Allocates a device for exclusive use by a process and its
    subprocesses. No other process can allocate the device or assign
    channels to it until the image that called $ALLOC exits or
    explicitly deallocates the device with the Deallocate Device
    ($DALLOC) service.

    Format

      SYS$ALLOC  devnam ,[phylen] ,[phybuf] ,[acmode] ,[flags]

    C Prototype

      int sys$alloc  (void *devnam, unsigned short int *phylen, void

                     *phybuf, unsigned int acmode, unsigned int

                     flags);