11 Mar
2021
11 Mar
'21
4:54 p.m.
I just read part of the Grant Saviers interview from CHM, where near the end he gives a
bit of history of DECtape. In particular, the fact that it was derived from LINCtape
though the format details are quite different.
A question popped into my mind, prompted by having read Guy Fedorkow's paper about
Whirlwind just a few days earlier: the Whirlwind tape format has 6 physical tracks but 3
logical tracks (each logical track is recorded redundantly on two physical tracks) and one
of those tracks is a clock track. LINCtape and DECtape have the same redundant recording
scheme, and also have a clock track; the difference is that they add a mark track to
enable the recording of block numbers and in-place block writing.
That made me wonder if LINCtape was, in part, inspired by the Whirlwind tape system, or if
those analogies are just a concidence.
Incidentally, it's probably not widely known that LINCtape/DECtape is not the only
tape system with random block write capability. Another one that does this is the
Electrologica X1 tape system, which uses 1/2 inch 10 track tapes, which include a clock
and a mark track. An interesting wrinkle is that the X1 tape system lets you chose the
block size when formatting the tape, and then data block writes allow for the writing of
any block size up to the formatted block size. I'm not sure when that device was
introduced; the documentation I have is from 1964. There's no sign the designers knew
of DECtape (or vice versa).
paul
11 Mar
11 Mar
5:05 p.m.
Paul,
Very interesting, thanks for sharing.
Bill
On Thu, Mar 11, 2021 at 7:54 PM Paul Koning via cctalk <
cctalk at classiccmp.org> wrote:
I just read part of the Grant Saviers interview from
CHM, where near the
end he gives a bit of history of DECtape. In particular, the fact that it
was derived from LINCtape though the format details are quite different.
A question popped into my mind, prompted by having read Guy Fedorkow's
paper about Whirlwind just a few days earlier: the Whirlwind tape format
has 6 physical tracks but 3 logical tracks (each logical track is recorded
redundantly on two physical tracks) and one of those tracks is a clock
track. LINCtape and DECtape have the same redundant recording scheme, and
also have a clock track; the difference is that they add a mark track to
enable the recording of block numbers and in-place block writing.
That made me wonder if LINCtape was, in part, inspired by the Whirlwind
tape system, or if those analogies are just a concidence.
Incidentally, it's probably not widely known that LINCtape/DECtape is not
the only tape system with random block write capability. Another one that
does this is the Electrologica X1 tape system, which uses 1/2 inch 10 track
tapes, which include a clock and a mark track. An interesting wrinkle is
that the X1 tape system lets you chose the block size when formatting the
tape, and then data block writes allow for the writing of any block size up
to the formatted block size. I'm not sure when that device was introduced;
the documentation I have is from 1964. There's no sign the designers knew
of DECtape (or vice versa).
paul
6:12 p.m.
My recollection was the Linc was inspired by the Whirlwind. Whirlwind
predates it by a few years.
AND of course the Linc inspired the PDP5 and PDP8. Of course some one
correct me if this is wrong, but my sources were DEC when I worked
there 1969-1980, stories with Don White and others, as well as my
sabbatical on site at Lincoln Lab building working on a project, some
interaction with associates of Charles Molnar et al, and their
recollections.
And Linc8 and Link12 were related too.
Here is a link (pardon the pun) to the Digibarn recollection story on LINC.
https://www.digibarn.com/stories/linc/documents/LINC-Personal-Workstation/L…
On Thu, Mar 11, 2021 at 8:05 PM Bill Degnan via cctalk
<cctalk at classiccmp.org> wrote:
Paul,
Very interesting, thanks for sharing.
Bill
On Thu, Mar 11, 2021 at 7:54 PM Paul Koning via cctalk <
cctalk at classiccmp.org> wrote:
> I just read part of the Grant Saviers interview from CHM, where near the
> end he gives a bit of history of DECtape. In particular, the fact that it
> was derived from LINCtape though the format details are quite different.
>
> A question popped into my mind, prompted by having read Guy Fedorkow's
> paper about Whirlwind just a few days earlier: the Whirlwind tape format
> has 6 physical tracks but 3 logical tracks (each logical track is recorded
> redundantly on two physical tracks) and one of those tracks is a clock
> track. LINCtape and DECtape have the same redundant recording scheme, and
> also have a clock track; the difference is that they add a mark track to
> enable the recording of block numbers and in-place block writing.
>
> That made me wonder if LINCtape was, in part, inspired by the Whirlwind
> tape system, or if those analogies are just a concidence.
>
> Incidentally, it's probably not widely known that LINCtape/DECtape is not
> the only tape system with random block write capability. Another one that
> does this is the Electrologica X1 tape system, which uses 1/2 inch 10 track
> tapes, which include a clock and a mark track. An interesting wrinkle is
> that the X1 tape system lets you chose the block size when formatting the
> tape, and then data block writes allow for the writing of any block size up
> to the formatted block size. I'm not sure when that device was introduced;
> the documentation I have is from 1964. There's no sign the designers knew
> of DECtape (or vice versa).
>
> paul
>
>
6:16 p.m.
On 3/11/21 6:12 PM, Bob Smith via cctalk wrote:
My recollection was the Linc was inspired by the
Whirlwind. Whirlwind
predates it by a few years.
Tom Stockebrand is the connecting link between all of these tape systems.
I don't know if he was involved with the original Raytheon tape design
for Whirlwind but he is known to have work on the tape system for the
Memory Test Computer and certainly would be aware of how the WW tape
system worked.
6:18 p.m.
