Path: utzoo!attcan!uunet!lll-winken!lll-tis!helios.ee.lbl.gov!pasteur!ucbvax!hplabs!nsc!voder!apple!landon
From: landon@Apple.COM (Landon Dyer)
Newsgroups: comp.sys.mac.programmer
Subject: 32K global data limit in MPW
Message-ID: <11064@apple.Apple.Com>
Date: 26 May 88 16:23:22 GMT
Organization: Apple Computer Inc, Cupertino, CA
Lines: 32

> ...  The MPW implementation is pretty
> much the same as the UNIX implementation, except the *stupid* compiler
> and the *stupid* linker will not allow more than 32k of global data.
> ...
>
>      I didn't realize this until trying to link Flex, but a maximum of
> 32k for global data in a real program is *stupid* (imagine the word
> *stupid* in bold, italic, shadowed style of the system font, size 24).
> Get with it, Apple.

The Mac architecture uses the 32K "below" A5 to hold global data.
16-bit register-relative references are much cheaper than any kind of
32-bit reference, which probably made a big difference in what would
fit into the 128K RAM available on the original Mac.

The current compilers generate 16-bit A5-relative references to global
data.  The code generation imposes the 32K limit; the linker doesn't
have much to do with it.

The Pascal and C compilers in the Beta release of MPW 3.0 will have
options to generate 32-bit A5-relative references to global data.
Since the 68000 doesn't have a corresponding addressing mode, several
instructions must be generated for each data reference.  It's rather
expensive, but it may help you get ported software off the ground.

Again: If you have more than 32K of global data, chances are that
you're doing something wrong.


-Landon
-- 

I speak for me.