Friday, November 15, 2002

Introduction

The newest processor from Intel's fabs is out, and ready to change the world of CPU technology. Its name is the 3.06GHz Intel Pentium 4 Hyper-Threading processor, and it is the first desktop processor to incorporate Hyper-Threading technology, which allows a single chip to do the work of two, or almost. Hyper-Threading allows for the sharing and partitioning of resources, with only a 5% increase in the die size of the underlying CPU.

Hyper-Threading

Hyper-Threading (HT) is new to the IA-32 family, and is peculiar to those that are based on the NetBurst architecture (which is to say, the Pentium 4). Operating systems, and applications that have been built on previous IA-32 systems will have no compatibility problems with hyper-threaded CPUs either, which comes as a great relief to programmers the world over. In fact, what Intel has done is create a second "logical" CPU within the confines of the new P4; multiple processing tasks are completed more quickly by executing two or more threads at the same time.

The result? From the point of view of the operating system, or the user, multiple tasks can be processed as if there were really two processors at work.

From a micro-architectural perspective, the logical processors of an HT chip can execute threads of code more quickly by simply bundling them together, and running them through the pipeline simultaneously. This can be done, in no small part, because each logical CPU has a complete set of information regarding the state of the system. This is essentially contained within the general registers, the control registers, the APIC, and a few system state registers.

From a program's perspective, once these essential resources are duplicated, the CPU looks like two separate chips. Better yet, the number of transistors required to pull it off is only a tiny fraction of the CPU's total. The two logical CPUs share nearly all the resources of the "real" CPU, including the caches, the execution units, the branch predictors, the logic controls, and the system bus.

Each logical processor also comes with its own APIC, which means that any interrupts sent to one logical unit need not effect the other at all.

Next: The Hyper-Threading architecture.

3.06Ghz Intel Pentium 4 HT