Identify JOYP_SGB_MLT_REQ constant

Author: Rangi42

constants/misc_constants.asm

@@ -10,3 +10,6 @@ DEF TRUE  EQU 1
 
 ; input
 DEF NO_INPUT EQU 0
+
+; SGB command MLT_REQ can be used to detect SGB hardware
+DEF JOYP_SGB_MLT_REQ EQU %00000011

engine/gfx/palettes.asm

@@ -462,8 +462,8 @@ CheckSGB:
 	ei
 	call Wait7000
 	ldh a, [rJOYP]
-	and $3
-	cp $3
+	and JOYP_SGB_MLT_REQ
+	cp JOYP_SGB_MLT_REQ
 	jr nz, .isSGB
 	ld a, JOYP_SGB_ZERO
 	ldh [rJOYP], a
@@ -494,8 +494,8 @@ CheckSGB:
 	call Wait7000
 	call Wait7000
 	ldh a, [rJOYP]
-	and $3
-	cp $3
+	and JOYP_SGB_MLT_REQ
+	cp JOYP_SGB_MLT_REQ
 	jr nz, .isSGB
 	call SendMltReq1Packet
 	and a