[int_19h]

Most BASICs had something like that. Here's something from QBASIC documentation:

   Transfers control to a machine-language procedure.

   CALL ABSOLUTE ([argumentlist,] offset%)

       ▪ argumentlist    Arguments passed to a machine-language procedure
                         as offsets from the current data segment.
       ▪ offset%         The offset from the current code segment, set by
                         DEF SEG, to the starting location of the procedure.

   Example:
       'Calls routine for printing the screen to a local printer.
       DIM a%(2)
       DEF SEG = VARSEG(a%(0))
       FOR i% = 0 TO 2
          READ d%
          POKE VARPTR(a%(0)) + i%, d%
       NEXT i%
       DATA 205, 5, 203  : ' int 5  retf  'Machine-language code
                                          'for printing screen.
       CALL ABSOLUTE(VARPTR(a%(0)))
       DEF SEG

And before that, there was DEF USR: http://www.antonis.de/qbebooks/gwbasman/defusr.html