Conley, Rose & Tayon
Apparatus and method for accessing special registers without serialization
 A microprocessor employing an apparatus for performing special register writes without serialization is provided. The apparatus detects special register write instructions when the instructions are dispatched, and stores an indication of the write in a special register dependency block. Instructions subsequent to the special register write instruction are examined for both explicit and implicit dependencies upon the special register write. If a dependency is detected with respect to a particular instruction, the instruction is dispatched to a reservation station along with an indication of the dependency. Instructions subsequent to the special register write instruction which are not dependent upon the special register are dispatched without an indication of special register dependency. Instructions without dependencies may speculatively execute prior to instructions with dependencies, or even prior to the special register write instruction. In one particular embodiment employing the x86 microprocessor architecture, the microprocessor detects updates to the DS, ES, FS, and GS segment registers (i.e. the data segment registers). Updates to other segment registers are serialized. 
Advanced Micro Devices, Inc.
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William M. Thang M. Rupaka
Johnson Tran Mahalingaiah