diff --git a/common/hw-gfx-gma-pcode.ads b/common/hw-gfx-gma-pcode.ads
new file mode 100644
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+++ b/common/hw-gfx-gma-pcode.ads
@@ -0,0 +1,60 @@
+--
+-- Copyright (C) 2016, 2019 secunet Security Networks AG
+--
+-- This program is free software; you can redistribute it and/or modify
+-- it under the terms of the GNU General Public License as published by
+-- the Free Software Foundation; either version 2 of the License, or
+-- (at your option) any later version.
+--
+-- This program is distributed in the hope that it will be useful,
+-- but WITHOUT ANY WARRANTY; without even the implied warranty of
+-- MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+-- GNU General Public License for more details.
+--
+
+with HW.GFX.GMA.Registers;
+
+private package HW.GFX.GMA.PCode is
+
+   -- We have to ensure that previous usage of the mailbox finished
+   -- (Wait_Ready) or know that we already did so (Mailbox_Ready).
+   --
+   -- If we wait for the other side to acknowledge (Wait_Ack), we
+   -- know that it's ready (=> Mailbox_Ready).
+
+   -- XXX: Supposed to be a `Ghost` variable, but GNAT seems too broken?
+   Mailbox_Ready : Boolean with Part_Of => HW.GFX.GMA.State;
+
+   -- Just send a command, discard the reply.
+   procedure Mailbox_Write
+     (MBox        : in     Word32;
+      Command     : in     Word64;
+      Wait_Ready  : in     Boolean := False;
+      Wait_Ack    : in     Boolean := True;
+      Success     :    out Boolean)
+   with
+      Pre => Mailbox_Ready or Wait_Ready,
+      Post => (if Wait_Ack and Success then Mailbox_Ready);
+
+   -- Repeatedly send a request command the expected reply is received.
+   procedure Mailbox_Request
+     (MBox        : in     Word32;
+      Command     : in     Word64;
+      Reply_Mask  : in     Word64;
+      Reply       : in     Word64 := 16#ffff_ffff_ffff_ffff#;
+      TOut_MS     : in     Natural := Registers.Default_Timeout_MS;
+      Wait_Ready  : in     Boolean := False;
+      Success     :    out Boolean)
+   with
+      Pre => Mailbox_Ready or Wait_Ready,
+      Post => (if Success then Mailbox_Ready);
+
+   -- For final mailbox commands that don't have to wait.
+   procedure Mailbox_Write
+     (MBox        : Word32;
+      Command     : Word64;
+      Wait_Ready  : Boolean := False)
+   with
+      Pre => Mailbox_Ready or Wait_Ready;
+
+end HW.GFX.GMA.PCode;