Implementation Concepts
The following are the concepts necessary to the implementation of Tomasulo's Algorithm.
- 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).
- All general-purpose and reservation station registers hold either real or virtual values. If a real value is unavailable to a destination register during the issue stage, a virtual value is initially used. The functional unit that is computing the real value is assigned as the virtual value. The virtual register values are converted to real values as soon as the designated functional unit completes its computation.
- Functional units use reservation stations with multiple slots. Each slot holds information needed to execute a single instruction, including the operation and the operands. The functional unit begins processing when it is free and when all source operands needed for an instruction are real.
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