Mailinglist Archive: opensuse (5130 mails)

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Re: [SLE] Totally OT: 64/32,... -- memory models are _everything_ to programs
  • From: Jerry Feldman <gaf@xxxxxxx>
  • Date: Wed, 17 May 2006 08:45:30 -0400
  • Message-id: <200605170845.31686.gaf@xxxxxxx>
On Tuesday 16 May 2006 6:44 pm, Bryan J. Smith wrote:

> Yes, in the Windows world -- where intelligent Microsoft architects
> design good APIs and then Microsoft's own tool developers and
> application divisions _ignore_ them (despite Microsoft always blaming
> the problem on ISVs, it's Microsoft itself) -- it's a major issue (and
> always has been).
>
> But the IEEE POSIX and X/Open SUS has addressed the split 32-bit/64-bit
> memory model on Alpha, MIPS, SPARC and now x86-64. There is no reason
> not to follow the specifications, and the GNU compiler targets do
> exactly that.
>
> Compilers and toolchains will _never_ solve memory model issues.
Just as an aside.
There are a number programming memory models in use. In the Unix?Linux
world, we use LP32 for 32-bit programming where long integers and pointers
are 32bits. In the 64-bit world, we chose the LP64 model where long
integers and pointers are 64-bits and integers remain at 32-bits. I say we
as this is the model that Digital chose for Tru64 Unix (previously Digital
Unix and OSF/1) back in the early 1990s.
http://www.unix.org/version2/whatsnew/lp64_wp.html

LP32 and LP64 are not the only models in use. Microsoft chose LLP64 which
defines both the int and long int datatypes as 32-bits using only long long
and pointers at 64-bits. (Note that all of the models define
single-precision floating point as 32-bits and double precision floating
point at 64-bit (IEEE 754 standard).

One thing that comes into play here when talking about memory models is
virtual memory. A 32-bit program can only directly address 4GB of data
(segmented into stack, text, initialized data, heap, and mmapped segments)
where a 64-bit program can address 16 exabytes. (Certainly there are
physical limitations of the chips - PAE-52 for instance), and the 2.6
kernel permits addresses up to 64GB.

There are different needs in the community. Today's servers have need to
much larger physical memory, especially with multi-core, multi-CPU
architectures. The AMD-64 and EM64T architectures work well on the desktops
and in many server configurations where the larger servers need
architectures, such as IA64. The IA64 has a much different physical memory
model than does the X86-64 chips, but I'm not going to get into the
architecture.

--
Jerry Feldman <gaf@xxxxxxx>
Boston Linux and Unix user group
http://www.blu.org PGP key id:C5061EA9
PGP Key fingerprint:053C 73EC 3AC1 5C44 3E14 9245 FB00 3ED5 C506 1EA9

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