| Author |
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Guest
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 Posted:
Sat Oct 22, 2005 3:18 pm Post subject:
Re: Random Access Tape? |
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In article <20051021144030.4fc4794c.steveo@eircom.net>,
Steve O'Hara-Smith <steveo@eircom.net> wrote:
| Quote: | On Fri, 21 Oct 2005 12:03:09 GMT
Carl Pearson <jman@hal_no_spam_pc.org> wrote:
He keeps claiming that tape *can* have random access. Were this the
case, of course discs would be more like a tape recorder, as the random
vs sequential access argument would be thrown out of the discussion.
Well I have heard of a UNIX file system being mounted directly
from a DEC tape drive - apparently the superblock accesses became very
visible :)
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I recall a case where a -10 was run from a DECtape to make room
for a program and data that couldn't fit. It took all night
and stand alone to do the computing job.
| Quote: |
Strictly speaking random access should imply that the time taken
to access any data item is independent of the location of that data item.
A tape is clearly not random access nor is a disc, but a disc is a closer
approximation to random access than a tape.
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We're not talking about any old tape, but a DECtape.
/BAH |
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Steve O'Hara-Smith
Guest
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| Posted:
Sat Oct 22, 2005 8:53 pm Post subject:
Re: Random Access Tape? |
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On Sat, 22 Oct 05 10:18:56 GMT
jmfbahciv@aol.com wrote:
| Quote: | In article <20051021144030.4fc4794c.steveo@eircom.net>,
Steve O'Hara-Smith <steveo@eircom.net> wrote:
Strictly speaking random access should imply that the time taken
to access any data item is independent of the location of that data item.
A tape is clearly not random access nor is a disc, but a disc is a closer
approximation to random access than a tape.
We're not talking about any old tape, but a DECtape.
|
Not even a DECtape achieved anything like constant time to find
a word.
--
C:>WIN | Directable Mirror Arrays
The computer obeys and wins. | A better way to focus the sun
You lose and Bill collects. | licences available see
| http://www.sohara.org/ |
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thvv
Guest
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| Posted:
Thu Oct 27, 2005 7:17 pm Post subject:
Re: Random Access Tape? |
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Peter Flass wrote:
| Quote: | I believe the Multics backup worked this way. Multics tapes had
software-assigned block numbers, and the backup recorded
the starting block number for each file.
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Not to my knowledge. Yes, Multics standard tape format had
a block number in each block, and the tape DIM noticed an error if
the number was not monotone increasing. It did not provide
this block number to its caller, and the hierarchy backup
software did not record the block number in the dump map,
and the retriever/reloader software did not try fancy positioning
tricks to find a particular file.
The tape format described in
http://www.multicians.org/mspm-bb-3-01.html
had some cool attributes though. If an error occurred when writing
a tape block, the tape DIM just wrote the block again, with the
same block number. To read a block, it read until past the desired
block. This meant that while a tape was reading, an operator could
put the drive in standby, dismount the tape, mount it on another
drive, switch unit numbers, and ready the new drive: and the
tape DIM would notice the block number mismatch, resynch, and
continue> reading without indicating an error.
Multics tape format was also specified to have a single tape mark
every 128 blocks, so that spacing forward to a given record number
would be fast. We never used this feature and I think after a while
eliminated it: it just made it difficult to copy a Multics tape on a
360. |
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Scott Lurndal
Guest
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| Posted:
Sat Oct 29, 2005 5:14 am Post subject:
Re: Random Access Tape? |
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Carl Pearson <jman@hal_no_spam_pc.org> writes:
| Quote: | Howdy, Group,
Been having a conversation with this guy regarding tape vs disc.
He asked if a hard or floppy disk was more like a tape recorder, or a
record player.
I'm siding with record player, due to tape's inability to have random
access.
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One word DECtape.
scott |
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sqrfolkdnc
Guest
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| Posted:
Sat Oct 29, 2005 9:26 pm Post subject:
Re: Random Access Tape? |
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Of course the LGP-30 had main memory AND the accumulator on DRUM, and
addresses were NOT sequentially assigned. A good programmer located
his working storage so that it could be most often fetched in the same
revolution as the instruction, then fetch the next instruction wihout
wasting a whole revolution of the drum. There was a chart showing what
memory locations could be accessed from an instruction at any location,
without losing a revolution. It had 4096 words of memory, IIRC.
