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The IBM System/360 Model 91 was an early machine that supported out-of-order execution of instructions; it used the Tomasulo algorithm, which uses register renaming. The POWER1 from 1990 is the first microprocessor that used register renaming and out-of-order execution. This processor implemented register renaming only for floating-point loads.
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.
On hardware where software pipelining is necessary to improve performance alongside loop unrolling (i.e. hardware which lacks register renaming or implements in-order superscalar execution), additional registers may need to be used to store temporary variables from multiple iterations that could otherwise reuse the same register. [7]
the Tomasulo algorithm, which uses register renaming, allowing continual issuing of instructions The task of removing data dependencies can be delegated to the compiler, which can fill in an appropriate number of NOP instructions between dependent instructions to ensure correct operation, or re-order instructions where possible.
In computer engineering, instruction pipelining is a technique for implementing instruction-level parallelism within a single processor. Pipelining attempts to keep every part of the processor busy with some instruction by dividing incoming instructions into a series of sequential steps (the eponymous "pipeline") performed by different processor units with different parts of instructions ...
While process advances will allow ever greater numbers of execution units (e.g. ALUs), the burden of checking instruction dependencies grows rapidly, as does the complexity of register renaming circuitry to mitigate some dependencies. Collectively the power consumption, complexity and gate delay costs limit the achievable superscalar speedup.
It has up to 4 copies of integer register files (future, retired, scaled, and scratched, each containing 7 read and 4 write ports) and 2 copies of the floating point register file. However, unlike Alpha and x86, they are located in the backend as a retire unit right after the out-of-order unit and the renaming of register files.
Register pressure measures the availability of free registers at any point in time during the program execution. Register pressure is high when a large number of the available registers are in use; thus, the higher the register pressure, the more often the register contents must be spilled into memory. Increasing the number of registers in an ...