/sys$common/syshlp/helplib.hlb Sys Parameters, TTY_DMASIZE  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    TTY_DMASIZE specifies a number of characters in the output
    buffer. Below this number, character transfers are performed;
    above this number, DMA transfers occur if the controller is
    capable of DMA I/O.

    TTY_DMASIZE is a DYNAMIC parameter.