Library /sys$common/syshlp/dbg$help.hlb DEBUG, SET, BREAK, Qualifiers, /SYSEMULATE  *Conan The Librarian (sorry for the slow response - running on an old VAX)

       /SYSEMULATE[=mask]

    (Alpha only) Stops program execution and returns control to the
    debugger after the operating system emulates an instruction.
    The optional argument mask is an unsigned quadword with bits
    set to specify which emulated instruction groups shall cause
    breakpoints. The only emulated instruction group currently
    defined consists of the BYTE and WORD instructions. Select this
    instruction group by setting bit 0 of mask to 1.

    If mask is not specified or if mask = FFFFFFFFFFFFFFFF, the
    debugger stops program execution when the operating system
    emulates any instruction.