Try to clean up PrepareOAMData.

engine/oam_dma.asm

@@ -0,0 +1,25 @@
+WriteDMACodeToHRAM:
+; Since no other memory is available during OAM DMA,
+; DMARoutine is copied to HRAM and executed there.
+	ld c, $ff80 % $100
+	ld b, DMARoutineEnd - DMARoutine
+	ld hl, DMARoutine
+.copy
+	ld a, [hli]
+	ld [$ff00+c], a
+	inc c
+	dec b
+	jr nz, .copy
+	ret
+
+DMARoutine:
+	; initiate DMA
+	ld a, $c3
+	ld [$ff46], a
+
+	; wait for DMA to finish
+	ld a, $28
+.wait	dec a
+	jr nz, .wait
+	ret
+DMARoutineEnd:

engine/overworld/oam.asm

@@ -0,0 +1,178 @@
+PrepareOAMData:
+; Determine OAM data for currently visible
+; sprites and write it to wOAMBuffer.
+
+	ld a, [$cfcb]
+	dec a
+	jr z, .asm_4b1e
+
+	cp 0 - 1
+	ret nz
+	ld [$cfcb], a
+	jp HideSprites
+
+.asm_4b1e
+	xor a
+	ld [$ff90], a
+.asm_4b21
+	ld [$ff8f], a
+
+	ld d, wSpriteStateData1 / $100
+	ld a, [$ff8f]
+	ld e, a
+	ld a, [de] ; c1x0
+	and a
+	jp z, .asm_4bad
+
+	inc e
+	inc e
+	ld a, [de] ; c1x2 (facing/anim)
+	ld [$d5cd], a
+	cp $ff ; off-screen (don't draw)
+	jr nz, .visible
+
+	call Func_4bd1
+	jr .asm_4bad
+
+.visible
+	cp $a0
+	jr c, .usefacing
+	and $f
+	add $10
+	jr .asm_4b48
+
+.usefacing
+	and $f
+.asm_4b48
+	ld l, a
+
+	push de
+	inc d
+	ld a, e
+	add $5
+	ld e, a
+	ld a, [de] ; c2x7
+	and $80
+	ld [$ff94], a ; temp store sprite priority
+	pop de
+
+	ld h, 0
+	ld bc, SpriteFacingAndAnimationTable
+	add hl, hl
+	add hl, hl
+	add hl, bc
+
+	ld a, [hli]
+	ld c, a
+	ld a, [hli]
+	ld b, a
+	ld a, [hli]
+	ld h, [hl]
+	ld l, a
+
+	call Func_4bd1
+
+	ld a, [$ff90]
+	ld e, a
+	ld d, wOAMBuffer / $100
+.tile
+	ld a, [$ff92]            ; temp for sprite Y position
+	add $10                  ; Y=16 is top of screen (Y=0 is invisible)
+	add [hl]                 ; add Y offset from table
+	ld [de], a               ; write new sprite OAM Y position
+	inc hl
+	ld a, [$ff91]            ; temp for sprite X position
+	add $8                   ; X=8 is left of screen (X=0 is invisible)
+	add [hl]                 ; add X offset from table
+	inc e
+	ld [de], a               ; write new sprite OAM X position
+	inc e
+	ld a, [bc]               ; read pattern number offset (accomodates orientation (offset 0,4 or 8) and animation (offset 0 or $80))
+	inc bc
+	push bc
+	ld b, a
+
+	ld a, [$d5cd]            ; temp copy of c1x2
+	swap a                   ; high nybble determines sprite used (0 is always player sprite, next are some npcs)
+	and $f
+
+	; Sprites $a and $b have one face (and therefore 4 tiles instead of 12).
+	; As a result, sprite $b's tile offset is less than normal.
+	cp $b
+	jr nz, .offset
+	ld a, $a * 12 + 4
+	jr .gotoffset
+
+.offset
+	; a *= 12
+	sla a
+	sla a
+	ld c, a
+	sla a
+	add c
+.gotoffset
+	add b ; which frame
+	pop bc
+	ld [de], a ; tile id
+	inc hl
+	inc e
+	ld a, [hl]
+	bit 1, a ; sprite priority
+	jr z, .fg
+	ld a, [$ff94] ; facing priority
+	or [hl]
+.fg
+	inc hl
+	ld [de], a
+	inc e
+	bit 0, a ; OAMFLAG_ENDOFDATA
+	jr z, .tile
+
+	ld a, e
+	ld [$ff90], a
+
+.asm_4bad
+	ld a, [$ff8f]
+	add $10
+	cp $100 % $100
+	jp nz, .asm_4b21
+
+	; Clear unused OAM.
+	ld a, [$ff90]
+	ld l, a
+	ld h, wOAMBuffer / $100
+	ld de, $4
+	ld b, $a0
+	ld a, [$d736]
+	bit 6, a
+	ld a, $a0
+	jr z, .clear
+	ld a, $90
+.clear
+	cp l
+	ret z
+	ld [hl], b
+	add hl, de
+	jr .clear
+
+Func_4bd1: ; 4bd1 (1:4bd1)
+	inc e
+	inc e
+	ld a, [de] ; c1x4
+	ld [$ff92], a
+	inc e
+	inc e
+	ld a, [de] ; c1x6
+	ld [$ff91], a
+	ld a, $4
+	add e
+	ld e, a
+	ld a, [$ff92]
+	add $4
+	and $f0
+	ld [de], a ; c1xa (y)
+	inc e
+	ld a, [$ff91]
+	and $f0
+	ld [de], a  ; c1xb (x)
+	ret