Tomasulo algorithm

Tomasulo’s algorithm is a computer architecture hardware algorithm for dynamic scheduling of instructions that allows out-of-order execution, designed to efficiently utilize multiple execution units. It was developed by Robert Tomasulo at IBM in 1967, and 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:

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. If a real value is unavailable to a destination register during the issue stage, a placeholder value is initially used. The placeholder value is a tag indicating which reservation station will produce the real value. When the unit finishes and broadcasts the result on the CDB, the placeholder will be replaced with the real value.

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