[Simh] PDP-10 DECtapes

[Johnny Billquist]

On 2013-03-28 03:08, Bob Supnik wrote:
> I believe the PDP-10 DECtape images that are floating around the web may
> not be accurate transcriptions.
>
> All the images are 289KB long. An 18b (36b) DECtape consists of 18b
> words in a 32b container (a 36b word in a 72b container). Thus, each
> 256W x 18b (128W x 36b) DECtape block requires 1KB of dusj storage.
> Therefore, the unage length, in KB, should equal the number of blocks,
> which is 578. And in fact, all extent 18b PDP DECtape images are 578KB
> long.
>
> A 16b DECtape consists of 16b words in a 16b container. Thus, each 256W
> x 16b DECtape block requires 1/2 KB of storage. Therefore, the image
> length, in KB, should equal half the number of blocks, or 578 / 2 = 289.
> And that's what we have.
>
> This is confirmed by the fact that if you let the simulator "autosize"
> the 289KB images, it comes up with PDP-11 format.
>
> I verified, by patching the simulator, that there is data in both halves
> of each 32b word in the image. This too points to a PDP-11 image.
>
> Even if the PDP-10 DECtape were packed with no  padding, it should be
> 18/16 = 9/8 the size of a PDP-11 image, or 325+KB. So I think the images
> were transcribed on a TC11 and are missing 2b in every 18b.
>
> This is not to say that the TD8E simulator is correct. I cannot get it
> to zero (initialize) a fresh 18b DECtape through PIP10, so something is
> broken. However, the TD8E is so arcane that it will take a considerable
> period of time to figure out what the software is trying to do and why
> the simulator is responding incorrectly.

I don't know if I'd call the TD8E arcane. Like I said before, it is the 
most stupid controller ever invented.
I had to write code last year to extract 18b images from a PDP-15 using 
the TD8E. Reading through the documentation, and reading through the 
TD8E driver as well as PIP10 was enough for me to understand how it 
worked, so you should be able to do the same.
Basically, the TD8E gives you almost nothing at all. You need to do just 
about all the fiddling yourself.
It is horribly stupid and primitive.

	Johnny