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.\" ========================================================================
.\"
.IX Title "OPENSSL_instrument_bus 3"
.TH OPENSSL_instrument_bus 3 "2019-12-20" "1.0.2u" "OpenSSL"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
.if n .ad l
.nh
.SH "NAME"
OPENSSL_instrument_bus, OPENSSL_instrument_bus2 \- instrument references to memory bus
.SH "SYNOPSIS"
.IX Header "SYNOPSIS"
.Vb 4
\& #ifdef OPENSSL_CPUID_OBJ
\& size_t OPENSSL_instrument_bus (int *vector,size_t num);
\& size_t OPENSSL_instrument_bus2(int *vector,size_t num,size_t max);
\& #endif
.Ve
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
It was empirically found that timings of references to primary memory
are subject to irregular, apparently non-deterministic variations. The
subroutines in question instrument these references for purposes of
gathering entropy for random number generator. In order to make it
bus-bound a 'flush cache line' instruction is used between probes. In
addition probes are added to \fBvector\fR elements in atomic or
interlocked manner, which should contribute additional noise on
multi-processor systems. This also means that \fBvector[num]\fR should be
zeroed upon invocation (if you want to retrieve actual probe values).
.PP
OPENSSL_instrument_bus performs \fBnum\fR probes and records the number of
oscillator cycles every probe took.
.PP
OPENSSL_instrument_bus2 on the other hand \fBaccumulates\fR consecutive
probes with the same value, i.e. in a way it records duration of
periods when probe values appeared deterministic. The subroutine
performs at most \fBmax\fR probes in attempt to fill the \fBvector[num]\fR,
with \fBmax\fR value of 0 meaning \*(L"as many as it takes.\*(R"
.SH "RETURN VALUE"
.IX Header "RETURN VALUE"
Return value of 0 indicates that \s-1CPU\s0 is not capable of performing the
benchmark, either because oscillator counter or 'flush cache line' is
not available on current platform. For reference, on x86 'flush cache
line' was introduced with the \s-1SSE2\s0 extensions.
.PP
Otherwise number of recorded values is returned.

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