SETFIELD         Set record field value                             Extension

  SETFIELD field_name destination_r/m, source_reg

    Generates code that sets a value in a record field. Sets the field in
    the destination register or memory address with the contents of a
    source register. Source register must be a register of the same size
    as destination_r/m or smaller; if smaller, it must be the least
    significant part of another register that is the same size as the
    destination.
    To perform its function, SETFIELD generates a series of XOR, XCHG,
    ROL, ROR, OR, and MOVZX instructions. The contents of source_reg
    are changed.

    If you're using SETFIELD when your source and target registers are
    the same, the instruction does not OR the source value to itself.
    Instead, SETFIELD ensures that the fields of the target register not
    being set will be zero.

    SETFIELD does not attempt to clear the target field before ORing the
    new value. If this is necessary, you must explicitly clear the field
    (using MASKFLAG).

    Example:    RECORD FiDate year:7, month:4, day:5
                sub      dx,dx                ; Clear all fields
                setfield year   dx,bx         ; (bx changed)
                setfield month  dx,cx         ; (cx changed)
                setfield day    dx,al