diff --git a/constants/SP2000.inc b/constants/SP2000.inc
index 4305f49..9177364 100644
--- a/constants/SP2000.inc
+++ b/constants/SP2000.inc
@@ -1837,12 +1837,20 @@ CtrlByte:						; Bytes for IDE.Read.Status
 
 ; device/head register bits
 DeviceHeadByte:
-.LBA			EQU	#40
-.Slave			EQU	#10
+.Obsolute		EQU	#A0
+.LBA			EQU	#40 + .Obsolute
+.CHS			EQU	#00 + .Obsolute
+.Master			EQU	#00 + .Obsolute
+.Slave			EQU	#10 + .Obsolute
 ;
 DeviceHeadBit:
 .LBA			EQU	6	; LBA = 1
 .Slave			EQU	4	; Slave = 1
+;
+Drive:
+.Master			EQU	DeviceHeadByte.Master
+.Slave			EQU	DeviceHeadByte.Slave
+
 
 DeviceControlBit:
 .Always_Set		EQU	3
@@ -1874,10 +1882,8 @@ ATAPI			EQU     2
 .UNKNOWN		EQU	#1F
 	ENDMODULE
 ;
-Drive:
-.Master			EQU	%1010'0000
-.Slave			EQU	%1011'0000
 
+;
 	MODULE	CMD
 ATAPI:
 .Reset			EQU #08