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OPENSSL_ia32cap(3)                                 OpenSSL                                OPENSSL_ia32cap(3)



NAME
       OPENSSL_ia32cap - finding the IA-32 processor capabilities

SYNOPSIS
        unsigned long *OPENSSL_ia32cap_loc(void);
        #define OPENSSL_ia32cap (*(OPENSSL_ia32cap_loc()))

DESCRIPTION
       Value returned by OPENSSL_ia32cap_loc() is address of a variable containing IA-32 processor
       capabilities bit vector as it appears in EDX register after executing CPUID instruction with EAX=1
       input value (see Intel Application Note #241618). Naturally it's meaningful on IA-32[E] platforms
       only. The variable is normally set up automatically upon toolkit initialization, but can be
       manipulated afterwards to modify crypto library behaviour. For the moment of this writing six bits
       are significant, namely:

       1. bit #28 denoting Hyperthreading, which is used to distiguish
          cores with shared cache; 2. bit #26 denoting SSE2 support; 3. bit #25 denoting SSE support; 4. bit
       #23 denoting MMX support; 5. bit #20, reserved by Intel, is used to choose between RC4 code
          pathes; 6. bit #4 denoting presence of Time-Stamp Counter.

       For example, clearing bit #26 at run-time disables high-performance SSE2 code present in the crypto
       library. You might have to do this if target OpenSSL application is executed on SSE2 capable CPU, but
       under control of OS which does not support SSE2 extentions. Even though you can manipulate the value
       programmatically, you most likely will find it more appropriate to set up an environment variable
       with the same name prior starting target application, e.g. on Intel P4 processor 'env
       OPENSSL_ia32cap=0x12900010 apps/openssl', to achieve same effect without modifying the application
       source code. Alternatively you can reconfigure the toolkit with no-sse2 option and recompile.



50                                               2013-03-05                               OPENSSL_ia32cap(3)