Subject: Re: Why a pdp-11

From: [email-masked]
Date: Thu, 22 Dec 94 02:32:14 EST

>Was that what the RS08 was? I thought the DF32 was the fixed head drive of 
>choice for the early 8's
 
DF32/DS32 was 32KW and smallish.  RF08/RS08 was 256KW and took up a whole
rack (500 lbs. total), although add'l RS08 drives were only 10.5" a pop
(you could have up to four).  The RF/RS was sort of, kind of, a little
compatible with the DF/DS, and TSS/8 had conditionals to use either for
swapping.
 
>I keep wondering as to why I have never SEEN a RS03/04 
 
If they're anything like the size/weight of an RS08, then as soon as
someone tried to wheel one across an asphalt parking lot on a summer day,
it sank full height into the ground and disappeared.  That would make
them a little harder to find...
 
Seriously, when we moved my RS08 into my parents' basement by sliding it
down the stairs on a sheet of plywood, THREE of us were unable to stop it
and it got downstairs before we did.  Luckily this was 1983 and the local
field circus office still had their RS08 formatter box and the two guys
they sent over had just spent a week on service contract time re-learning
how to format RS08s on a PDP-12 at MIT, so I didn't have to pay them $115
per hour to learn how to format mine.  Although, they *had* forgotten how
to load diags from paper tape.  They got it running again and were laid
off a couple of weeks later, never got around to sending me the bill.
All around a good deal...  The drive flaked out again a couple of years
later, shortly before the TSS/8.24 disk date format would have run out
(the PUTR DECtape format already had, it kept only the low digit of the
year since the 1970s seemed to last forever).
 
I can't believe my PDP-8 has doubled in age since I got it, it was a
dinosaur *then*!
 
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).
 
John Wilson   0,3

---

From: Charles Lasner <[email-masked]>
Date: Thu, 22 Dec 1994 09:31:11 -0500 (EST)

> compatible with the DF/DS, and TSS/8 had conditionals to use either for

DEC had been experimenting with head/track disks and drums for swapping 
etc.  The DF32 was originally intended to be the *memory* plane for the 
PDP-8/s, then code-named PDP-10!  Even in the final version it's 32Kx13 
or the (then) maximum memory of the PDP-8 architecture complete with 
memory parity.  This proved to be too slow for even the dreadfully slow 
-8/s, which eventually acquired the first ever quad cards as a 4K at a 
time memory plane, etc.  The DF32 was born as a result.  It's a DMA 
peripheral that can pass a word at a time if desired, since the transfer 
count can be set to 0001-4096 words, etc.

The RF08 is a larger device based on the same head/disk technology.  It 
isn't compatible, however they are similar enough that there is a 
bootstrap convention that serves both.  (It depends on power-clearing of 
register extensions, etc.)  The device code is the same in both, thus 
they never appear together in the same system.

The DF32 can be expanded with DS32 expander disks that plug into the DF32 
while the main DF32 has the buss interface, etc.  With the maximum of 
three DS32's, the total capacity is 128 KW, one half that of a single 
larger RF08.

The RF08 can be expanded with up to three RS08 expanders in a similar 
fashion for a total capacity of 1 MW, the logical maximum device under 
OS/8.  However, the RF08 has an obscure option to double this capacity 
using up to seven expanders coupled with what is known as the DV08 
option.  In this configuration, extra programming is needed to address an 
additional select bit for the (up to) four extra units.  The DV08 
automatically does additional DMA transfers to verity writes without 
program intervention.  Interrupts can occur on read-verify errors after 
the program-initiated write.  No program buffer or other operations are 
required, just initializing the DV08 itself, etc.

> changed it (not as far as I know).

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

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.

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).

cjl