This is a post version of my single page article published in the free technical magazine Paged Out! released under the Creative Commons License 4.0

This is a very simple but fun effect that can be achieved in just a couple of lines of assembly. This effect is done using the same Nintendo logo that is left on VRAM by the boot rom. In our program we change the horizontal and vertical scroll for each scan line of the LCD using a lookup table. For this, we only need a toolchain that outputs instructions for the Sharp LR35902 CPU. RGBDS (https://rednex.github.io/rgbds/) will be our weapon of choice. It includes an assembler, linker and rom header fixer.

	; wobble.asm
section "HEADER", ROM0[$0100]
	nop
	jp wobble_main
	; The ROM header needed by
	; the system to validate the rom.
	db $CE,$ED,$66,$66,$CC,$0D,$00,$0B
	db $03,$73,$00,$83,$00,$0C,$00,$0D
	db $00,$08,$11,$1F,$88,$89,$00,$0E
	db $DC,$CC,$6E,$E6,$DD,$DD,$D9,$99
	db $BB,$BB,$67,$63,$6E,$0E,$EC,$CC
	db $DD,$DC,$99,$9F,$BB,$B9,$33,$3E
	db "WOBBLE",$00
	; Entry point
section "WOBBLE", ROM0[$0150]
wobble_main:
	; E is our LUT offset
	ld e,$00
	; Initialize H to the
	; MSB of WOBBLE_DATA
	ld h,$20
wobble_loop:
	; B will be the scanline we
	; want to transform
	ld b,$00
.inner_loop:
	; Load the current scanline
	; position and compare it to B
	ldh a,[$44]
	cp b
	jr nz,.inner_loop
	ld a,b
	inc b
	add a,e
	and $1F
	; We use the current scanline
	; position and the LUT offset
	; to calculate the index into
	; the LUT.
	; idx = (scanline + lut_ofs) & 0x1F
	ld l,a
	ld a,[hl]
	; scroll_x = LUT[idx]
	ldh [$43],a
	ld a,l
	add a,$09
	and $1F
	ld l,a
	ld a,[hl]
	; idx = (idx + 9) & 0x1F
	; scroll_y = LUT[idx]
	ldh [$42],a
	ldh a,[$44]
	; Finally we check if we've
	; reached vblank to
	; break the loop
	cp $90
	jr nz,.inner_loop
	; Increment the LUT offset so
	; we can have motion.
	inc e
	jr wobble_loop
	; Lookup table of simple sine wave
section "WOBBLE_DATA", ROM0[$2000]
	db $00,$00,$01,$01,$02,$02,$02,$02
	db $02,$02,$02,$02,$01,$01,$00,$00
	db $00,$00,$FF,$FF,$FE,$FE,$FE,$FE
	db $FE,$FE,$FE,$FE,$FF,$FF,$00,$00

For compiling this with RBGDS we use the following commands:

rgbasm -o wobble.o wobble.asm

rgblink -o wobble.gb wobble.o

rgbfix -v -p0 wobble.gb

Now that the rom is built we can run it on the BGB emulator or a physical system. Sadly it won’t work with a Game Boy Color because that system clears the logo from VRAM after booting up.