Date: Sat, 13 Mar 1999 14:39:31 -0800 From: Jason Thorpe <thorpej@nas.nasa.gov> To: Dennis <dennis@etinc.com> Cc: hackers@FreeBSD.ORG Subject: Re: 'C' language question Message-ID: <199903132239.OAA12983@lestat.nas.nasa.gov>
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On Sat, 13 Mar 1999 17:41:04 -0500
Dennis <dennis@etinc.com> wrote:
> I have a memory mapped controller that requires that all of its
> register reads and writes be 32 bits....the follow code:
>
> *reg |= 0x80000000;
>
> generates a byte OR (ORB instruction) which trashes the register with
> gcc 2.9.2....a stupid "optimization" that happens to be wrong in this
> case.
For gcc's definition of `volatile':
volatile u_int32_t val, *reg = ...;
val = *reg;
val |= 0x80000000;
*reg = val;
...but, you should really be using bus_space (hey, even FreeBSD has it
now! :-), as the operation primitives there are specifically written
to do access in the explcitly specified size.
For best results, also make sure there are barrier operations in there,
so the system core logic isn't allowed to do write piggybacking.
> is there a declaration that will force the compiler to generate a 32bit
> OR (ORL instruction) on this?
Actually, you get it to do a load and a store, and you place your own
OR operation in between.
-- Jason R. Thorpe <thorpej@nas.nasa.gov>
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