mac-rom/Tidbits/PreventSwitchLaunch.a

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;
; File: PreventSwitchLaunch.a
;
; Contains: 'proc' -16495 code: Prevents switch launching from System 6 to System 7.
;
; Written by: Dean Yu
;
; Copyright: <09> 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 that<61>s 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 it<69>s 6.x code. If it isn<73>t, 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 doesn<73>t 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 it<69>s 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 we<77>re 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 there<72>s a nice big
; parameter block already there, there<72>s 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 don<6F>t 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 InitFontsPtch<63>s 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 we<77>re the most recent patch on _InitFonts
bne.s gotPatched ; If not, we do some fancy byte saving
_RecoverHandle ,SYS
_DisposHandle ; Free up the memory we<77>re using
move.w #kInitFontsTrap,d0
move.l oldInitFont,a0
_SetTrapAddress ; Restore _InitFonts
jmp (a0) ; Continue with _InitFonts
;
; There<72>s a patch in front of us on _InitFont, so we can<61>t remove ourselves,
; but we can reduce the space we<77>re 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 it<69>s
; 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
; wouldn<64>t 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 isn<73>t handled, and
; will generate a dsOldSystem system error.
;
FinderTooOld
moveq #dsOldSystem,d0
_SysError
ENDPROC
END