On 3/11/21 6:16 PM, Al Kossow via cctalk wrote:
On 3/11/21 6:12 PM, Bob Smith via cctalk wrote:
He also did the magtape system for TX-2
My recollection was the Linc was inspired by the
Whirlwind. Whirlwind
predates it by a few years.
Tom Stockebrand is the connecting link between all of these tape systems.
I don't know if he was involved with the original Raytheon tape design
for Whirlwind but he is known to have work on the tape system for the
Memory Test Computer and certainly would be aware of how the WW tape
system worked.
6:23 p.m.
On 3/11/21 6:18 PM, Al Kossow via cctalk wrote:
On 3/11/21 6:16 PM, Al Kossow via cctalk wrote:
there are memos in the Whirlwind archives that show the magnetic tape they
were getting had so many imperfections that they had to double up on the
tracks to make it reliable
On 3/11/21 6:12 PM, Bob Smith via cctalk wrote:
?He also did the magtape system for TX-2
My recollection was the Linc was inspired by the
Whirlwind. Whirlwind
predates it by a few years.
Tom Stockebrand is the connecting link between all of these tape systems.
I don't know if he was involved with the original Raytheon tape design
for Whirlwind but he is known to have work on the tape system for the
Memory Test Computer and certainly would be aware of how the WW tape
system worked.
12 Mar
12 Mar
8:38 a.m.
THanks Al! I forgot about that chapter of Tom's ..worked for him for a
few years, Fantastic Boss, and one helluv an engineer.
bb
On Thu, Mar 11, 2021 at 9:23 PM Al Kossow via cctalk
<cctalk at classiccmp.org> wrote:
On 3/11/21 6:18 PM, Al Kossow via cctalk wrote:
On 3/11/21 6:16 PM, Al Kossow via cctalk wrote:
there are memos in the Whirlwind archives that show the magnetic tape they
were getting had so many imperfections that they had to double up on the
tracks to make it reliable
On 3/11/21 6:12 PM, Bob Smith via cctalk wrote:
He also did the magtape system for TX-2
My recollection was the Linc was inspired by the
Whirlwind. Whirlwind
predates it by a few years.
Tom Stockebrand is the connecting link between all of these tape systems.
I don't know if he was involved with the original Raytheon tape design
for Whirlwind but he is known to have work on the tape system for the
Memory Test Computer and certainly would be aware of how the WW tape
system worked.
20 Mar
20 Mar
1:07 p.m.
Why did DEC not use the LINCtape format for the PDP-8? I assume maintaining
format compatibility between their low, mid, and high range systems was
important to them? I suppose there was no other good solution to
transferring large files across different DEC platforms than to use
magnetic tape...so I may have answered my own question here.
Kyle
1:13 p.m.
On Mar 20, 2021, at 4:07 PM, Kyle Owen via cctalk
<cctalk at classiccmp.org> wrote:
Why did DEC not use the LINCtape format for the PDP-8? I assume maintaining
format compatibility between their low, mid, and high range systems was
important to them? I suppose there was no other good solution to
transferring large files across different DEC platforms than to use
magnetic tape...so I may have answered my own question here.
Kyle
Speculating here since I have no direct knowledge: the DECtape format allows read and
write in either direction, while LINCtape only allows read and write forward. The
bidirectional I/O capability was part of DECtape format from the start, and I suspect the
desire was to keep that.
Also, the DECtape format (ignoring the funny bit order in the PDP-1 case) is the same
across all models except for the block size and count in the PDP-8 case. Chances are the
desire was to reuse all that design.
paul
1:21 p.m.
On Sat, Mar 20, 2021, 16:13 Paul Koning <paulkoning at comcast.net> wrote
Speculating here since I have no direct knowledge: the
DECtape format
allows read and write in either direction, while LINCtape only allows read
and write forward. The bidirectional I/O capability was part of DECtape
format from the start, and I suspect the desire was to keep that.
What systems took advantage of the bidirectional nature?
Kyle
1:33 p.m.
On Mar 20, 2021, at 4:21 PM, Kyle Owen via cctalk
<cctalk at classiccmp.org> wrote:
On Sat, Mar 20, 2021, 16:13 Paul Koning <paulkoning at comcast.net> wrote
DOS-11 for one (and thus RSTS, which reuses that format). DOS DECtape files are linked
lists; each block contains a link to the next block. To allow for reading one block at a
time, start/stop mode, DOS interleaves 4:1. If you're allocating a long file and the
allocation reaches end of tape, allocation then continues in the reverse direction. The
extreme case of a single file that takes up the whole drive looks like two up/down passes
over the tape, each one touching 1/4th of the blocks in each direction. When a file is
read, blocks are read in the same direction as they were written. The direction is given
by the sign of the block number in the link word, negative means reverse.
As Grant pointed out in the oral history interview, bidirectional DECtape I/O in the sense
that you could read a block in the opposite direction it was written isn't all that
useful. While the PDP-11 controller does the obverse complement thing, that just means
you get the bits in the word correct but the 256 words are still in the opposite order.
That could be handled, of course, but I haven't seen programs that do so.
Well, one exception: the tape formatter does the write timing/mark forward, then write all
in reverse, then reads (to check) forward. But those are test patterns so the job of
dealing with the direction change is easy.
paul
Speculating here since I have no direct
knowledge: the DECtape format
allows read and write in either direction, while LINCtape only allows read
and write forward. The bidirectional I/O capability was part of DECtape
format from the start, and I suspect the desire was to keep that.
What systems took advantage of the bidirectional nature?
Kyle
1141
days inactive
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10 comments
5 participants
participants (5)
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aek＠bitsavers.org -
billdegnan＠gmail.com -
bobsmithofd＠gmail.com -
kylevowen＠gmail.com -
paulkoning＠comcast.net