check out this thread, I think I got it from here…but not sure (mine is v2.19 for windows)…CalculiX i8 and mingw - #7 by jbr
anyway 64GB of ram should be enough for about 2 Million equations (dofs) according to the manual, so keep it in mind.
from the installation guide http://www.dhondt.de/ccx_2.20.README.INSTALL :
USING CALCULIX with 8 byte per integer
This may be necessary for really big models. Right now, there is a file
Makefile_i8 showing you how it works. The integers in the FORTRAN files must
be compiled with the option -fdefault-integer-8 (for the GNU compiler). This
tells the compiler to consider all integers in the FORTRAN files as
8-byte integers. For the C-files, the flag -DLONGLONG must be added to the
CFLAGS. Right now, the CalculiX distribution assumes that you also generate
a integer8 version of ARPACK (use the above FORTRAN flag) and an
integer8 version of PARDISO (of course only if you use any if these, i.e.
if you have the flags -DPARDISO and -DARPACK in your Makefile). The
SPOOLES distribution, however, is assumed to be an integer 4, since it
is not known that an integer8 version of SPOOLES has already been made.