CPU Fast String – The Tech ARP BIOS Guide

CPU Fast String - The Tech ARP BIOS Guide

CPU Fast String

Common Options : Enabled, Disabled

 

Quick Review of CPU Fast String

The CPU Fast String BIOS feature controls the processor’s fast string feature.

When enabled, the processor will perform the REP string instruction in the “fast string” mode whenever the necessary conditions are met.

When disabled, the processor will not perform the REP string instruction in the “fast string” mode.

It is recommended that you enable CPU Fast String for better performance. There is currently no reason why you should disable CPU Fast String.

 

Details of CPU Fast String

Intel processors from the Intel P6 family onwards can modify their operation during string store (or REP string) operations to maximize performance. This ability is called fast string processing.

When certain “fast string” conditions are met, the processor will try to transfer as many pieces of data as possible in 16 byte chunks. Otherwise, the processor will only transfer the data individually in byte / word / dword / qword granularity. For fast string operations to occur, these five conditions must be met :

  • The source and destination address must be 16-byte aligned.
  • The string operation (REP MOVS) must operate on the data in ascending order
  • The initial count (ECX) must be at least 76
  • The source and the destination can’t overlap by less than a cache line
  • The memory types of both source and destination must either be write back cacheable or write combining.

The Intel Nehalem microarchitecture improved on the performance of fast string processing by reducing the startup overhead and increasing the data transfer throughput. It also removed the first condition above – that the source and destination address must be aligned. From the Nehalem microarchitecture onwards, REP string can operate in fast string mode even if the address isn’t aligned to 16-bytes.

The CPU Fast String BIOS feature controls the processor’s fast string feature.

When enabled, the processor will perform the REP string instruction in the “fast string” mode whenever the necessary conditions are met.

When disabled, the processor will not perform the REP string instruction in the “fast string” mode.

It is recommended that you enable CPU Fast String for better performance. There is currently no reason why you should disable CPU Fast String.

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