NAME
       SetWriteMask -- Set the pixel write mask value for a RastPort (V39).


   SYNOPSIS
       success=SetWriteMask ( rp, msk )
	  d0	               a0   d0

	ULONG SetWriteMask(struct RastPort *,ULONG)

   FUNCTION
	Set the current value of the bit write mask for the rastport.
	bits of the pixel with zeros in their mask will not be modified by
	subsequent drawing operations.

   INPUTS
       rp  =  a pointer to a valid RastPort structure.
	msk =  a longword mask value.

	Graphics devices which do not support per-bit masking will
	return 0 (failure).
   BUGS

   NOTES

   SEE ALSO
	graphics/gfxmacros.h