To end, we see that Intel promises compatibility with the 32-bit software (IA-32). They should run without any change since the operating system and the firmware have features for that. It should be possible to run software in real mode (16 bits), protected mode (32 bits) and virtual mode 86 (16 bits). They mean that the CPU will be able to operate in IA-64 mode or IA-32 mode. There are special instructions to go from one mode to the other, as it is shown in Figure 3.
The three instructions that make the transition between the instruction sets are:
- JMPE (IA-32): jumps to a 64-bit instruction and changes to IA-64 mode;
- br.ia (IA-64): moves to a 32-bit instruction and changes to IA-32 mode;
- Interruptions transit to IA-64 mode, allowing the fulfillment of all interruption conditions and
- rfi (IA-64): it is the return of the interruption; the return happens both to an IA-32 situation and to an IA-64, depending on the situation present at the moment when the interruption is invoked.