A 20 MHz CMOS reorder buffer for a superscalar microprocessor

Superscalar processors can achieve increased performance by issuing instructions out-of-order from the original sequential instruction stream. Implementing an out-of-order instruction issue policy requires a hardware mechanism to prevent incorrectly executed instructions from updating register values. A reorder buffer can be used to allow a superscalar processor to issue instructions out-of-order and maintain program correctness. This paper describes the design and implementation of a 20MHz CMOS reorder buffer for superscalar processors. The reorder buffer is designed to accept and retire two instructions per cycle. A full-custom layout in 1.2 micron has been implemented, measuring 1.1058 mm by 1.3542 mm.