diff --git a/cpu/callRet.asm b/cpu/callRet.asm
index b0b438f..464dd08 100644
--- a/cpu/callRet.asm
+++ b/cpu/callRet.asm
@@ -18,14 +18,16 @@ SECTION "home", ROM0
 ProgramStart:
 	call waitVBlank
 
-	ld b, $ff  ; Fill color.
+	ld b, $ff  ; Fill pattern.
 
 	call clearLogo
 
+	; Simple call.
 	call waitVBlank
 	ld hl, $8010
 	call fillTile
 
+	; Conditional returns.
 	call waitVBlank
 	call retNz
 	ld hl, $8040
@@ -46,30 +48,43 @@ ProgramStart:
 	ld hl, $80a0
 	call fillTile
 
+	; Conditional calls.
 	call waitVBlank
 	ld a, 2
 	dec a
-	ld hl, $8110
+	ld hl, $80f0
 	call nz, fillTile
 
 	call waitVBlank
 	ld a, 1
 	dec a
-	ld hl, $8130
+	ld hl, $8110
 	call z, fillTile
 
 	call waitVBlank
 	ld a, 200
 	add a, 55
-	ld hl, $8150
+	ld hl, $8130
 	call nc, fillTile
 
 	call waitVBlank
 	ld a, 255
 	add a, 1
-	ld hl, $8170
+	ld hl, $8150
 	call c, fillTile
 
+	; Stack pointer validation.
+	ld hl, sp+$00
+	ld a, h
+	cp $ff
+	jr nz, loop
+	ld a, l
+	cp $fe
+	jr nz, loop
+	ld hl, $8180
+	call waitVBlank
+	call fillTile
+
 loop:
 	jr loop