AIX Version 4.3 Differences Guide

On AIX 4.3, the same algorithm is used for loading 64-bit modules, but the work is
split between kernel code and user-space code. The kernel part of the 64-bit
loader is responsible for mapping the modules of a process into the 64-bit user
address space. The user-space part processes the symbol tables and performs
the relocation of the data sections.
Kernel extensions are still in XCOFF32 modules and they are entirely loaded and
resolved by the kernel. The user-space processing of the shared library
segments is handled by a privileged process running in 64-bit mode called the
shared library loader assistant process or SHLAP.
The user-space processing of privately-loaded modules is handled by code that
is loaded into a system-wide segment that is shared by all 64-bit processes. This
code is called user-space loader assistant (USLA) and runs in the process of
loading the module. The USLA is implemented as a loadable module that is
prelinked at a fixed address, so that it will not have to be relocated. When an
execve() system call leaves the kernel, it transfers control to the USLA that
performs symbol resolution and relocation for privately-loaded modules. After
load() calls, library code will be responsible for calling the USLA to complete the
relocation of any newly-loaded modules.
Because the kernel is not performing symbol resolution and relocation for 64-bit
processes, only a small portion of a 64-bit module needs to be addressable in the
kernel. The kernel only needs to examine the XCOFF header, the section
headers, the loader section headers, and the loader section import IDs. Even for
extremely large programs, the size of these areas will be small. Only the import
ID section is variable length, and its length depends on the number of
dependents a module has, not on the size of the module itself. These portions of
a module can be read into allocated memory, avoiding addressability problems
inherent in the existing 32-bit loader.
3.2.6 Virtual Memory Manager
The AIX 4.3 design point is a 60-bit user address space. The areas impacted in
the VMM are the address space code, the shared memory code, teaching VMM
code to understand remapped addresses, and the remapping services
themselves.
3.2.6.1 Executing a 64-Bit Program
The size of a user-address space only changes as a result of exec(). A 32-bit
program may exec a 32 or 64-bit program, and conversely, all combinations are
possible.
The VMM provides support routines for exec() processing to switch between a
32-bit and 64-bit executable. The routine vm_makeme64() is called when the
exec()’d program is a 64-bit program. This routine pins and initializes the 64-bit
u-block extension, initializes the 64-bit address space structures, initializes the
Address Space Register (ASR) in the Machine State (MST) extension with the
real address of the segment table, sets the sbreak and stack sizes, and marks the
process as a 64-bit executable.
The routine vm_makeme32() is called whenever a 64-bit program execs(). It is
called even if the program to be executed is 64-bits. This routine initializes a
32-bit user address space from the 64-bit one, clears the 64-bit MST extension
52 AIX Version 4.3 Differences Guide