Path: utzoo!utgpu!water!watmath!clyde!att!pacbell!ames!ncar!noao!mcdsun!fishpond!fnf
From: fnf@fishpond.UUCP (Fred Fish)
Newsgroups: comp.sys.amiga.tech
Subject: Re: Another question on the 80286- er, 68000 memory models
Message-ID: <47@fishpond.UUCP>
Date: 10 Jun 88 01:19:47 GMT
References: <8805182223.AA20918@cory.Berkeley.EDU> <2653@louie.udel.EDU> <2869@polya.STANFORD.EDU> <2069@sugar.UUCP> <527@sas.UUCP>
Reply-To: fnf@fishpond.UUCP (Fred Fish)
Organization: occasionally
Lines: 26

In article <527@sas.UUCP> toebes@sas.UUCP (John Toebes) writes:
>             Because of the C language definition (and even Modula2)
>you do not have any idea of the total amount of data that a program might
>contain.  Given this, the code must be generated knowing AHEAD of time
>how big the data is going to be.  The ONLY real optimization that can be
>done to do this without the programmer giving information ahead of time is
>to delay generation of code until link time.  There are indeed articles
>discussing this type of optimization, but it is far from acceptable in
>performance for a simple piece of information.

As a side note to this discussion, since it's not really relevant
to the Amiga (yet :-), the M88000 linker solves the small/large
model problem by actually patching the object code as necessary.
If a particular data item is in the lower 64K of memory, or within
64K of any known base pointer, the load or store access can be done
with a single instruction (small model equivalent).  If not, then
the access can be done with either a two or three instruction sequence
(large model equivalent), which the linker synthesizes and uses to replace
the original instruction.  The actual code to do this is only a few
pages of C code, and took me about two days to write and debug.

-Fred

-- 
# Fred Fish    hao!noao!mcdsun!fishpond!fnf     (602) 921-1113
# Ye Olde Fishpond, 1346 West 10th Place, Tempe, AZ 85281  USA