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diff --git a/unix/gdev/iism75/zwrm75.x b/unix/gdev/iism75/zwrm75.x
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+# Copyright(c) 1986 Association of Universities for Research in Astronomy Inc.
+
+include	<mach.h>
+include	"m75.h"
+include	"iis.h"
+
+# ZWRM75 -- Initiate an asynchronous write to the IIS.  We are called to
+# output the header of all instructions sent to the IIS.  There are 3 types
+# of instructions; those which consist only of a header write, those which
+# consist of a header write followed by a data write, and those which
+# consist of a header write followed by a data read.  Translation of an M70
+# instruction into an M75 instruction may involve moving information between
+# the header and data block, hence we must save the headers of the read and
+# write instructions until the data has been read or written.  The STATE
+# variable in the channel descriptor is used to keep track of the instruction
+# processing state.
+
+procedure zwrm75 (ifcb, buf, nbytes, offset)
+
+int	ifcb			# pointer to channel descriptor passed as int
+char	buf[ARB]		# input buffer
+int	nbytes			# number of  bytes to write
+long	offset			# not used for this device
+
+pointer	fcb
+int	xferid, and()
+
+begin
+	fcb = ifcb
+
+	if (FCB_STATE(fcb) == READY) {
+	    # Start a new instruction.
+
+	    if (nbytes != SZB_IISHDR) {
+		FCB_STATUS(fcb) = ERR
+		return
+	    }
+
+	    # Save the M70 header in the descriptor.
+	    call amovs (buf, FCB_IISHDR(fcb), LEN_IISHDR)
+	    xferid = XFERID(buf)
+
+	    # Determine the state for the new instruction.
+
+	    if (THINGCT(buf) == 0)
+		FCB_STATE(fcb) = READY
+	    else if (and (xferid, IREAD) != 0)
+		FCB_STATE(fcb) = DATA_READ
+	    else
+		FCB_STATE(fcb) = DATA_WRITE
+
+	    # If the new state is READY, no data read or write is needed,
+	    # so just translate and output the header.
+
+	    if (FCB_STATE(fcb) == READY)
+		call m75put (fcb, buf, nbytes, offset)
+	    else {
+		# Set up a channel status as if we had just written the new
+		# header, so that the next ZWTM75 will not return an error.
+
+		FCB_STATUS(fcb) = IIS_INACTIVE
+		FCB_NBYTES(fcb) = SZB_IISHDR
+	    }
+	    
+	} else if (FCB_STATE(fcb) == DATA_WRITE) {
+	    # This is the second zwrm75 call for a hdr+data output
+	    # instruction.
+
+	    call m75put (fcb, buf, nbytes, offset)
+	    FCB_STATE(fcb) = READY
+
+	} else {
+	    # ZRDM75 should have been called, set error on the channel.
+	    FCB_STATUS(fcb) = ERR
+	}
+end