AIX Version 4.3 Differences Guide

Figure 4. Register Implementation of 32-Bit and 64-Bit PowerPC Processors
In the 64-bit implementation of PowerPC, existing machine instructions do not
change drastically. Many instructions simply work the same in 64-bit mode. That
is, they can manage 64-bit long data and use/generate 64-bit-long addresses.
New instructions, that were not implemented in the previous PowerPC chips, are
included for the handling of 64-bit data.
A 64-bit PowerPC can also work in 32-bit mode. In this way, any application that
currently runs on the 32-bit PowerPCs can run unchanged. For example,
arithmetic instructions running in 32-bit mode operate only on the lower-half of
the CPU register involved and consider only that half of the register in the result.
32-bit addresses are handled in the same way.
The virtual address space is the amount of virtual memory that an application can
address independent of the size of the physical memory actually installed in the
machine on which it is running. Figure 5 shows a simplified representation of the
virtual address space that the PowerPC architecture can manage in 32-bit and in
64-bit mode. As shown, the 32-bit implementation is already capable of
addressing a very large (2 52 bytes, refer also to Table 11) address space. The
64-bit implementation goes up to 2 80 bytes (a huge number that signifies nearly
64-Bit Enablement 31