[Simh] PIP10 on PDP-8 SIM

Tue Mar 19 09:47:37 EDT 2013

Non-power-of-2 word sizes are in many applications today. 12- and 14-bit instruction width PIC processors are sold by the billions.  24-bit data width DSP's are extremely common, and other sizes including
28-bit, 36-bit, 48-bit, and 56-bit data widths abound in DSP audio/video processing and other applications.

Tim.

From: simh-bounces at trailing-edge.com [mailto:simh-bounces at trailing-edge.com] On Behalf Of Armistead, Jason
Sent: Tuesday, March 19, 2013 9:30 AM
To: simh at trailing-edge.com
Subject: Re: [Simh] PIP10 on PDP-8 SIM

I had trouble with Timothe's link to the USPTO, but found this same patent in PDF form at

http://www.brouhaha.com/~eric/retrocomputing/dec/dectape/3387293.pdf

As a relative newbie who started my serious journey into computing with an Apple ][  I've never fully understood DEC's fascination with word lengths that weren't multiples of 2 (even though I dabbled with the PDP-11s at the local council my father worked at as a civil engineer - mainly to play Mugwump and Wumpus after giving up on trying to learn FORTRAN).  Maybe someone can give a brief history behind the 12-bit, 18-bit and 36-bit sizing of words.  All I've ever had to deal with are 8-bit, 16-bit, 32-bit and 64-bit processors and their O/S. (But yes, I do remember the 4004 !!!)

From: simh-bounces at trailing-edge.com<mailto:simh-bounces at trailing-edge.com> [mailto:simh-bounces at trailing-edge.com] On Behalf Of Timothe Litt
Sent: Tuesday, 19 March 2013 9:03 AM
To: simh at trailing-edge.com<mailto:simh at trailing-edge.com>
Subject: Re: [Simh] PIP10 on PDP-8 SIM

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 is ftp://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

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