toms385, a Fortran77 code which implements ACM toms algorithm 385, for evaluating the exponential integral Ei(x).
The text of many ACM TOMS algorithms is available online through ACM: https://www.acm.org/pubs/calgo or NETLIB: https://www.netlib.org/toms/index.html.
The information on this web page is distributed under the MIT license.
toms385 is available in a Fortran77 version.
test_values, a Fortran77 library which supplies test values of various mathematical functions.