Provide introduction for the arch(7) manpage.

Start with some words about linear address space and its layout, then
explain pointers models and ABIs, providing explanation to the
structure of the tables.

Reviewed by:	emaste, imp
'Future-proof' cheri wording by:	brooks
Sponsored by:	The FreeBSD Foundation
MFC after:	2 weeks
Differential revision:	https://reviews.freebsd.org/D10596

share/man/man7/arch.7

@@ -26,7 +26,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd May 3, 2017
+.Dd May 4, 2017
 .Dt ARCH 7
 .Os
 .Sh NAME
@@ -35,9 +35,92 @@
 .Sh DESCRIPTION
 Differences between CPU architectures and platforms supported by
 .Fx .
-.Pp
+.Ss Introduction
 If not explicitly mentioned, sizes are in bytes.
+.Pp
+FreeBSD uses flat address space for program execution, where
+pointers have the same binary representation as
+.Vt unsigned long
+variables, and
+.Vt uintptr_t
+and
+.Vt size_t
+types are synonyms for
+.Vt unsigned long .
+.Pp
+In order to maximize compatibility with future pointer integrity mechanisms,
+manipulations of pointers as integers should be performed via
+.Vt uintptr_t
+or
+.Vt intptr_t
+and no other types.
+In particular,
+.Vt long
+and
+.Vt ptrdiff_t
+should be avoided.
+.Pp
+On some architectures, e.g.
+.Dv sparc64 ,
+.Dv powerpc
+and AIM variants of
+.Dv 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.
+.Pp
+On each architecture, the main user mode thread's stack starts near
+the highest user address and grows down.
 .Ss Type sizes
+All FreeBSD architectures use some variant of the ELF (see
+.Xr elf 5 )
+.Sy Application Binary Interface
+(ABI) for the machine processor.
+All supported ABIs can be divided into two groups:
+.Bl -tag -width "Dv ILP32"
+.It Dv ILP32
+.Vt int ,
+.Vt long ,
+.Vt void *
+types machine representations all have 4-byte size.
+.It Dv LP64
+.Vt int
+type machine representation uses 4 bytes,
+while
+.Vt long
+and
+.Vt void *
+are 8 bytes.
+.El
+Compilers define
+.Dv _LP64
+symbol when compiling for an
+.Dv LP64
+ABI.
+.Pp
+Some machines support more that one FreeBSD ABI.
+Typically these are 64-bit machines, where the
+.Dq native
+.Dv LP64
+execution environment is accompanied by the
+.Dq legacy
+.Dv ILP32
+environment, which was historical 32-bit predecessor for 64-bit evolution.
+Examples are:
+.Bl -column -offset indent "powerpc64" "Sy ILP32 counterpart"
+.It Sy LP64        Ta Sy ILP32 counterpart
+.It Dv amd64       Ta Dv i386
+.It Dv powerpc64   Ta Dv powerpc
+.It Dv mips64*     Ta Dv mips*
+.El
+.Dv arm64
+currently does not support execution of
+.Dv armv6
+binaries, even if the CPU implements
+.Dv AArch32
+execution state.
+.Pp
 On all supported architectures:
 .Bl -column -offset -indent "long long" "Size"
 .It Sy Type Ta Sy Size
@@ -48,6 +131,9 @@ On all supported architectures:
 .It float Ta 4
 .It double Ta 8
 .El
+Integers are represented as two-complement.
+Alignment of integer and pointer types is natural, that is,
+the address of the variable must be congruent to zero modulo type size.
 .Pp
 Machine-dependent type sizes:
 .Bl -column -offset indent "Sy Architecture" "Sy void *" "Sy long double" "Sy time_t"