The x86 Feature Flags On using the CPUID instruction for processor identification and feature determination Some features of interest In our course we focus on EM64T and VT A majority of x86 processors today do not support either of these features e g our classroom and Lab machines lack them But machines with Intel s newest CPUs such as Core 2 Duo and Pentium D 9xx do have both of these capabilities built in Software needs to detect these features or risk crashing in case they re missing Quotation NOTE Software must confirm that a processor feature is present using feature flags returned by CPUID prior to using the feature Software should not depend on future offerings retaining all features IA 32 Intel Architecture Software Developer s Manual volume 2A page 3 165 The CPUID instruction It exists if bit 21 in the EFLAGS register the ID bit can be toggled by software It can be executed in any processor mode and at any of processor s privilege levels It returns two categories of information Basic processor functions Extended processor functions It s documented online see class website An example using CPUID Here is a code fragment that uses CPUID to obtain basic processor information vid section data asciz xxxxxxxxxxxx Vendor Identification String section text Using CPUID to obtain the processor s Vendor Identification String xor eax eax setup 0 as input value cpuid then execute CPUID mov ebx vid 0 save bytes 0 3 mov edx vid 4 save bytes 4 7 mov ecx vid 8 save bytes 8 11 CPU feature information You can execute CPUID with input value 1 to get some processor feature information as well as processor version information The feature information is returned in the EDX and ECX registers with individual bitsettings indicating whether or not specific features are present in the processor These bits are documented in volume 2A Register EDX 31 30 29 28 R 27 26 25 24 23 22 21 20 M M X H T R 19 18 17 16 15 14 13 12 P S N M T R R 11 10 R 9 8 7 6 P A E 5 4 3 P S E Legend partial HT Hyperthreading Technology 1 yes 0 no MMX MultiMedia eXtensions 1 yes 0 no PSN Processor Serial Number 1 yes 0 no MTRR Memory Type Range Registers 1 yes 0 no PAE Page Address Extensions 1 yes 0 no PSE Page Size Extensions 1 yes 0 no FPU Floating Point Unit on chip 1 yes 0 no R Intel reserved bit 2 1 0 F P U Register ECX 31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 RRRRRRRRRRRRRRRRRR 11 10 RR 9 R 8 7 6 5 V RM X 4 3 2 RR Legend partial VMX Virtualization Technology eXtensions 1 yes 0 no R Intel reserved bit 1 0 AMD s extensions The Advanced Micro Devices corporation pioneered the 64 bit architectural design in their x86 processors e g Athlon Opteron They implemented some extended inputvalues for their CPUID instruction and to be compatible Intel has followed suit These extended input values represent negative integers in two s complement New example using CPUID Here is a code fragment that uses CPUID to get the highest extended function inputvalue that the processor understands highin section data int 0 for highest CPUID input value section text Using CPUID to obtain the processor s highest valid CPUID input value mov 0x80000000 eax setup the input value cpuid then execute CPUID mov eax highin save the The 64 bit feature AMD uses CPUID input value 0x80000001 for obtaining their extended features bits returned in the ECX and EDX registers so Intel processors follow this convention too section data ext features space section mov cpuid mov mov 8 text 0x80000001 eax edx ext features 0 ecx ext features 4 for extended features bits setup input value in EAX then execute CPUID save feature bits from EDX save feature bits from ECX Intel s extended features bits 31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 L ECX R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R S F 31 30 29 28 EDX R R E M 6 4 T 27 26 25 24 23 22 21 20 RRRRRRRR 19 18 17 16 15 14 13 12 X RRRRRRRR D 11 10 S Y S C A L L 9 8 7 6 5 4 3 2 1 0 RRRRRRRRRRR Legend EM64T Extended Memory 64 bit Technology 1 yes 0 no XD eXecute Disable paging bit implemented 1 yes 0 no SYSCALL fast SYSCALL SYSRET 64 bit mode 1 yes 0 no LSF LAHF SAHF implemented in 64 bit mode 1 yes 0 no R Intel reserved bit The asm construct When using C C for systems programs we sometimes need to employ processorspecific instructions e g to access CPU registers or the current stack area Because our high level languages strive for portability across different hardware platforms these languages don t provide direct access to CPU registers or stack gcc g extensions The GNU compilers support an extension to the language which allows us to insert assembler code into our instruction stream Operands in registers or global variables can directly appear in assembly language like this as can immediate operands int count 4 global variable asm mov count eax asm imul 5 eax ecx Local variables Variables defined as local to a function are more awkward to reference by name with the asm construct because they reside on the stack and require the generation of offsets from the ebp register contents A special syntax is available for handling such situations in a manner that gcc g can decipher Template The general construct format is as follows asm instruction sequence output operand s input operand s clobber list Example from usecpuid cpp int regEBX regECX regEDX local variables some high level code could go here now here is an example of using the asm construct asm mov 3 eax n cpuid n mov ebx 0 n mov edx 1 n mov ecx 2 n m regEBX m regEDX m ECX i 0 ax bx cx dx further high level code ccould go here How to see your results You can ask the gcc compiler to stop after translating your C C source file into x86 assembly language gcc S myprog cpp Then you can view the compiler s outputfile named myprog s by using the cat command or by using an editor cat myprog s more The processor s brand string One of the most interesting and helpful capabilities of the CPUID instruction that recent Intel and AMD x86 processors implement is the brand string feature It allows software to determine the CPU s complete and official marketplace name The string can have up to 48 characters …
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