Reservation stations

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Reservation Stations are decentralized features of the microarchitecture of a CPU that allow for register renaming, and are used by the Tomasulo algorithm for dynamic instruction scheduling.

Reservation stations permit the CPU to fetch and re-use a data value as soon as it has been computed, rather than waiting for it to be stored in a register and re-read. When instructions are issued, they can designate the reservation station from which they want their input to read. When multiple instructions need to write to the same register, all can proceed and only the (logically) last one need actually be written.

In Tomasulos algorithm, instructions are issued in sequence to Reservation Stations which buffer the instruction as well as the operands of the instruction. If the operand is not available, the Reservation Station listens on a Common Data Bus for the operand to become available. When the operand becomes available, the Reservation Station buffers it, and the execution of the instruction can begin.

Functional Units (such as an adder or a multiplier), each have their own corresponding Reservation Station. The output of the Functional Unit connects to the Common Data Bus, where Reservation Stations are listening for the operands they need.