Subject: Re: Why a pdp-11

From: [email-masked]
Date: 27 Dec 94 23:44:27 PST (Tue)

>
>
>On Thu, 22 Dec 1994 [email-masked] wrote:
[...]
>>  
>> The RF08 backplane contains 96 flip chips with not a single IC anywhere.
>> My favorite feature of the RS08 is the bank of switches that let you
>> write protect it 1/8th of a disk at a time.  Great for when you accidentally
>> blow the system away, just enable only the first 1/8, do a LOAD, and it'll
>> bomb out just before it would have overwritten your files.  Also the disk
>> (like the DF/DS) is addressable to the word, really neat.  TSS used blocks
>> anyway for the directory structure, but as a 100% artificial concept.
>> There was a system call to retrieve the block size, in case they ever
>> changed it (not as far as I know).
>
And cjl responded:
>The Disk Monitor System is based on the physical size of DECtape, which 
>is 129 words.  The data is stored in the first 128 words, just like all 
>other PDP-8 DECtape systems, but the 129th word is used as a link pointer 
>to the next logical block.  This severely makes it dependent on the 
>hardware, thus the DMS was only ever implemented on the few devices that 
>could be adapted to this odd size:
>
>1)	TC01/08 DECtape
>
>2)	DF32
>
>3)	RF08
>
>4)	RK08 (wasting 127 words/sector; this is a 256 word/sector disk 
>with the ability to set a word count from 001-256 words)
>
>5)	PDP-12 LINCtape formatted to 128 words/block

And of course, the 9-track magtape (TU10) in core-dump mode and 7-track magtape
(I remember it as TU20, but I think my memory is flawed).

Using the scheme
  EOF with long gap
  two's complement block number
  129 words (link word last)
  EOF with long gap
  ...
and rewriting blocks in the middle of the tape.

Twisted, but it worked. 4KDMS running on a pair of magtape drives was a
sight to behold, especially with one of the drives a 9-track and the
other a 7-track with clunking and wheezing and reels-a-spinning. The
pain was how many places drivers for devices existed in the 4KDMS, e.g.,
in PIP, and how hard it was to patch in a driver for the magtape in the
place of the DECtape driver in each required place.

:-)

OS/8 on magtape-only systems was a kick, too.

>
>While I've never seen it, there is allegedly an ALGOL for the PDP-8 that 
>is based on simulating the data structure of some Burroughs model on an 
>RF08.  The RF is necessary due to the need for some unwieldy size of data 
>block to match the PDP-8-translated ALGOL environment, etc.  Apparently 
>the compiler was compiled on a running Burroughs systems, and the binary 
>was transferred to the RF08 along with an -8 support program for 
>execution, etc.

There was also ROGALGOL. I have a listing somewhere, but the tapes are
long ago turned to dust.

>
>While head/track disks are generally faster, especially at seeking, most 
>PDP-8 systems were geared towards standard sized disk media.  All newer 
>peripherals are designed this way even today, thus signaling the end of 
>software locked into odd disk sizes, etc.  All PDP-8 disks, from the RK8E 
>through the more recent SCSI-based designs, cannot perform odd-length 
>transfers.
>
>Additionally, the DF32 and RF08 were never properly designed to be 
>powered down!  (Yes, they expected you to perpetually power them!)  Each 
>power-down sequence is essentially a head crash.  The drives even have a 
>few spare heads/tracks so you can switch to an auxiliary instead of 
>powering down, etc.  This was never necessary conceptually, and various 
>drum manufacturers solved the problem with a head-lift mechanism, but DEC 
>never accomplished this, even with the PDP-11 variants (I think called 
>RS64? and RF11).

I seem to remember that the DF32/DS32 were single sided, even though
the platter was surfaced on both sides, and, if times were really hard,
the platter could be flipped once and then reformatted. Those were sick days...

>cjl

Regards,
Mike
--
Mike Murphy  [email-masked]  ucsd!sceard!mrm  +1 619 598 5874
Better is the enemy of Good

---

From: Charles Lasner <[email-masked]>
Date: Fri, 30 Dec 1994 21:11:20 -0500 (EST)

> (I remember it as TU20, but I think my memory is flawed).

TU10.  Correct the first time

> OS/8 on magtape-only systems was a kick, too.

Yes, this was also done on non-DEC magtapes as well.  I think the company 
was Iomega (Iomedia?  One of them is for bournoulli disks.  This is the 
other one :-).)  They are in turn a spin-off of Digitronics, the original 
company that made the big clunky rubber-roller white-painted paper-tape 
reader such as found on the PDP-5.  They made drives in 7-track, 9-track, 
and 1600 PE 9t.  I worked for Western Union.  Each remote site had the 
9-track and perhaps some set of the others.  My main machine had all of 
them (1 1600, 2 800-9 and 2 800-7).  (I had no LPT, but I could write a 
listing to a 7-track at 556 and then take the tape to an inhouse univac 
machine to get printed.  A two-page OS/8 handler did it, etc.)

Our handler depended on the Omnibus machine hack where executing 7016 at 
relative offset 015 of the page loads the page address+16 into the AC.  
Note that the position is important because it could be on the -8/a CPU.  
(On the -8/e CPU it doesn't matter, but on the -8/a, the value is 
absolute address, not page address+16, so you must load the instruction 
at 015 relative, etc.) (Note: I didn't write that handler :-).)

> There was also ROGALGOL. I have a listing somewhere, but the tapes are

Please find the listing; it can be scanned!

> the platter could be flipped once and then reformatted. Those were sick days...

I heard the same.

cjl