Instruction Scheduling in Microprocessors

The Central Processing Unit (CPU) in a microprocessor is responsible for running machine instructions as fast as possible so that the machine performance is at its maximum level. While simple in design, in-order execution processors provide sub-optimal performance, because any delay in instruction processing blocks the entire instruction stream. To overcome this limitation, modern highperformance designs use out-of-order (OoO) instruction scheduling to better exploit available Instruction-Level Parallelism (ILP), and both static (compilerassisted) and dynamic (hardware-assisted) scheduling solutions are possible. The hardware-assisted scheduling integrates an OoO core that requires a complex dynamic instruction scheduler and additional datapath structures are utilized to hold the in-flight instructions in program order to support the reconstruction of precise program state. The logic becomes even more complex when superscalar (those capable of executing multiple instructions every clock cycle) designs are used. This chapter gives a brief introduction to instruction scheduling on pipelined superscalar architectures, and, then, explains some of the keystone static and dynamic instruction scheduling algorithms.