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Volume 3: General-Purpose and System Instructions - Stanford ...

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AMD64 Technology 24594 Rev. 3.10 February 2005field, the BIOS uses the 12-bit br<strong>and</strong> ID field to program the processor name stringthat is returned by extended functions 8000_0002h–8000_0004h. If the 8-bit br<strong>and</strong> IDfield is 0 (the value in EBX returned by st<strong>and</strong>ard function 1), then the 12-bit br<strong>and</strong> IDis used to program the processor name string. For more information on the 8-bit br<strong>and</strong>ID, see “St<strong>and</strong>ard Function 1 EBX: Initial APIC ID, Logical Processor Count,CLFLUSH Size, <strong>and</strong> 8-Bit Br<strong>and</strong> ID” on page 121.Figure 3-3 shows the format of the EBX register following execution of CPUIDextended function 8000_0001h.31 12 11 0Reserved12-Bit Br<strong>and</strong> IDBits Mnemonic Description31–12 Reserved11–0 12-Bit Br<strong>and</strong> IDFigure 3-3.Extended Function 8000_0001h EBX: 12-bit Br<strong>and</strong> IDFor information on using the 12-bit br<strong>and</strong> ID to program the processor name string,see the AMD Processor Recognition Application Note, order# 20734.Extended Function 8000_0001h ECX: AMD Feature Support. Extended function 8000_0001hreturns information about AMD features—those features that were originallyimplemented by AMD—in the ECX <strong>and</strong> EDX registers. The value of each bit returnedindicates whether support for a specific feature is present on the processorimplementation. If the value of a bit is 1, the feature is supported. If the value is 0, thefeature is not supported.Table 3-6 summarizes the feature bits returned in the ECX register by CPUIDextended function 8000_0001h.Table 3-6.ECX BitCPUID AMD Feature Support (Extended Function 8000_0001h—ECX)Feature(feature is supported if bit is set to 1)0 LAHF/SAHF Supported in 64-Bit Mode.1CMP Legacy. The HTT (Hyperthreading Technology) feature bit <strong>and</strong> the logical processor count field(returned in EBX by st<strong>and</strong>ard function 1) indicate multiple cores in the package instead of hyperthreading.(See “Logical Processor Count” on page 122.)126 CPUID

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