[Simh] "DECtape marks issue"????????

[Rich Alderson]

> Date: Thu, 02 Sep 2010 20:08:01 +0200
> From: "dott.Piergiorgio" <dott.piergiorgio at fastwebnet.it>

> (looking to the DECTape marks issue, for example)

What is "the DECtape marks issue"?  The DECtape emulation in SimH does not
simulate the mark track on a DECtape, so I don't understand how there could
be a "DECtape marks issue".

I suggested to Bob several years ago that the DECtape representation could
be made more realistic (and thereby more flexible).  His reasonable answer
was to point out that I could do the implementation, should I choose to do
so, and submit it for everyone's review; since I am not a C programmer, I
have never had the time to do that.  (Macro-20, sure.)

I'll outline my original vision, and my present simplification, and if you
(or anyone else) wants to pursue it, I'll be happy to provide insight into
the working of DECtape drives and controllers.

Represent the frames on a DECtape as 8-bit bytes; the timing tracks are
implicit in the order of the bytes in the resulting file.  Represent the
mark tracks with bits 0 and 7, one the inverse of the other (as a nod to
the automatic inversion of bit sense in a Read Backwards operation).  The
data tracks are 1 = bits 1 and 4, 2 = 2 and 5, and 3 = 3 and 6, where bits
1-2-3 are conjointly inverted with respect to bits 4-5-6.  We ignore the
fact that on a physical DECtape, 5 tracks' worth of data is redundantly
recorded on 10 tracks (T M 1 2 3 1A 2A 3A M T).

This can be simplified if we drop the inversion, and represent each frame
in an 8-bit byte where the mark track is bit 0 and data tracks are 5-6-7.

The controller (555, TC01, TC02, TC08, TC09, TD10, TD8E, TC11, or TC15)
is responsible for assembling 3 bits of data at a time into 12, 16, 18,
or 36 bit words and putting it into the CPU or memory, as appropriate.
That's where all the work is.  There's also the question of other brands
of computer (Data General comes to mind) and their controllers, as well.

Creating realistic DECtape emulations for SimH could turn into a cottage
industry.

But let's return to the question:  WHAT "DECtape marks issue"????????

                                                                Rich