Microprocessor Design/Out Of Order Execution

In a superscalar or similar processor design, there are multiple execution units that can be used to process pieces of data simultaneously. However, these execution units are not always used at the same time, and some processing power is lost. Sometimes, it is possible to feed instructions to all the execution units if we take the instructions out of their original order. Out of order execution (OOOE) is when a processor is capable of executing instructions out of their original order, in an attempt to do more work in parallel, and execute programs more quickly.

The Tomasulo scheduling algorithm is a OOOE algorithm that supports function units with variable latency, including data-dependent latency.

Hazards
OOOE comes with some significant hazards, and the hazard detection units in these processors are not trivial. The dependencies of all instructions need to be determined, and instructions cannot execute before or at the same time as instructions on which they are dependent.

Example: Intel Hyperthreading
Hyperthreading is the name for a technology developed by Intel for use in the Pentium 4 chip. Hyperthreading works by duplicating some architectural components of the processor, such as the status flags, the control registers, and the general purpose registers. Hyperthreading does not, however, duplicate any of the execution units.

In a hyperthreaded system, it appears to the operating system that there are two separate processors, when there is only one processor. The OOOE engine feeds instructions from the two separate execution threads to the execution cores, to try and keep all of the cores simultaneously busy. In general hyperthreading increases performance, although in some instances this additional complexity actually decreased performance.