[Simh] PIP10 on PDP-8 SIM

[Johnny Billquist]

On 2013-03-18 17:44, Bob Supnik wrote:
> I was trying to get a debug setup for PIP10, per Ian King's mail, when I
> discovered that none of my OS/8 images have PIP10 on them. This
> certainly explains why the feature has never been tested before. I
> suspect that ReadAll and WriteAll either are not working at all, or are
> not working when the DECtape format is 18b. Another possibility is that
> PDP-10 DECtape format is not the same as 18b format, at the nitty-gritty
> level (format of headers and trailers).

The DECtape format as such, with all the headers and so on, is the same 
on all tapes. A normal PDP-8 formatted tape will have 129 (12-bit) 
words, however, while a PDP-10 (or any other 18-bitter) would have 128 
18-bit words (if I remember right).
The PDP-8, when doing 12-bit formatted tapes, just packs data in a way 
that is rather different from an 18-bit machine. But at the tape as 
such, there is nothing odd about it.

But I can see lots of potential for errors when emulating this whole thing.

I couldn't give exact details on lots of bits without looking in 
manuals, but in essence a DECtape is always doing 18-bit words. That is 
done by doing 6 groups of 3 bits each.
A PDP-8 will pack three 12-bit words into two 18-bit words. This means 
that a DECtape block for a PDP-8 will only have 86 18-bit words.
So the blocks are shorter, but you have more of them, when the tape is 
formatted for a PDP-8.

I hope (assume) that you already know all of this. If not, let me know, 
and I can try helping out some more.
I actually did dump a few 18-bit tapes on my PDP-8 only a few months 
ago, which is when I actually had to dig rather deep into all of this.
PIP10 was one of the things I really looked into. But since my tapes had 
actually been written on a PDP-15, I had to write my own code in the 
end, to just dump the raw data.

> If anyone has a canned OS/8 V3C image with PIP10, please post it
> somewhere (like Mediafire) and let me know by email.

I think someone already posted this, but I know I have that software, 
including sources (if I remember right) somewhere. Let me know if it 
can't be located anywhere else.

	Johnny