Commodore 64

Approach	Memory addresses
Two character screens	$0400, $0800
Two bitmap screens	$2000, $4000
Bank switching	Different 16K banks

Amiga

Hardware designed for double buffering:

• Copper can change display pointers
• Blitter can clear/fill buffers quickly
• Triple buffering common for 50fps

NES

• Change nametable pointer during vblank
• PPU has two nametables
• Scrolling uses this naturally

Memory cost

Display type	Single buffer	Double buffer
C64 text (40×25)	1KB	2KB
C64 bitmap	8KB	16KB
Amiga lo-res	40KB	80KB

On memory-constrained systems, double buffering was a luxury.

Triple buffering

Three buffers enable:

• One displaying
• One ready to display
• One being drawn

Smoother when drawing time varies.

Page flipping vs copying

Method	Speed	Memory
Page flip	Instant	2× screen
Copy	Slow	1× screen + buffer
Dirty rectangles	Medium	Minimal

Vertical blank timing

Swap must happen during vertical blank:

wait_vblank:
    lda $d011
    bpl wait_vblank   ; Wait for vblank start
.wait_end:
    lda $d011
    bmi .wait_end     ; Wait for vblank end
    ; Now safe to swap

See also

• Raster Interrupts
• VIC-II
• Copper
• Blitter