AIX Version 4.3 Differences Guide

• Executable data and BSS
Following the text segments are working segments containing the
executable's initialized data and BSS (uninitialized data) areas. The data area
is vm_map()ed from the executable and relocated.
• Heap
The break value for the process is initialized just above the BSS area. As the
heap is grown (with brk() and sbrk() calls), new segments are allocated above
the current segment containing the current break value up to the segment
containing the new break value. The heap is not allowed to grow into segment
number 0x70000000 or beyond any shmat or mmap segment.
• shmat and mmap segments
Starting at segment number 0x70000000, shmat and mmap segments are
allocated if no address is specified. Segments are allocated in increasing
order in the first available segment at, or after, segment number 0x70000000.
The shmat and mmap segments are placed where requested if the segment
number is available and less than segment number 0x80000000.
• Explicitly-loaded modules
Explicitly-loaded modules (using the load() system call) were previously
loaded into the heap of the process. This creates complexity in dealing with
heap expansion/contraction and explicit loads.
Explicitly-loaded objects are now loaded into separate segments starting at
segment number 0x80000000. Segment numbers are allocated in increasing
order in the first available segment number at, or after, segment number
0x80000000.
Explicitly-loaded objects are limited to segment numbers 0x80000000 to
0x8FFFFFFF. To reduce segment table faults, multiple loaded modules are
vm_map()ed into these working storage segments. The data for the loaded
modules is loaded into these segments (and relocated) also.
• Shared library text and data segments
Starting at segment number 0x90000000, shared library text and data
segments are allocated. These segment numbers are allocated globally (at
the same address in all 64-bit address spaces) to allow sharing of the
segments (in the case of text segments) and vm_map()ing of the segments (in
the case of data segments). Global shared library text and data segments are
maintained by the loader using the current method. Shared library text and
data segments are limited to segment numbers 0x90000000 to 0x9FFFFFFF.
• User stack
The initial user stack is allocated starting at the top of segment number
0xFFFFFFFF and will consume additional segment numbers below this as the
stack grows. New segment numbers will be allocated at stack growth time.
Only the segment number below the segment number containing the current
top of stack is allocated. References to more than one segment number away
from the current top of the stack are treated as wild references.
Note: This restricts local variable allocation to less than 256 megabytes of the
total allocation in each function. The stack growth is limited to segment
numbers ranging from 0xF0000000 to 0xFFFFFFFF as segment numbers in
the range 0xA0000000 to 0xEFFFFFFF are reserved for future system use.
36 AIX Version 4.3 Differences Guide