VMS Help MACRO, VAX MACRO Assembler, Instructions, XFC  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    Purpose:        Extended function call - provides customer-defined
                    extensions to the instruction set
    Format:         opcode
    Operation:      {XFC fault}
    C. Codes:       N = 0, Z = 0, V = 0, C = 0
    Exceptions:     Opcode reserved to customer, customer reserved
                    exception
    Opcodes:        FC    XFC      Extended function call
    Description:    This instruction requests services of nonstandard
                    microcode or software. If no special microcode is
                    loaded, then an exception is generated to a kernel
                    mode software simulator. Typically the byte after
                    the XFC would request a specific service. Parameters
                    would be passed either as normal operands or in the
                    registers.