Don't modern tape drives have multiple tracks, going to the end and
back multiple times, so if you want to get to something 95% through the
complete linear image, you only have to scan down one track part way to
get it, not through 95% of the data? |
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Phil Weldon
Guest
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| Posted:
Sat Oct 29, 2005 9:51 pm Post subject:
Re: Random Access Tape? |
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'sqrfolkdnc' worte, in part:
| Of course the LGP-30 had main memory AND the accumulator on DRUM, and
| addresses were NOT sequentially assigned.
_____
While I never worked with an LGP-30, though I helped get replacement systems
for it up and running. In 1965-66 Pittsburgh Plate Glass Company replaced
the LGP-30 in each of their 5 paint plants with Univac 1050 systems (24 to
32 K 6-bit core memory.) I remember discussions of placing data and
instructions to avoid rotational delay. For the 1050 replacements we had
severe memory limitations even with the much larger size; we used very
complex instructions (that took a very long time to execute) to compress the
code. The main I/O was 800 bit-per-inch tape; even sequential access was
slow.
Phil Weldon
"sqrfolkdnc" <carey.schug@gmail.com> wrote in message
news:1130603164.345010.260910@g14g2000cwa.googlegroups.com...
| Of course the LGP-30 had main memory AND the accumulator on DRUM, and
| addresses were NOT sequentially assigned. A good programmer located
| his working storage so that it could be most often fetched in the same
| revolution as the instruction, then fetch the next instruction wihout
| wasting a whole revolution of the drum. There was a chart showing what
| memory locations could be accessed from an instruction at any location,
| without losing a revolution. It had 4096 words of memory, IIRC.
|
| Don't modern tape drives have multiple tracks, going to the end and
| back multiple times, so if you want to get to something 95% through the
| complete linear image, you only have to scan down one track part way to
| get it, not through 95% of the data?
| |
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Peter Flass
Guest
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| Posted:
Sun Oct 30, 2005 3:40 am Post subject:
Re: Random Access Tape? |
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sqrfolkdnc wrote:
| Quote: | Of course the LGP-30 had main memory AND the accumulator on DRUM, and
addresses were NOT sequentially assigned. A good programmer located
his working storage so that it could be most often fetched in the same
revolution as the instruction, then fetch the next instruction wihout
wasting a whole revolution of the drum. There was a chart showing what
memory locations could be accessed from an instruction at any location,
without losing a revolution. It had 4096 words of memory, IIRC.
|
Sounds like the IBM 650. |
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Michael J Kingston
Guest
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| Posted:
Sun Oct 30, 2005 6:41 pm Post subject:
Re: Random Access Tape? |
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In message <OTS8f.28348$Bv6.232@twister.nyroc.rr.com>, Peter Flass
<Peter_Flass@Yahoo.com> writes
| Quote: |
sqrfolkdnc wrote:
Of course the LGP-30 had main memory AND the accumulator on DRUM, and
addresses were NOT sequentially assigned. A good programmer located
his working storage so that it could be most often fetched in the same
revolution as the instruction, then fetch the next instruction wihout
wasting a whole revolution of the drum. There was a chart showing what
memory locations could be accessed from an instruction at any location,
without losing a revolution. It had 4096 words of memory, IIRC.
Sounds like the IBM 650.
My recollection is that IBM 650 drum had sequential addresses. Ferranti |
Pegasus drum had addresses in a sequence arranged so as to optimise
continuous reading of 8-word blocks.
--
Michael J Kingston |
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Guest
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| Posted:
Sun Oct 30, 2005 7:00 pm Post subject:
Re: Random Access Tape? |
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In article <NZyVADDV+LZDFA$R@digweed2.demon.co.uk>,
Michael J Kingston <mike.kingston@digweed2.demon.co.uk> wrote:
| Quote: | In message <OTS8f.28348$Bv6.232@twister.nyroc.rr.com>, Peter Flass
Peter_Flass@Yahoo.com> writes
sqrfolkdnc wrote:
Of course the LGP-30 had main memory AND the accumulator on DRUM, and
addresses were NOT sequentially assigned. A good programmer located
his working storage so that it could be most often fetched in the same
revolution as the instruction, then fetch the next instruction wihout
wasting a whole revolution of the drum. There was a chart showing what
memory locations could be accessed from an instruction at any location,
without losing a revolution. It had 4096 words of memory, IIRC.
