IA-64

IA-64 (Intel Architecture-64) is a new 64-bit-architecture developed by Intel for its next line of processors such as Itanium. Unlike previous Intel x86 processors, it will not support the old IA-32 instruction set.

This design relies on a concept called Explicitly Parallel Instruction Computing (EPIC), similar to the pre-existing VLIW design but with a number of enhanchements. In an EPIC design the instructions that have no dependencies and can be executed in parallel are segregated (based on a predefined set of templates) into Instruction Groups and fed into the processor as manageable chunks. These chunks are then capable of being executed in parallel. The deciding factor in segregating the data is that they should neither have Data Dependency nor Control Dependency.

The architecture tends to reduce memory latency by providing a rich set of registers for each type of data. Thus we have 128 64-bit integer (general purpose) registers and 128 82-bit registers for floating point data. This enables information to be maintained in the registers for quicker access rather than accessing memory every time data is modified.

The architecture also provides a versatile set of instructions that cater to a wide spectrum of usage. Thus we have explicit instructions for multimedia operations, and explicit instructions for floating point operations.

Whenever a function call is made the processor has to do significant work in push'ing all the current register values into stack and then pop'ing them back on completion. The IA-64 design reduces this memory latency by performing all the stack operations on the registers themselves. A special section of hardware called the Register Stack Engine handles the cases where the registers and the memory have to be synced.

Above all when a 32-bit instruction (32-bit application) is run on a 64-bit machine the processor temporarily switches to 32-bit mode and returns to 64-bit mode on completion.

At bootup, the system decides whether to enter IA32-mode or IA64-mode. It will need a 32-bit x86 compatible operating system to run in IA32-mode, and a 64-bit IA64 compatible OS to run in IA64-mode.