Difference between x86, x32, and x64 architectures?

Question

Please explain the difference between x86, x32 and x64? Its a bit confusing when it comes to x86 and x32 because most of the time 32-bit programs run on x86...

x32 is a new ABI being actively worked on. It is basically 32-bit code running in x86_64 (x64) mode on the CPU so that it has access to the additional 8 registers to boost program speed while remaining memory efficient via the use of 32-bit pointers. See sites.google.com/site/x32abi and lwn.net/Articles/456731. — gps
The x32 ABI is part of Linux 3.4, released on the 20th May 2012 — Frédéric Grosshans
@jww I don't have a hammer. Also, I didn't say that this wasn't a real question, just that it's off-topic. It's a little bit broad, and probably would have been better asked on Super User. Still, Hans' answer can be improved without reopening the question. — Bill the Lizard
@jww Ah, I see. I added a reopen vote. I guess there's no harm in SO hosting a borderline SO/SU question. — Bill the Lizard
x86 refers to the architectures compatible with intel 386, 486, etc. x32 and x64 are misnomers often used just to mean "32-bit" or "64-bit" — martinkunev

jww jww · Accepted Answer · 2015-09-19T01:37:55

Hans and DarkDust answer covered i386/i686 and amd64/x86_64, so there's no sense in revisiting them. This answer will focus on X32, and provide some info learned after a X32 port.

x32 is an ABI for amd64/x86_64 CPUs using 32-bit integers, longs and pointers. The idea is to combine the smaller memory and cache footprint from 32-bit data types with the larger register set of x86_64. (Reference: Debian X32 Port page).

x32 can provide up to about 30% reduction in memory usage and up to about 40% increase in speed. The use cases for the architecture are:

vserver hosting (memory bound)
netbooks/tablets (low memory, performance)
scientific tasks (performance)

x32 is a somewhat recent addition. It requires kernel support (3.4 and above), distro support (see below), libc support (2.11 or above), and GCC 4.8 and above (improved address size prefix support).

For distros, it was made available in Ubuntu 13.04 or Fedora 17. Kernel support only required pointer to be in the range from 0x00000000 to 0xffffffff. From the System V Application Binary Interface, AMD64 (With LP64 and ILP32 Programming Models), Section 10.4, p. 132 (its the only sentence):

10.4 Kernel Support
Kernel should limit stack and addresses returned from system calls between 0x00000000 to 0xffffffff.

When booting a kernel with the support, you must use syscall.x32=y option. When building a kernel, you must include the CONFIG_X86_X32=y option. (Reference: Debian X32 Port page and X32 System V Application Binary Interface).

Here is some of what I have learned through a recent port after the Debian folks reported a few bugs on us after testing:

the system is a lot like X86
the preprocessor defines __x86_64__ (and friends) and __ILP32__, but not __i386__/__i686__ (and friends)
you cannot use __ILP32__ alone because it shows up unexpectedly under Clang and Sun Studio
when interacting with the stack, you must use the 64-bit instructions pushq and popq
once a register is populated/configured from 32-bit data types, you can perform the 64-bit operations on them, like adcq
be careful of the 0-extension that occurs on the upper 32-bits.

If you are looking for a test platform, then you can use Debian 8 or above. Their wiki page at Debian X32 Port has all the information. The 3-second tour: (1) enable X32 in the kernel at boot; (2) use debootstrap to install the X32 chroot environment, and (3) chroot debian-x32 to enter into the environment and test your software.

Difference between x86, x32, and x64 architectures?

4 Answers