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[oweals/openssl.git] / doc / crypto / OPENSSL_ia32cap.pod

diff --git a/doc/crypto/OPENSSL_ia32cap.pod b/doc/crypto/OPENSSL_ia32cap.pod
index 46afd1988083ed20de053fd6a0ede2dd23c3bc10..2e659d34a5c43b29e36b2e622a30fd165c99aa1f 100644 (file)
--- a/doc/crypto/OPENSSL_ia32cap.pod
+++ b/doc/crypto/OPENSSL_ia32cap.pod
@@ -2,33 +2,42 @@
 
 =head1 NAME
 
-OPENSSL_ia32cap
+OPENSSL_ia32cap - finding the IA-32 processor capabilities
 
 =head1 SYNOPSIS
 
- extern unsigned long OPENSSL_ia32cap;
- unsigned long *OPENSSL_ia32cap_loc();
+ unsigned long *OPENSSL_ia32cap_loc(void);
+ #define OPENSSL_ia32cap (*(OPENSSL_ia32cap_loc()))
 
 =head1 DESCRIPTION
 
-OPENSSL_ia32cap is 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 defined/meaningful on IA-32 platforms only.
-The variable is normally set up automatically upon toolkit
-initialization and can be manipulated afterwards to modify crypto
-library behaviour. For the moment of this writing only two bits are
-significant, namely bit #26 denoting SSE2 support, and bit #4 denoting
-presence of Time-Stamp Counter. Resetting bit #26 at run-time for
-example 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 programmatically
-manipulate the value, you most likely will find it more appropriate to
-set up an environment variable with the same name prior starting target
-application, e.g. 'env OPENSSL_ia32cap=0x10 apps/openssl', to achieve
-same effect without modifying the application source code.
-Alternatively you can reconfigure the toolkit with no-sse2 option and
-recompile.
+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.
 
 =cut