Global variables have been introduced by various Prolog implementations recently. The implementation of them in SWI-Prolog is based on hProlog by Bart Demoen. In discussion with Bart it was decided that the semantics of hProlog nb_setval/2, which is equivalent to nb_linkval/2, is not acceptable for normal Prolog users as the behaviour is influenced by how built-in predicates that construct terms (read/1, =../2, etc.) are implemented.
GNU-Prolog provides a rich set of global variables, including arrays. Arrays can be implemented easily in SWI-Prolog using functor/3 and setarg/3 due to the unrestricted arity of compound terms.