Path: utzoo!attcan!uunet!seismo!sundc!pitstop!sun!imagen!atari!portal!cup.portal.com!chrisj
From: chrisj@cup.portal.com
Newsgroups: comp.sys.mac.programmer
Subject: TN200 SlotVRemove Is Wrong
Message-ID: <9852@cup.portal.com>
Date: 8 Oct 88 04:58:28 GMT
Organization: The Portal System (TM)
Lines: 45
XPortal-User-Id: 1.1001.2041

Macintosh Tech Note 200, "MPW 2.0.2 Bugs", page 14, shows the following
"corrected" code for the SlotVRemove glue:

;----------------------------------------------------------------------
;FUNCTION SlotVRemove(vblTaskPtr: QElemPtr; theSlot: Integer): OSErr;
;----------------------------------------------------------------------
SlotVRemove PROC EXPORT
	MOVEA.L	(SP)+,A1	;Get the return address
;===>	MOVEA.L	(SP)+,A0	;==OOPS, popped args in wrong order!
	MOVE.W	(SP)+,D0	;The slot number
	MOVEA.L	(SP)+,A0	;The VBL task ptr
	MOVE.L	A1,-(SP)	;Put the return address back
	_SlotVRemove		;Call the register based trap
	MOVE.W	D0,(SP) 	;Set the result code
	JMP	(A1) 		;And go home

Unfortunately, the correction itself requires a correction.  The MOVE
preceeding the trap macro saves the return address on the stack, the
MOVE following the trap stores the result code on top of the high word
of the saved return address, and the JMP returns to the caller with 4
more bytes left on the stack than it should.  Since IM I-94 says that
the trap dispatcher saves and restores A1 around register-based traps,
it appears that we can just get rid of the instruction right before the
trap macro:
...
;===>	MOVE.L	A1,-(SP)	;unnecessary to save A1
	_SlotVRemove		;Call the register based trap
	MOVE.W	D0,(SP) 	;Set the result code
	JMP	(A1) 		;And go home

An alternative would be to replicate the first instruction of the
procedure, placing the duplicate immediately after the trap macro.
...
	MOVE.L	A1,-(SP)	;Put the return address back
	_SlotVRemove		;Call the register based trap
	MOVEA.L	(SP)+,A1	;Get the return address (again)
	MOVE.W	D0,(SP) 	;Set the result code
	JMP	(A1) 		;And go home

Would one of the Apple folks please confirm that the first alternative
shown above is the right way to glue to SlotVRemove?  Thank you.

Christopher T. Jewell   chrisj@cup.portal.com   sun!cup.portal.com!chrisj
"I figure the people who close zoos at night know what they're doing."
                        Mike Royko (on night baseball at Wrigley Field)