ARCH(7) FreeBSD Miscellaneous Information Manual ARCH(7)
NAME
arch - Architecture-specific details
DESCRIPTION
Differences between CPU architectures and platforms supported by FreeBSD.
Introduction
This document is a quick reference of key ABI details of FreeBSD
architecture ports. For full details consult the processor-specific ABI
supplement documentation.
If not explicitly mentioned, sizes are in bytes. The architecture
details in this document apply to FreeBSD 13.0 and later, unless
otherwise noted.
FreeBSD uses a flat address space. Variables of types unsigned long,
uintptr_t, and size_t and pointers all have the same representation.
In order to maximize compatibility with future pointer integrity
mechanisms, manipulations of pointers as integers should be performed via
uintptr_t or intptr_t and no other types. In particular, long and
ptrdiff_t should be avoided.
On some architectures, e.g., AIM variants of powerpc64, the kernel uses a
separate address space. On other architectures, kernel and a user mode
process share a single address space. The kernel is located at the
highest addresses.
On each architecture, the main user mode thread's stack starts near the
highest user address and grows down.
FreeBSD architecture support varies by release. This table shows
currently supported CPU architectures along with the first FreeBSD
release to support each architecture.
Architecture Initial Release
aarch64 11.0
amd64 5.1
armv7 12.0
powerpc64 9.0
powerpc64le 13.0
riscv64 12.0
Discontinued architectures are shown in the following table.
Architecture Initial Release Final Release
alpha 3.2 6.4
arm 6.0 12.4
armeb 8.0 11.4
armv6 10.0 14.x
ia64 5.0 10.4
i386 1.0 14.x
mips 8.0 13.5
mipsel 9.0 13.5
mipselhf 12.0 13.5
mipshf 12.0 13.5
mipsn32 9.0 13.5
mips64 9.0 13.5
mips64el 9.0 13.5
mips64elhf 12.0 13.5
mips64hf 12.0 13.5
pc98 2.2 11.4
powerpc 6.0 14.x
powerpcspe 12.0 14.x
riscv64sf 12.0 13.5
sparc64 5.0 12.4
Type sizes
All FreeBSD architectures use some variant of the ELF (see elf(5))
Application Binary Interface (ABI) for the machine processor. All
supported ABIs can be divided into two groups:
ILP32 int, long, void * types machine representations all have 4-byte
size.
LP64 int type machine representation uses 4 bytes, while long and
void * are 8 bytes.
Some machines support more than one FreeBSD ABI. Typically these are
64-bit machines, where the "native" LP64 execution environment is
accompanied by the "legacy" ILP32 environment, which was the historical
32-bit predecessor for 64-bit evolution. Examples are:
LP64 ILP32 counterpart
amd64 i386
powerpc64 powerpc
aarch64 armv7
aarch64 will support execution of armv7 binaries if the CPU implements
AArch32 execution state. Binaries targeting armv6 and earlier are no
longer supported by FreeBSD.
On all supported architectures:
Type Size
short 2
int 4
long sizeof(void*)
long long 8
float 4
double 8
Integers are represented in two's complement. Alignment of integer and
pointer types is natural, that is, the address of the variable must be
congruent to zero modulo the type size. Most ILP32 ABIs, except arm,
require only 4-byte alignment for 64-bit integers.
Machine-dependent type sizes:
Architecture void * long double time_t
aarch64 8 16 8
amd64 8 16 8
armv7 4 8 8
i386 4 12 4
powerpc 4 8 8
powerpcspe 4 8 8
powerpc64 8 8 8
powerpc64le 8 8 8
riscv64 8 16 8
time_t is 8 bytes on all supported architectures except i386.
