Path: utzoo!mnetor!uunet!oddjob!hao!husc6!tut.cis.ohio-state.edu!ut-sally!utah-cs!utah-gr!uplherc!sp7040!obie!wes
From: wes@obie.UUCP (Barnacle Wes)
Newsgroups: comp.lang.c
Subject: Re: register unions
Message-ID: <77@obie.UUCP>
Date: 26 Feb 88 06:52:29 GMT
References: <1229@eneevax.UUCP> <10075@ulysses.homer.nj.att.com> <41964@sun.uucp>
Distribution: comp.lang.c
Organization: UinTech, Layton, UT
Lines: 29
Keywords: registers, unions
Summary: Varying register usage...

In article <41964@sun.uucp>, guy@gorodish.Sun.COM (Guy Harris) writes:
> > The official AT&T version of the K&R portable C compiler only
> > supports register allocation of int's.  All other types are
> > silently assigned to core.
> 
> I presume you mean "integral types", not "int"s.  The version of PCC2 that
> comes with S5R3.1 on 3B2s, for instance, quite cheerfully puts "short"s,
> "char"s, and "long"s into registers as well.

A lot of this type of problem is caused by the underlying machine
architecture.  On the M68000, you usually have 5 or 6 address
registers available, so most 68K C compilers let you put pointers in
registers as well as integral types.  The one I use the most (MWC for
Atari ST) does not allow floats of any flavor to be put in registers.

On Intel chips, a pointer in a register isn't really defined; there
are no real general-purpose registers, and there aren't any registers
big enough to hold a full address.  I guess you could probably put an
address is ES:DI or something like that, but I don't know of any
compilers that do.  MicroPort's pcc surely doesn't!

----
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