How can I see how %*% is implemented?

I would like to see how the matrix multiplication operator %*% is 
implemented (because I want to see which external Fortran/C routines are 
used). How can I do so? Best S??ren

[This is probably a R-devel question: please study the posting guide.]

get("%*%")

.Primitive("%*%")

so get your R sources out (I'll use current R-devel), go to 
src/main/names.c and search.  You find

{"%*%",         do_matprod,     0,      1,      2,      {PP_BINARY, 
PREC_PERCENT,0}},

So this is OP 0 of do_matprod.  Search for that in src/main/*.c: it is is 
array.c. Find

     if (PRIMVAL(op) == 0) {			/* op == 0 : matprod() */

and the meat is

 	if (mode == CPLXSXP)
 	    cmatprod(COMPLEX(CAR(args)), nrx, ncx,
 		     COMPLEX(CADR(args)), nry, ncy, COMPLEX(ans));
 	else
 	    matprod(REAL(CAR(args)), nrx, ncx,
 		    REAL(CADR(args)), nry, ncy, REAL(ans));

Now look for matprod and cmatprod.  The answer is that if there are no 
IEEE754 specials, dgemm or zgemm are used.  Those are level-3 BLAS 
routines.

Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595