AIX Version 4.3 Differences Guide

• Big data programs
The maxdata field is used for 32-bit programs to indicate that the heap should
be placed in a different place where a larger heap can be accommodated. In
64-bit processes, the address space will always be able to accommodate a
large heap, so no indication is necessary. The maxdata and maxstack fields, if
set, are used to set the soft rlimit value for 32-bit and 64-bit applications. If the
limit specified is greater than the hard rlimit values, the exec() will fail.
3.2.2 System Calls
Because the AIX kernel remains 32-bit, the interfaces to the various system calls
must be through the types and structures of 32-bit mode C. 64-bit mode
applications, however, are compiled with 64-bit mode types and structures. This
section explains how the different types are communicated to the kernel.
On a 64-bit PowerPC, AIX will run both 32-bit-mode processes and 64-bit-mode
processes. The problem is that 64-bit applications compiled with the same AIX
header files that 32-bit processes use (but compiled for 64-bit execution mode)
build data structures, arrays, and scalars using the rules of 64-bit C, though the
kernel expects data structures, arrays, and scalars that match the ones built by
32-bit applications using 32-bit C. If a 64-bit application builds, for example, an
ioveC structure containing an address and a length, this structure cannot be
passed directly to the kernel's kreadv() system call routine because that routine
cannot interpret the 64-bit address and 64-bit length.
Clearly, some code must be placed between the 64-bit application's system calls
and the 32-bit kernel (see Figure 6). AIX takes advantage of the following two
features in implementing this interface code:
• The 32-bit and 64-bit name spaces are completely separate, and calls from
64-bit applications are never resolved to 32-bit entry points. Specifically, the
calls from 64-bit applications to traditional system call entry points should not
be resolved to the 32-bit entry points of the same names exported by the
kernel.
• Since UNIX no longer makes a distinction between system calls and
subroutines, it is no longer necessary to strictly follow the old UNIX semantics.
If a caller passes a bad address to any system-supplied subroutine (whether
system call or not), it is permissible to end the calling process, as will happen
if a library routine dereferences an invalid pointer. This means that routines,
traditionally considered to be system calls, can reside in subroutine libraries,
and almost all of the system call interface code needed for 64-bit processes
can be placed in a user-mode library such as libc.a.
64-Bit Enablement 37