diff --git a/hardware.inc b/hardware.inc
index e1e3b2c..ef53f34 100644
--- a/hardware.inc
+++ b/hardware.inc
@@ -1037,10 +1037,9 @@ def BOOTUP_A_MGB equ $FF
     def BOOTUP_A_SGB  equ BOOTUP_A_DMG
     def BOOTUP_A_SGB2 equ BOOTUP_A_MGB
 
-; Register B = CPU qualifier (if A is BOOTUP_A_CGB)
+; Register B = CPU qualifier
+def BOOTUP_B_DMG0  equ $FF
 def B_BOOTUP_B_AGB equ 0
-    def BOOTUP_B_CGB equ 0 << B_BOOTUP_B_AGB
-    def BOOTUP_B_AGB equ 1 << B_BOOTUP_B_AGB
 
 ; Register C = CPU qualifier
 def BOOTUP_C_DMG equ $13
diff --git a/hardware_compat.inc b/hardware_compat.inc
index 704777d..e89c523 100644
--- a/hardware_compat.inc
+++ b/hardware_compat.inc
@@ -385,6 +385,11 @@ def OAM_B equ OAM_SIZE
 
 def BOOTUPB_B_AGB equ B_BOOTUP_B_AGB
 
+; These values should not be relied upon; see
+; https://gbdev.io/pandocs/Power_Up_Sequence.html#cgbdmg_b
+    def BOOTUP_B_CGB equ 0 << B_BOOTUP_B_AGB
+    def BOOTUP_B_AGB equ 1 << B_BOOTUP_B_AGB
+
 
 ;******************************************************************************
 ; Aliases