diff --git a/hardware.inc b/hardware.inc
index e1e3b2c..4e18013 100644
--- a/hardware.inc
+++ b/hardware.inc
@@ -1031,16 +1031,15 @@ def INT_HANDLER_JOYPAD equ $0060 ; joypad interrupt handler address
 ;******************************************************************************
 
 ; Register A = CPU type
-def BOOTUP_A_DMG equ $01
+def BOOTUP_A_DMG equ $01 ; DMG or SGB
 def BOOTUP_A_CGB equ $11 ; CGB or AGB
 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)
-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 B = CPU qualifier
+def BOOTUP_B_DMG0  equ $FF
+def B_BOOTUP_B_AGB equ 0 ; 0 = CGB, 1 = AGB
 
 ; Register C = CPU qualifier
 def BOOTUP_C_DMG equ $13
diff --git a/hardware_compat.inc b/hardware_compat.inc
index 140b481..b8e6834 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
 
+; Warning: 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