. ▁It ▁supports ▁G CC ▁or ▁Cl ang / LL VM ▁comp ilers ▁as ▁back ends . ▁It ▁diver ges ▁from ▁other ▁run times ▁in ▁terms ▁of ▁syntax , ▁semantics ▁and ▁A BI ▁compat ability . ▁It ▁supports ▁Linux , ▁Free B SD , ▁and ▁Windows . ▁ ▁Port able ▁Object ▁Comp iler ▁Besides ▁the ▁G CC / Ne XT / App le ▁implementation , ▁which ▁added ▁several ▁extensions ▁to ▁the ▁original ▁Step stone ▁implementation , ▁another ▁free , ▁open - source ▁Object ive - C ▁implementation ▁called ▁the ▁Port able ▁Object ▁Comp iler ▁also ▁exists . ▁The ▁set ▁of ▁extensions ▁implemented ▁by ▁the ▁Port able ▁Object ▁Comp iler ▁diff ers ▁from ▁the ▁G CC / Ne XT / App le ▁implementation ; ▁in ▁particular , ▁it ▁includes ▁Small t alk - like ▁blocks ▁for ▁Object ive - C , ▁while ▁it ▁la cks ▁protocol s ▁and ▁categories , ▁two ▁features ▁used ▁extens ively ▁in ▁Open Step ▁and ▁its ▁derivatives ▁and ▁relatives . ▁Over all , ▁P OC ▁represents ▁an ▁older , ▁pre - Ne XT ▁stage ▁in ▁the ▁language ' s ▁evolution , ▁roughly ▁conform ant ▁to ▁Brad ▁C ox ' s ▁ 1 9 9 1 ▁book . ▁ ▁It ▁also ▁includes ▁a ▁runtime ▁library ▁called ▁Object P ak , ▁which ▁is ▁based ▁on ▁C ox ' s ▁original ▁I CP ak 1 0 1 ▁library ▁( which ▁in ▁turn ▁der ives ▁from ▁the ▁Small t alk - 8 0 ▁class ▁library ), ▁and ▁is ▁quite ▁rad ically ▁different ▁from ▁the