Path: utzoo!attcan!uunet!husc6!bloom-beacon!gatech!rutgers!ucsd!ucsdhub!hp-sdd!hplabs!well!brecher
From: brecher@well.UUCP (Steve Brecher)
Newsgroups: comp.sys.mac.programmer
Subject: Re: Multifinder: how do I patch GetNextEvent?
Message-ID: <7374@well.UUCP>
Date: 14 Oct 88 20:46:11 GMT
References: <747@ttrdf.UUCP> <10050025@eecs.nwu.edu> <2456@spray.CalComp.COM>
Reply-To: brecher@well.UUCP (Steve Brecher)
Organization: Software Supply, Sunnyvale, CA
Lines: 34


In article <2456@spray.CalComp.COM>, anson@spray.CalComp.COM (Ed Anson) writes,
> As I understand it (now), the only traps which must not be tail patched are
> those which:
> 
> 1. Are relatively small, and
> 2. Are called from other (bigger) traps.

Add:
3. Are patched by Apple to fix bugs using "come from" tests.

A "come from" test examines the stack to determine the location of the
caller of the trap, e.g.

    if returnAddress = aParticularROMaddress then
         doSomethingToFixTheROMbug
    else
         jump to original trap code

Apple uses this technique to avoid consuming large amounts of RAM for
ROM bug fixes; in some cases, without "come from" tests, entire managers
would have to be replaced in RAM.

A tail patch wipes out the ROM bug fix patch because the return address
will never satisfy the test, since by definition a tail patch executes
the previous trap code with a JSR.

The wish to
> ...see the rules modified to something like:
> 1. Avoid tail patches, except for the following traps...

cannot be granted by Apple until such time as it discovers how to
write bug-free ROMs, or how to legally protect its operating system
code without resorting to delivering it in ROM.