Tomasulo's algorithm

Tomasulo's algorithm is a computer architecture hardware algorithm for dynamic scheduling of instructions that allows out-of-order execution and enables more efficient use of multiple execution units. It was developed by Robert Tomasulo at IBM in 1967 and was first implemented in the IBM System/360 Model 91’s floating point unit. The major innovations of Tomasulo’s algorithm include register renaming in hardware, reservation stations for all execution units, and a common data bus (CDB) on which computed values broadcast to all reservation stations that may need them. These developments allow for improved parallel execution of instructions that would otherwise stall under the use of scoreboarding or other earlier algorithms. Robert Tomasulo received the Eckert–Mauchly Award in 1997 for his work on the algorithm. The following are the concepts necessary to the implementation of Tomasulo's algorithm: The Common Data Bus (CDB) connects reservation stations directly to functional units. According to Tomasulo it "preserves precedence while encouraging concurrency". This has two important effects: Functional units can access the result of any operation without involving a floating-point-register, allowing multiple units waiting on a result to proceed without waiting to resolve contention for access to register file read ports. Hazard Detection and control execution are distributed. The reservation stations control when an instruction can execute, rather than a single dedicated hazard unit. Instructions are issued sequentially so that the effects of a sequence of instructions, such as exceptions raised by these instructions, occur in the same order as they would on an in-order processor, regardless of the fact that they are being executed out-of-order (i.e. non-sequentially). Tomasulo's algorithm uses register renaming to correctly perform out-of-order execution. All general-purpose and reservation station registers hold either a real value or a placeholder value.