AIX Version 4.3 Differences Guide

where it can be directly manipulated, should provide dramatic performance
improvements. Table 11 shows the size of the address spaces that can be
managed as a function of the length of the address that the CPU generates.
Table 11. Size of Address Space as a Function of Address Length
Address Length
8-bit
16-bit
32-bit
52-bit
64-bit
Flat Address Space
256 Bytes
64 Kilobytes
4 Gigabytes
4000 Terabytes
16,384,00 Terabytes
3.1.2 64-Bit Challenges
As previously mentioned, 64-bit architectures can prove advantageous in many
areas of application. It should be noted, however, that these advantages can
come at a cost. Extra addressability must be accompanied by very large amounts
of system memory to work effectively. Applications compiled in 64-bit mode also
consume more disk space than their 32-bit equivalents, adding to the cost of
system storage. It is also important to remember that 32-bit applications that
cannot or do not take advantage of the features previously mentioned should
remain as 32-bit binaries. If compiled in 64-bit mode without change, they will
probably not see any performance improvement. It is possible that the application
will run slightly slower due to cache effects and longer code-path length.
Although the vast majority of current applications will not fully utilize the functions
and capabilities of a 64-bit architecture, the applications of the near future will
increasingly view 32-bit technology as a limiting factor.
3.1.3 64-Bit PowerPC Design
The PowerPC architecture is, by its nature, an open, extendable design. There is
nothing in the chip architecture itself that would affect binary compatibility as you
migrate across different PowerPC implementations. The PowerPC processor
architecture was defined from the start as a 64-bit architecture that is a superset
of the 32-bit architecture implemented in the 601, 603, and 604 processors.
An important aspect of the 64-bit version of PowerPC is its binary compatibility
with the previous PowerPC processors. From the standpoint of the 32-bit and
64-bit specifications, there are a few differences, as shown in Figure 4. The
number of CPU registers (the basic storage cell where the CPU stores the data
on which it performs its computations) remains the same, but these registers are
now 64 bits long instead of 32 bits. A few other control registers also move from
32 to 64 bits in length. Note that the floating point registers do not change in size,
since they already conform to industry standards for floating-point that require 32-
or 64-bit-long data.
30 AIX Version 4.3 Differences Guide