AIX Version 4.3 Differences Guide

that can be larger than 2 32 for large files. In this case, the system call is
directed to the 32-bit llseek() interface that is prepared to handle long integers.
• float, double - Passed without change.
• pointer - Converted from 64-bit effective address to 32-bit effective address as
described in Section 3.2.2.2, “64-Bit to 32-Bit Address Remapping” on page
40.
Many system calls involve passing the address of one or more structures in
storage. If the structures involve any data types whose sizes differ between 32-bit
and 64-bit mode (int, long, pointer), the library routine must pass a (32-bit) pointer
to a local 32-bit copy of the data constructed on its own stack.
Some system calls are 64-bit-enabled, meaning they understand 64-bit pointers
or longs. In such cases, the library code typically passes these by value in
adjacent registers. The kernel code understands to parse the input as such. The
shmat() call is an example of a service that understands a 64-bit address in
adjacent registers.
If the kernel is going to look at the data being passed, the library routine allocates
32-bit versions of the same structure(s) and copies the data, field by field,
through assignment statements. This results in automatic truncation of the int and
long fields, some of which may need testing for magnitude before the conversion
is done. Pointers are converted as described in Section 3.2.2.2, “64-Bit to 32-Bit
Address Remapping” on page 40.
If the kernel fills in data as the result of a system call, the data must be widened
by the library routine on return from the kernel extension. Space for the 32-bit
version of the structures to be filled-in must be allocated by the library routine
before calling the kernel extension. Assignment, field by field, will do the proper
widening (zero extension or sign extension, as appropriate).
Returned pointers, such as from sbrk() or shmat(), require special handling
between the library routine and kernel extension routine to ensure that the proper
64-bit values are returned to the library routine.
Note that some system calls involve data passing in both directions, into and out
of the kernel, and thus require action on the part of the library routine before, and
after, the system call.
3.2.2.2 64-Bit to 32-Bit Address Remapping
As shown in Figure 7, the PowerPC architecture divides the Effective Address
(EA) into three fields:
• The Effective Segment ID (ESID)
• A 16-bit page number within the segment
• A 12-bit byte offset within the page
The width of the ESID varies with execution mode:
• In 32-bit mode, the ESID is 4 bits and is often referred to as the Segment
Register number.
• In 64-bit mode, the ESID is 36 bits.
40 AIX Version 4.3 Differences Guide