HP Vectra XU 6/XXX HP Vectra XU 6/XXX - Guide to Optimization Performance - Page 36

Out-of-order Execution, Speculative Execution

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and addresses. The Pentium Pro provides five execution units, giving it increased flexibility for instruction execution. To ensure that the pipelines are used to their greatest efficiency, the Pentium Pro must keep its instruction units busy. For this reason, the Pentium Pro implements two additional features for instruction execution: out-of-order execution; and speculative execution. Out-of-order Execution When executing x86 family instructions, previous generations of processors had to be faithful to the order in which the instructions were received. This meant that if a particularly slow or complicated instruction was being executed, the processor was obliged to stall its pipeline until the instruction had completed. By breaking instructions down into micro-ops, the Pentium Pro is able to change the order in which it executes instructions.When a particular instruction requires longer than average to execute, the Pentium Pro will fetch and execute another instruction while waiting for the first instruction to complete. The status of the first instruction is stored in registers while the new instruction is executed, ensuring that no conflicts arise. This facility is called out-of-order execution, and ensures that the Pentium Pro's three pipelines are used to their greatest effect. Speculative Execution All programs contain branches. These are produced by conditional instructions which can change the thread of instruction execution. Depending on the conditions when a conditional instruction is executed, different instructions will be required next by the program. Like the Pentium processor, the Pentium Pro provides a branch prediction unit that can identify program branches and load their instructions into the level-one instruction cache memory. However, unlike the Pentium, the Pentium Pro is also able to execute these instructions in case they are needed. This feature is called speculative execution, and it can significantly reduce the time required for program branches.

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and addresses. The Pentium Pro provides five execution units, giving it increased flexibility for
instruction execution.
To ensure that the pipelines are used to their greatest efficiency, the Pentium Pro must keep its
instruction units busy. For this reason, the Pentium Pro implements two additional features for
instruction execution: out-of-order execution; and speculative execution.
Out-of-order Execution
When executing x86 family instructions, previous generations of processors had to be faithful
to the order in which the instructions were received. This meant that if a particularly slow or
complicated instruction was being executed, the processor was obliged to stall its pipeline until
the instruction had completed.
By breaking instructions down into micro-ops, the Pentium Pro is able to change the order in
which it executes instructions.When a particular instruction requires longer than average to
execute, the Pentium Pro will fetch and execute another instruction while waiting for the first
instruction to complete. The status of the first instruction is stored in registers while the new
instruction is executed, ensuring that no conflicts arise. This facility is called out-of-order
execution, and ensures that the Pentium Pro’s three pipelines are used to their greatest effect.
Speculative Execution
All programs contain branches. These are produced by conditional instructions which can
change the thread of instruction execution. Depending on the conditions when a conditional
instruction is executed, different instructions will be required next by the program.
Like the Pentium processor, the Pentium Pro provides a branch prediction unit that can identify
program branches and load their instructions into the level-one instruction cache memory.
However, unlike the Pentium, the Pentium Pro is also able to execute these instructions in case
they are needed. This feature is called speculative execution, and it can significantly reduce
the time required for program branches.