AIX Version 4.3 Differences Guide

Figure 7. Effective Segment IDs in 32-Bit and 64-Bit Mode
For each process, the kernel maintains an array of sixteen entries, each holding a
segment register value that defines the mapping of user effective addresses to
system virtual addresses at the point of a system call. When a 32-bit application
issues a system call and passes pointers to data in user space, the entries in this
array are loaded into segment registers by the copyin() and copyout() routines to
access user-space data. The kernel never accesses user-space data directly, but
only through copyin() and copyout().
The 32-bit AIX kernel does not generally understand 64-bit address spaces. To
perform kernel services for 64-bit address spaces, the 16-element mapping array
previously mentioned must be set up so that the relevant user-space data for a
system call can all be accessed through 32-bit addresses.
The VMM provides services to remap 64-bit pointers passed through system calls
into a 32-bit address space. The 64-bit library code fills out a data structure
describing the 64-bit pointers to be remapped using one of the __remap*()
services. This structure is passed to one of the remap*_64() kernel services by
the 64-bit kernel extension to remap the specified segments in the user address
space into a 32-bit address space for use by the normal kernel system call
routines.
The general form of the remapping structure is an array of effective segment
identifiers and an integer indicating the number of such ESIDs to be remapped. A
pointer to this remapping structure is passed to the remap*_64() routine. An ESID
is 36 bits long and is represented as two integers; the high-order 32-bits of the
ESID in the first word, and the remaining four bits of the ESID in the high-order
four bits of the second word. To optimize the remapping of a single ESID, the two
words describing the ESID are passed in registers rather than passing a pointer
to a remapping structure. Additional bits are defined in the unused bits of the
second word to indicate which form is being used – pointer or in-line registers.
Remapped addresses are only valid in the kernel for the duration of the system
call. Subsequent system calls may not rely on 32-bit remapped addresses from
previous system calls. Extensions with such requirements should save the
segment register value and use the long-form virtual address instead or call
as_unremap64() to obtain the 64-bit unremapped address to store.
3.2.2.3 Remap Library Data Structures
All the remap data structures are defined in the header file ,
except the kernel MST-extension remap region that was discussed earlier. This
header file is shipped with AIX because all of these structures may be required by
the user.
64-Bit Enablement 41