usr/src/cmd/apl/apl_s.s

#
#       This module contains the stack push routine and the fltused
#       symbol definition.  Push() doesn't have to be machine coded
#       but it does get called alot.  The fltused definition will
#       prevent the loader from adding some printf-associated modules.
#
#       If APL is to be loaded from a library, then some undefined
#       symbol must be present in the module which contains the
#       fltused definition.  In that sense, it is necessary to
#       have _push (or something) in machine code.
#
#                                       Ross Harvey 18-May-78
#
.text
.globl  _push
_push:  .word   0
        movl    _sp, r0
#       movl    4(ap),(r0)
#       acbl    $4,_sp,_staktop,ok
        movl    4(ap),(r0)+
        cmpl    r0, _staktop
        bgtr    toobig
        movl    r0, _sp
        ret
toobig: movl    r0,_sp
        calls   $0,_newstak
        ret