AIX Version 4.3 Differences Guide

For AIX, and the applications that run on AIX, the 64-bit PowerPCs have two
important attributes. They are very fast when running as 32-bit processors, and
they offer the opportunity of running a 64-bit environment. AIX 4.3 exploits these
attributes separately. There are two different execution environments in AIX 4.3,
the 32-bit execution environment and the 64-bit execution environment. These
two environments are only available for 64-bit hardware. There is no 64-bit
execution environment on 32-bit hardware.
Generally, the AIX 4.3 kernel remains 32-bit, and only selected parts, such as the
Virtual Memory Manager, are upgraded to be aware of the 64-bit address space.
This means that the number of AIX kernels remains two (uniprocessor and
multiprocessor).
Although there were a number of choices regarding the basic data types of 64-bit
processes, the choice that was made by the Aspen working group, formed by
XOPEN, and a consortium of hardware vendors is called LP64, short for
Long-Pointer 64. This is commonly also called the 4/8/8 model, which stands for
the Integer/Long/Pointer type sizes. The benefit of this configuration is that it
provides 64-bit addressing with 8-byte pointers, large object support (8-byte
longs), and backward compatibility (4-byte integers). Other alternatives included
an 8/8/8 solution, called ILP64, and the LLP64 that were not adapted. Obviously,
the choice for AIX was LP64.
3.2 64-Bit Core Design
As stated, the kernel of the AIX operating systems remains 32-bit. To allow 64-bit
applications to run and use memory in the 64-bit address space, extensions to
the kernel were introduced. This is discussed in Section 3.2.2, “System Calls” on
page 37. A fundamental change was also introduced in the object module format
that basically enables executables to overcome the size limit of 2 32 bytes. The
subsection on XCOFF addresses these changes.
The implications for device drivers of 64-bit and 32-bit devices in a 64-bit
execution environment are explained in Section 3.2.4, “Device Drivers” on page
49.
The loading of 64-bit modules is shared between kernel-space code and
user-space code. The changes to the loader are discussed in 3.2.5, “Loader” on
page 51.
The Virtual Memory Manager is one of the key parts of the operating system
when it comes to large address space and mapping of virtual to real memory. The
consequences are summarized in the last part of this section.
3.2.1 Segment Register Mapping
Since the 64-bit hardware uses 64-bit effective addresses while operating in
64-bit execution mode, keeping sixteen segment registers would have required
increasing the size of segments proportionally to allow for the increased address
space. This is less useful to applications than increasing the number of segments
in a process' address space. Therefore, segmentation on the 64-bit hardware
remains in units of 256 megabytes. In 64-bit execution mode, the sixteen
segment registers used for 32-bit addressing are replaced with a segment table
(analogous to the hardware page table) that contains up to 256 of the most
64-Bit Enablement 33