[Simh] PDP-10 DECtapes

[Bob Supnik]

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.

/Bob