Endianness and Char Signedness
Architecture Endianness char Signedness
aarch64 little unsigned
amd64 little signed
armv7 little unsigned
i386 little signed
powerpc big unsigned
powerpcspe big unsigned
powerpc64 big unsigned
powerpc64le little unsigned
riscv64 little signed
Page Size
Architecture Page Sizes
aarch64 4K, 64K, 2M, 1G
amd64 4K, 2M, 1G
armv7 4K, 1M
i386 4K, 2M (PAE), 4M
powerpc 4K
powerpcspe 4K
powerpc64 4K
powerpc64le 4K
riscv64 4K, 2M, 1G
User Address Space Layout
Architecture Maximum Address Address Space Size
aarch64 0x0001000000000000 256TiB
amd64 (LA48) 0x0000800000000000 128TiB
amd64 (LA57) 0x0100000000000000 64PiB
armv7 0xbfc00000 3GiB
i386 0xffc00000 4GiB
powerpc 0xfffff000 4GiB
powerpcspe 0x7ffff000 2GiB
powerpc64 0x000fffffc0000000 4PiB
powerpc64le 0x000fffffc0000000 4PiB
riscv64 (Sv39) 0x0000004000000000 256GiB
riscv64 (Sv48) 0x0000800000000000 128TiB
The layout of a process' address space can be queried via the
KERN_PROC_VM_LAYOUT sysctl(3) MIB.
Historically, amd64 CPUs were limited to a 48-bit virtual address space.
Newer CPUs support 5-level page tables, which extend the significant bits
of addresses to 57 bits (LA57 mode). The address space layout is
determined by the CPU's support for LA57. Setting the vm.pmap.la57
tunable to 0 forces the system into 4-level paging mode, even on hardware
that supports 5-level paging. In this mode, all processes get a 48-bit
address space. The vm.pmap.prefer_la48_uva tunable determines whether
processes running on a LA57 system are limited to a 48-bit address space
by default. Some applications make use of unused upper bits in pointer
values to store information, and thus implicitly assume they are running
in LA48 mode. To avoid breaking compatibility, all processes run in LA48
mode by default. The elfctl(1) utility can be used to request LA48 or
LA57 mode for specific executables. Similarly, proccontrol(1) can be
used to configure the address space layout when executing a process.
The RISC-V specification permits 3-level (Sv39), 4-level (Sv48), and
5-level (Sv57) page tables. Hardware is only required to implement Sv39;
implementations which support Sv48 must also support Sv39, and
implementations which support Sv57 must also support Sv48. The
vm.pmap.mode tunable can be used to select the layout. FreeBSD currently
supports Sv39 and Sv48 and defaults to using Sv39.
Floating Point
Architecture float, double long double
aarch64 hard soft, quad precision
amd64 hard hard, 80 bit
armv7 hard hard, double precision
i386 hard hard, 80 bit
powerpc hard hard, double precision
powerpcspe hard hard, double precision
powerpc64 hard hard, double precision
powerpc64le hard hard, double precision
riscv64 hard hard, quad precision
Default Tool Chain
FreeBSD uses clang(1) as the default compiler on all supported CPU
architectures, LLVM's ld.lld(1) as the default linker, and LLVM binary
utilities such as objcopy(1) and readelf(1).
MACHINE_ARCH vs MACHINE_CPUARCH vs MACHINE
MACHINE_CPUARCH should be preferred in Makefiles when the generic
architecture is being tested. MACHINE_ARCH should be preferred when
there is something specific to a particular type of architecture where
there is a choice of many, or could be a choice of many. Use MACHINE
when referring to the kernel, interfaces dependent on a specific type of
kernel or similar things like boot sequences.
MACHINE MACHINE_CPUARCH MACHINE_ARCH
arm64 aarch64 aarch64
amd64 amd64 amd64
arm arm armv7
i386 i386 i386
powerpc powerpc powerpc, powerpcspe, powerpc64,
powerpc64le
riscv riscv riscv64
Predefined Macros
The compiler provides a number of predefined macros. Some of these
provide architecture-specific details and are explained below. Other
macros, including those required by the language standard, are not
included here.
The full set of predefined macros can be obtained with this command:
cc -x c -dM -E /dev/null
Common type size and endianness macros:
Macro Meaning
__LP64__ 64-bit (8-byte) long and pointer, 32-bit (4-byte) int
__ILP32__ 32-bit (4-byte) int, long and pointer
BYTE_ORDER Either BIG_ENDIAN or LITTLE_ENDIAN. PDP11_ENDIAN is
not used on FreeBSD.
