> Date: Wed, 11 Sep 2002 10:01:21 -0700 (PDT)
> From: Carl Lowenstein <cdl@mpl.ucsd.edu>
> To: port-vax@netbsd.org
> Subject: Re: asc_vsbus.c: the 64K DMA problem
> 
> A quick cruise through old handbooks finds that the only Unibus device
> with a 16-bit Byte Count register was the TM11 magtape controller.
> Disk and DECtape and newer magtape controllers had Word Count registers,
> until they got to the idea of using in-memory command packets.
> 
> I have some really ancient TM11 documentation at home, I will try to
> see if there is any discussion of what happens if the TM11 is given a
> byte count of 0.

September 10, 1970
Magnetic Tape Control Unit in the PDP-11
(Preliminary Specification)

Section 2.2  Byte Record Counter - MTBRC
(about half a page of text) 
My summary:  the value in the MTBRC is "level detected" not "edge
detected".  When it is zero, data transfer does not take place, and
incrementing of the MTBRC and the MTCMA (data pointer) is suspended.
And the READY flag is set in the MTCSR.

Wonder how they did that in the hardware.  As a some-time hardware
person I would have used the carry out from the MTBRC as it went from
-1 to 0.

Is this document (16 pages) something of sufficient historical value
to be scanned and archived somewhere?

    carl
-- 
    carl lowenstein         marine physical lab     u.c. san diego
                                                 clowenst@ucsd.edu