diff --git a/src/4cade.init.a b/src/4cade.init.a
index e8b35f220..e821747c2 100644
--- a/src/4cade.init.a
+++ b/src/4cade.init.a
@@ -20,8 +20,10 @@
          jsr   DisableAccelerator    ; set to 1 MHz (supports IIgs and many common accelerator cards)
          jsr   IsGS                  ; check for IIgs (allows super hi-res artwork)
          ror   zpMachineStatus
+         sta   SETC3ROM
          jsr   HasVidHDCard          ; check for VidHD card (allows super hi-res artwork even on non-IIgs machines)
          ror   zpMachineStatus
+         sta   CLRC3ROM
          jsr   Has128K               ; check for 128K (allows DHGR slideshows and 128K games)
          ror   zpMachineStatus
          jsr   HasJoystick           ; check for joystick (absence is OK but we filter out some games that require a joystick)
diff --git a/src/constants.a b/src/constants.a
index aa713ef51..e145fdb49 100644
--- a/src/constants.a
+++ b/src/constants.a
@@ -28,6 +28,8 @@ READMAINMEM =  $C002      ; STA to read from main mem
 READAUXMEM =   $C003      ; STA to read from aux mem
 WRITEMAINMEM = $C004      ; STA to write to main mem
 WRITEAUXMEM =  $C005      ; STA to write to aux mem
+CLRC3ROM =     $C00A      ; STA to use internal Slot 3 ROM (required to use 128K and DHGR)
+SETC3ROM =     $C00B      ; STA to use external Slot 3 ROM (required to detect VidHD in slot 3)
 CLR80VID =     $C00C      ; 40 columns (also used to get out of DHGR mode)
 SET80VID =     $C00D      ; 80 columns (also used to get into DHGR mode)
 PRIMARYCHARSET= $C00E     ; no mousetext for you