Path: utzoo!utgpu!water!watmath!clyde!att!pacbell!ames!pasteur!ucbvax!decwrl!labrea!polya!rokicki
From: rokicki@polya.STANFORD.EDU (Tomas G. Rokicki)
Newsgroups: comp.sys.amiga.tech
Subject: Re: Another question on the 80286- er, 68000 memory models
Message-ID: <2869@polya.STANFORD.EDU>
Date: 20 May 88 18:13:56 GMT
References: <8805182223.AA20918@cory.Berkeley.EDU> <2653@louie.udel.EDU>
Reply-To: rokicki@polya.Stanford.EDU (Tomas G. Rokicki)
Organization: Stanford University
Lines: 62

>    No, i have no problem with your discussion. I knew all that 
> stuff anyway. But the fun i am having with xenix kernel hacking
> has me a little worried, even though i have not been burned yet ...
> For example, i put a pointer into a structure. Now if i am
> in the small data mode, then maybe what gets put in is 16 bits. 
> Then i go to the library which is large model, ... bang!
>    Anyway, i will try to work up a more concrete example and see
> if this scenario is possible. I sure hope not.And i have not
> seen it yet, or been burned by it, but it bothers me somehow.

Let's get this all straight.  There are three independent things
that affect the `model' on the Lattice and Manx compilers:

	* data addressing.  This can either be 32-bit absolute
	  or 16-bit relative.  *Pointers are 32-bits in either
	  case*.  In the latter case, you are only allowed
	  65K of data.  You can mix these two models, so long as
	  any data used by the 16-bit relative stuff is within
	  the first 65K of the data segment (right?)  You should
	  always use 16-bit relative, and if your code has more
	  than 65K of static data, malloc it anyway.  Please.
	  It's real simple to do.  Another problem is in interrupt
	  code, where you need to set up the base register with
	  a call to _geta4() in Manx, and something similar in
	  Lattice (I've forgotten what it is) at the very
	  beginning.

	* code addressing (jsr's.)  These can either be 32-bit
	  absolute or 16-bit relative.  Again, you can mix
	  models.  If using 16-bit relative, then you have
	  a small problem with branches that are farther than
	  +/- 32K.  Manx solves this by indirecting off an
	  automatically created vector list in the data portion
	  of the program.  Lattice does something similar (but
	  I seem to be having troubles with it somehow . . .)
	  Always use 16-bit relative; no sense not to, since
	  the long ones are taken care of automatically, and
	  smaller code size is usually good.  Again, in
	  interrupt code, you need to set up a4 to get access
	  to these fake long branches in 16-bit offset mode.

	* int size.  Both Manx and Lattice allow 16 and 32-bit
	  ints.  Again, you can mix models, but be *very*
	  careful when you do so, and pull in the correct
	  libraries, due to the way args are passed on the
	  stack and the way printf/scanf work.  You should
	  use one or the other and stick with it.

In summary?  The only thing that really matters is int size, and that
you use the correct libraries for the int size you are working with.
The other models should mix freely, assuming you don't have more than
65K of statically allocated data that pushes the compiler's data out
of range.

Someone please correct me where I'm wrong.

-tom
-- 
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