Sounds like the IBM 650.
My recollection is that IBM 650 drum had sequential addresses. Ferranti
Pegasus drum had addresses in a sequence arranged so as to optimise
continuous reading of 8-word blocks.
|
Could you be confusing retrieval addressing and physcial addressing?
There is a huge difference.
Part of the art of writing disk device drivers is to
arrange the retrieval/storage addressing so that the
physical specs of the device don't interfere with
efficiency. This is where the seek times, revolution
rates and controller specs are used.
/BAH |
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Guest
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| Posted:
Mon Oct 31, 2005 1:24 am Post subject:
Re: Random Access Tape? |
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jmfbahciv@aol.com wrote:
| Quote: | Part of the art of writing disk device drivers is to
arrange the retrieval/storage addressing so that the
physical specs of the device don't interfere with
efficiency. This is where the seek times, revolution
rates and controller specs are used.
|
recnet posting in bit.listserv.bim-main on redoing the implementation
for multiple transfers per revolution
http://www.garlic.com/~lynn/2005s.html#22 MVCIN instruction
one was redoing the cp67 implemention supporting the 2301 "drum"
increasing the peak 4k page transfers from about 80/sec to 300/sec. the
other was the handling of the 3330 disk when trying to do sequential
transfers of 4k pages that might reside on different track ... but at
the same arm/cyl. position.
i also did some further stuff when doing the page mapped filesystem
support ... playing games with re-ordering requests for optimal
revolution and arm motion ... even when the requests were originally
presented in some totally different ordering. recent post in the same
referenced thread
http://www.garlic.com/~lynn/2005s.html#25 MVCIN instruction
note that the cms filesystem i/o was something left over from real i/o
paradigm ... that then had to be simulated in a virtual memory
environment. the "simulation" process would execute the disk i/o in the
order pass to it. the changes for page mapping allowed the i/o ordering
to be re-organized for optimal execution. some more on that subject
http://www.garlic.com/~lynn/2005s.html#28 MVCIN instruction |
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Peter Flass
Guest
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| Posted:
Mon Oct 31, 2005 6:03 am Post subject:
Re: Random Access Tape? |
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Michael J Kingston wrote:
| Quote: | In message <OTS8f.28348$Bv6.232@twister.nyroc.rr.com>, Peter Flass
Peter_Flass@Yahoo.com> writes
sqrfolkdnc wrote:
Of course the LGP-30 had main memory AND the accumulator on DRUM, and
addresses were NOT sequentially assigned. A good programmer located
his working storage so that it could be most often fetched in the same
revolution as the instruction, then fetch the next instruction wihout
wasting a whole revolution of the drum. There was a chart showing what
memory locations could be accessed from an instruction at any location,
without losing a revolution. It had 4096 words of memory, IIRC.
Sounds like the IBM 650.
My recollection is that IBM 650 drum had sequential addresses. Ferranti
Pegasus drum had addresses in a sequence arranged so as to optimise
continuous reading of 8-word blocks.
|
I think that's correct, but what I was alluding to was that each 650
instruction included the drum (disk?) address of the "next" instruction.
I guess it must have been quite an art to organize your program so that
the system was ready for the next instruction just as that location
passed under the R/W head so as not to lose a rotation. |
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Jeff Jonas
Guest
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| Posted:
Sun Nov 06, 2005 4:18 am Post subject:
Re: Random Access Tape? |
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| Quote: | Of course the LGP-30 had main memory AND the accumulator on DRUM, and
addresses were NOT sequentially assigned. A good programmer located
his working storage so that it could be most often fetched in the same
revolution as the instruction, then fetch the next instruction wihout
wasting a whole revolution of the drum. There was a chart showing what
memory locations could be accessed from an instruction at any location,
without losing a revolution. It had 4096 words of memory, IIRC.
|
I played with a LGP-21 for a while: same architecture, etc.
but transistorized and a fixed head single surface disk PLATTER instead of a drum.
The 4 "registers" were continuously read/written on the outermost tracks
by heads that were a word apart.
Yes, the disk was used as a delay line,
kinda like the way 3-head tapes read/erase/record.
That led me to appreciate what my dad said about drum programming
and the marvels of "SOAP": the self-optimizing assembler.
And a deeper appreciation of "The Story Of Mel". |
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