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