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diff --git a/sys/fmio/fmlfbwr.x b/sys/fmio/fmlfbwr.x
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+# Copyright(c) 1986 Association of Universities for Research in Astronomy Inc.
+
+include	<knet.h>
+include	<mach.h>
+include	"fmio.h"
+
+# FM_LFBINWRITE -- Asynchronous blocked binary write to an lfile.  We deal
+# only with binary data here; unpacking of text must be done at a higher
+# level.  The basic procedure is to convert the indicated lfile segment into
+# a range of lfile pages, then use the pagemap for the lfile to map these onto
+# physical datafile pages.  I/O is done in chunks of contiguous pages until
+# the requested amount of data has been transferred.  When writing at or
+# beyond EOF, new pages are automatically allocated upon demand.
+
+procedure fm_lfbinwrite (lf, buf, nbytes, offset)
+
+pointer	lf			#I lfile descriptor
+char	buf[ARB]		#I input data buffer
+int	nbytes			#I nbytes to write
+long	offset			#I lfile offset
+
+pointer	fm, pm
+int	status, chan, nleft, szbpage
+int	lfile, l1,l2, p1,p2, d1,d2, ip, nb, nt
+int	fmio_extend()
+
+begin
+	fm = LF_FM(lf)
+	pm = LF_PAGEMAP(lf)
+
+	# Verify descriptor.
+	if (fm == NULL || pm == NULL) {
+	    LF_STATUS(lf) = ERR
+	    return
+	} else
+	    LF_STATUS(lf) = 0
+
+	chan = FM_CHAN(fm)
+	szbpage = FM_SZBPAGE(fm)
+	lfile = (lf - FM_FTABLE(fm)) / LEN_FTE
+	nleft = nbytes
+
+	# Extend the pagemap?
+	while (offset + nbytes > LF_NPAGES(lf)*szbpage + 1)
+	    if (fmio_extend (fm, lfile, 1) == ERR) {
+		LF_STATUS(lf) = ERR
+		return
+	    } else
+		pm = LF_PAGEMAP(lf)
+
+	# Map lfile offset,nbytes into a range of lfile pages.
+	# I/O transfers are required to be aligned on page boundaries.
+
+	l1 = (offset - 1) / szbpage + 1
+	l2 = l1 + ((nleft + szbpage-1) / szbpage) - 1
+
+	# Write the data from the user buffer to the physical datafile,
+	# mapping lfile pages to physical offsets and moving data in chunks
+	# of as many contiguous pages as possible.
+
+	ip = 1
+	for (p1=l1;  nleft > 0 && p1 <= l2;  p1=p2) {
+	    # Get a contiguous range of datafile pages.
+	    d1 = Memi[pm+p1-1]
+	    for (p2=p1+1;  p2 <= l2;  p2=p2+1) {
+		d2 = Memi[pm+p2-1]
+		if (d2 - d1 != p2 - p1)
+		    break
+	    }
+
+	    # Compute the logical transfer size NB, and the amount of data
+	    # to be physically written NT.  The latter is always an integral
+	    # number of datafile pages in size.  NOTE that this requires that
+	    # the user buffer be an integral multiple of the page size, to
+	    # prevent referencing off the end of the buffer.
+
+	    nb = min (nleft, (p2 - p1) * szbpage)
+	    nt = (nb + szbpage-1) / szbpage * szbpage
+	    LF_LTSIZE(lf) = nb
+
+	    # Write the file segment.
+	    call zawrbf (chan, buf[ip], nt, (d1-1)*szbpage + FM_DATASTART(fm))
+	    LF_FLAGS(lf) = or (LF_FLAGS(lf), LFF_IOINPROGRESS)
+	    
+	    # Bump the i/o counters.
+	    ip = ip + nb / SZB_CHAR
+	    nleft = nleft - nb
+
+	    # If we didn't write all the data, wait until the write completes.
+	    if (nleft > 0) {
+		call zawtbf (chan, status)
+		LF_FLAGS(lf) = and (LF_FLAGS(lf), not(LFF_IOINPROGRESS))
+		if (status == ERR) {
+		    LF_STATUS(lf) = ERR
+		    return
+		} else if (status == 0) {
+		    break
+		} else
+		    LF_STATUS(lf) = LF_STATUS(lf) + min(LF_LTSIZE(lf),status)
+	    }
+	}
+
+	# Update the lfile size counter.
+	nb = offset + nbytes - 1
+	if (nb > LF_FSIZE(lf)) {
+	    LF_FSIZE(lf) = nb
+	    FM_DHMODIFIED(fm) = YES
+	}
+end