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From: rick@jessica.stanford.edu (Rick Wong)
Newsgroups: comp.sys.mac.programmer
Subject: 32K jump table limit
Keywords: MacApp
Message-ID: <3425@Portia.Stanford.EDU>
Date: 24 Aug 88 18:47:38 GMT
Sender: news@Portia.Stanford.EDU
Reply-To: rick@jessica.stanford.edu (Rick Wong)
Organization: Stanford University
Lines: 29


Much has been written here about the Mac's 32K global DATA limit.
I've got a nastier problem:  the 32K JUMP TABLE limit is too small.

On the Mac, code is divided into segments, to allow code to be
swapped into memory as needed.  Routines call routines in other
segments indirectly, via the jump table.  Also, Object Pascal uses
the jump table for method dispatching.

Since each entry in the jump table is 8 bytes, the maximum number
of entries is about 4K (4091, accounting for extra fields before
the jump table).

At first, over 4000 procedures seems a huge number.  It isn't.  I
am using MacApp, which is written in Object Pascal.  Every method
requires requires another jump table entry.  The simplest MacApp
application, which does "nothing," uses about 1100 entries with
debugging, 600 without.  The remaining entries don't seem so numer-
ous in large MacApp applications.

(By the way, the MPW 2.0.2 linker goes into an infinite loop when
an application goes over this 4K limit.)

Does anyone have any suggestions for dealing with this problem?
(This looks like a job for Apple.)

Rick Wong
Courseware Authoring Tools Project, Stanford University
rick@jessica.stanford.edu