[Simh] PIP10 on PDP-8 SIM

Tue Mar 19 09:03:15 EDT 2013

> 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). 
Pretty much right.  129 may be slightly misleading.  The format is 129, 
but the -8 used  only 128 of the 129 12-bit words/block for data.

The PDP-10 is 36-bit, not 18-bit.  A PDP-10 (actually, non-PDP-8) 
formatted DECtape would have 578 blocks of 128 36-bit words.  (256 
18-bit words at the hardware level.)

Blocks 0, 1, 2 are for DTBoot (hardware read-in bootstrap), and didn't 
contain user data.

Block 100 is the directory block.  Thus 574 blocks for user data. The 
directory holds up to 22 files, plus a map of which file owns each block.

The user data blocks have a one word header (LH = next block of file; RH 
= first block & words used in this block) + 127 words of payload.  This 
differed from disk files, where all 128 words were payload, so 
inattentive programmers could make a number of mistakes.  (E.g. random 
access block isn't word/128; you had to pay attention to the buffer's 
word count, etc.)

Data blocks of a file are (usually) not contiguous; this allowed the 
drive to stop and restart while reading or writing without having to 
reverse direction.  The spacing depended on what blocks were free when 
files were written, and the data mode.  (Files written in 'core dump' 
modes were assumed to be read by the monitor without stopping, so the 
gap was smaller, allowing larger programs to be read without reversing 
direction.)

The gory details of the format are in the Monitor Calls manual (TOPS-10).

The PDP-11 used 18-bit format, but ignored the high 2 bits of each word 
(except when reading PDP-10 tapes; the hardware for that was tricky as 
the high bits had to be read with programmed IO; the low 16 were DMAed 
on the TC-11.)

The low-level formatting that established the mark track, end zones and 
block delimiters was done via a stand-alone diagnostic. This differs 
between the two formats.  The directory block could be initialized under 
timesharing.

Directory structure given is for the PDP-10; other OSs used different 
formats.

 From an emulation point of view, the PDP-10's controller was the most 
interesting; the driver does dead reckoning; that is, it will start a 
drive spinning for a seek, disconnect, service other drives, and 
reconnect just before the desired block is expected to be over the read 
head.  So real-world timing matters.  The other controllers (and thus 
OSs) didn't support this.

Oh, all numbers above are radix 10.

The link for OS8 that I posted yesterday was via filewatcher; the direct 
link 
isftp://sunsite.unc.edu/pub/academic/computer-science/history/pdp-8/os8/ 
Sorry if this was confusing.

Of potential interest to low-level folks is 
http://patft.uspto.gov/netacgi/nph-Parser?Sect2=PTO1&Sect2=HITOFF&p=1&u=/netahtml/PTO/search-bool.html&r=1&f=G&l=50&d=PALL&RefSrch=yes&Query=PN/3387293

Hope this is useful.

This communication may not represent my employer's views,
if any, on the matters discussed.

On 18-Mar-13 16:26, Johnny Billquist wrote:
> 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
>
> _______________________________________________
> Simh mailing list
> Simh at trailing-edge.com
> http://mailman.trailing-edge.com/mailman/listinfo/simh

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mailman.trailing-edge.com/pipermail/simh/attachments/20130319/57ae2877/attachment-0002.html>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: smime.p7s
Type: application/pkcs7-signature
Size: 5159 bytes
Desc: S/MIME Cryptographic Signature
URL: <http://mailman.trailing-edge.com/pipermail/simh/attachments/20130319/57ae2877/attachment-0002.bin>