mac-rom/Tidbits/PreventSwitchLaunch.a

;
;	File:		PreventSwitchLaunch.a
;
;	Contains:	'proc' -16495 code:  Prevents switch launching from System 6 to System 7.
;
;	Written by:	Dean Yu
;
;	Copyright:	© 1990 by Apple Computer, Inc., all rights reserved.
;
;	Change History (most recent first):
;
;		 <3>	  6/6/90	DTY		Went back to using stack frame to save default directory, to get
;									around bug in FixSMgrWorld which trashed D1.
;		 <1>	  6/6/90	DTY		First checked in.  _InitFonts patch to prevent a switch launch
;									to System 7 from older Systems.  Closes the System 7 System
;									file, re-opens the previous System file, re-initialise the
;									system resource map, and attempt to remove patch from
;									_InitFonts.
;

		LOAD	'StandardEqu.d'

kInitFontsTrap	equ	$fe						; Trap word of _InitFonts
kJmpInstr		equ	$4ef9					; JMP instruction used for unpatching

InitFontsPtch	PROC	EXPORT

patchVars	RECORD	0,DECR
curVRefNum	ds.w	1					; Save current volume reference number
patchVarSize	equ	*
			ENDR
			
;
;	To reduce the amount of memory thatÕs used we attempt to remove this patch from _InitFonts
;	at the end of the patch.  If we determine that someone else patched _InitFonts before we
;	got called (unlikely, but it could happen), the following bra.s instruction is changed to
;	a jmp, and we reduce the size of our memory block down to 6 bytes, just enough to hold the
;	jmp instruction.
;

		WITH	patchVars
		bra.s	startInitFontsPatch
oldInitFont	dc.l	0					; Address of previous _InitFonts routine stored here

startInitFontsPatch

;
;	This code should only be run if the switch launch is coming from Finder 6.x  Check the code
;	at the return address on the stack to make sure itÕs 6.x code.  If it isnÕt, generate a
;	dsOldSystem System Error.  If it is Finder 6.x, add 4 bytes to the return address, since we
;	want to bypass the Finder changing the value in BootDrive, since we closed the System 7 System
;	file, and re-opened the old System file, so everything is the way it was before Finder started
;	switch launch, except Finder doesnÕt know that, so we have to fool it.
;
;	(SP) (Before) --->	31C3 0210		move.w	d3,BootDrive
;	(SP) (After)  --->	508f			addq.l	#8,sp
;
		move.l	(sp),a0					; Get the return address, and check the next instruction
		cmpi.l	#$31c30210,(a0)+		; to make sure itÕs a move.w D3,BootDrive
		bne.s	FinderTooOld			; If not, force a System Error.
		move.l	a0,(sp)					; Put our modified return address back on the stack
										
;	If weÕre here, then we should close the System 7 System file, and re-open the System 6
;	System file, and re-initialise the system resource map.
										
		clr.w	-(sp)
		_CloseResFile					; Close the System file
		
		link	a6,#patchVarSize
		sub.w	#ioFQElSize,sp

		move.l	sp,a0
		clr.l	ioCompletion(a0)
		clr.l	ioNamePtr(a0)
		_GetVol							; Get the current default volume
		move.w	ioVRefNum(a0),curVRefNum(a6)		; and save it
		
		move.w	BootDrive,ioVRefNum(a0)	; Set the volume to the previous System Folder
		_SetVol

;
;	_InitResources returns the reference number of the System file on the stack, but since thereÕs a nice big
;	parameter block already there, thereÕs no REAL reason to allocate a word for the reference number, since
;	the next call to _SetVol will reset that word anyway.
;	

		_InitResources					; _InitResources will open the System file, and rebuild the resource map
		
		move.l	sp,a0
		move.w	curVRefNum(a6),ioVRefNum(a0)		; Restore the default directory
		_SetVol

		add.w	#ioFQElSize,sp			; Get rid of our parameter block
		unlk	a6						; We donÕt need our stack frame any more
;
;	Remove this patch
;

		move.w	#kInitFontsTrap,d0
		_GetTrapAddress					; Get the address of _InitFonts routine
		move.l	a0,d0
		_StripAddress					; Clean this guy off
		move.l	d0,a0					; Save it
		lea		InitFontsPtch,a1		; Get InitFontsPtchÕs entry point
		move.l	a1,d0
		_StripAddress					; Cleanse ourselves of dirty bits

		exg		a0,d0					; Ensure that entry point of InitFontsPtch is in A0 for _RecoverHandle later
		cmp.l	d0,a0					; Make sure weÕre the most recent patch on _InitFonts
		bne.s	gotPatched				; If not, we do some fancy byte saving
		
		_RecoverHandle ,SYS
		_DisposHandle					; Free up the memory weÕre using
		
		move.w	#kInitFontsTrap,d0
		move.l	oldInitFont,a0
		_SetTrapAddress					; Restore _InitFonts
		jmp		(a0)					; Continue with _InitFonts

;
;	ThereÕs a patch in front of us on _InitFont, so we canÕt remove ourselves,
;	but we can reduce the space weÕre taking up to 6 bytes:
;	Since the address of the next _InitFonts routine is stored in bytes 2 thru 5 of this patch,
;	the preceding two byte bra.s instruction can be changed to a jmp, which would then use
;	oldInitFonts as the effective address to jump to.  So, when this patch gets called again, 
;	it will jump straight to the next _InitFonts patch/routine, since InitFontsPtch has done itÕs
;	job.

gotPatched

		move.w	#kJmpInstr,(a0)			; Replace bra.s with jmp
		_RecoverHandle ,SYS				; Handle to our block
		move.l	#startInitFontsPatch - InitFontsPtch,d0		; Size of jump/bra.s instruction
		_SetHandleSize					; Reduce our memory block down to size of jump.
										; This needs to be changed to something safer, because the Memory Manager
										; may move a block into this space when the patch gets cut back, and we
										; wouldnÕt be here any more.
		bra.s	InitFontsPtch			; Since this is a jump now, we can just branch back to it to call the next routine
		
;
;	InitFontsPtch checks specifically for code in Finder 6.x.  Anything else isnÕt handled, and
;	will generate a dsOldSystem system error.
;

FinderTooOld

		moveq	#dsOldSystem,d0
		_SysError
		ENDPROC
		END