netbsd man page for openssl_ia32cap

OPENSSL_ia32cap(3)						      OpenSSL							OPENSSL_ia32cap(3)

NAME
       OPENSSL_ia32cap - finding the IA-32 processor capabilities

LIBRARY
       libcrypto, -lcrypto

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.

1.0.1i								    2011-06-05							OPENSSL_ia32cap(3)