"The short answer is an Intel P4 processor that fools software
into thinking that there are two processors present.
Hyper-Threading is a form of simultaneous multithreading technology
(SMT), where multiple threads of software applications can be run
simultaneously on one processor. This is achieved by duplicating
the architectural state on each processor, while sharing one set of
processor execution resources. The architectural state tracks the
flow of a program or thread, and the execution resources are the
units on the processor that do the work: add, multiply, load, etc.
Hyper-Threading is a performance enhancement that was an after
thought of the original P4 design. Real world performance showed
that the P4 had some performance penalties (dead time) caused by
the optimizations and algorithms implemented in the pentium4
advanced architecture. With the fast L1 cache and some additional
registers, Intel put this dead time to good use. Hence,