Architecture-specific macros:
Architecture Predefined macros
aarch64 __aarch64__
amd64 __amd64__, __x86_64__
armv7 __arm__, __ARM_ARCH >= 7
i386 __i386__
powerpc __powerpc__
powerpcspe __powerpc__, __SPE__
powerpc64 __powerpc__, __powerpc64__
powerpc64le __powerpc__, __powerpc64__
riscv64 __riscv, __riscv_xlen == 64
Compilers may define additional variants of architecture-specific macros.
The macros above are preferred for use in FreeBSD.
Important make(1) variables
Most of the externally settable variables are defined in the build(7) man
page. These variables are not otherwise documented and are used
extensively in the build system.
MACHINE Represents the hardware platform. This is the same as
the native platform's uname(1) -m output. It defines
both the userland / kernel interface, as well as the
bootloader / kernel interface. It should only be used
in these contexts. Each CPU architecture may have
multiple hardware platforms it supports where MACHINE
differs among them. It is used to collect together all
the files from config(8) to build the kernel. It is
often the same as MACHINE_ARCH just as one CPU
architecture can be implemented by many different
hardware platforms, one hardware platform may support
multiple CPU architecture family members, though with
different binaries. For example, MACHINE of i386
supported the IBM-AT hardware platform while the MACHINE
of pc98 supported the Japanese company NEC's PC-9801 and
PC-9821 hardware platforms. Both of these hardware
platforms supported only the MACHINE_ARCH of i386 where
they shared a common ABI, except for certain kernel /
userland interfaces relating to underlying hardware
platform differences in bus architecture, device
enumeration and boot interface. Generally, MACHINE
should only be used in src/sys and src/stand or in
system imagers or installers.
MACHINE_ARCH Represents the CPU processor architecture. This is the
same as the native platforms uname(1) -p output. It
defines the CPU instruction family supported. It may
also encode a variation in the byte ordering of multi-
byte integers (endian). It may also encode a variation
in the size of the integer or pointer. It may also
encode a ISA revision. It may also encode hard versus
soft floating point ABI and usage. It may also encode a
variant ABI when the other factors do not uniquely
define the ABI. It, along with MACHINE, defines the ABI
used by the system. Generally, the plain CPU name
specifies the most common (or at least first) variant of
the CPU. This is why powerpc and powerpc64 imply 'big
endian' while armv7 and aarch64 imply little endian. If
we ever were to support the so-called x32 ABI (using
32-bit pointers on the amd64 architecture), it would
most likely be encoded as amd64-x32. It is unfortunate
that amd64 specifies the 64-bit evolution of the x86
platform (it matches the 'first rule') as almost
everybody else uses x86_64. The FreeBSD port was so
early, it predated processor name standardization after
Intel joined the market. At the time, each OS selected
its own conventions. Backwards compatibility means it
is not easy to change to the consensus name.
MACHINE_CPUARCH Represents the source location for a given MACHINE_ARCH.
It is generally the common prefix for all the
MACHINE_ARCH that share the same implementation, though
'riscv' breaks this rule. While amd64 and i386 are
closely related, MACHINE_CPUARCH is not x86 for them.
The FreeBSD source base supports amd64 and i386 with two
distinct source bases living in subdirectories named
amd64 and i386 (though behind the scenes there's some
sharing that fits into this framework).
CPUTYPE Sets the flavor of MACHINE_ARCH to build. It is used to
optimize the build for a specific CPU / core that the
binaries run on. Generally, this does not change the
ABI, though it can be a fine line between optimization
for specific cases.
TARGET Used to set MACHINE in the top level Makefile for cross
building. Unused outside of that scope. It is not
passed down to the rest of the build. Makefiles outside
of the top level should not use it at all (though some
have their own private copy for historical reasons).
TARGET_ARCH Used to set MACHINE_ARCH by the top level Makefile for
cross building. Like TARGET, it is unused outside of
that scope.
SEE ALSO
elfctl(1), proccontrol(1), sysctl(3), src.conf(5), build(7), simd(7)
HISTORY
An arch manual page appeared in FreeBSD 11.1.
FreeBSD 15.1-STABLE-HBSD July 14, 2025 ARCH(7)