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'[EE]: Do floppy drives have any error checking/cor'
2002\11\13@182704 by Steve Ruse

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I'm wondering about the data integrity of sensitive data files I'm moving
using floppies. If there is an error due to a bad sector, physical problem
with the disk, or any other factors, will the error/lost data be detected?
If so, how does the drive accomplish this detection?

Thanks!

Steve Ruse

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2002\11\13@202056 by cdb

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A floopy drive doesn't, it will be done by (in DOS) IOS.sys from the
operating system or other similar O/S software.

The floopy only knows if a disk is inserted and where its head is.

colin
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2002\11\13@203035 by ards, Justin P

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I know there is a verify /v option for copy in dos.  IE "copy /v c:test.dat
a:".

It detect a problem by reading back the new file and comparing with the
original.

I think the c64 drives did a comparison 5 times so they were extra slow
until someone came up with a way to turn verify off.

Perhaps

{Original Message removed}

2002\11\14@043309 by Alan B. Pearce

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>I know there is a verify /v option for copy in
>dos.  IE "copy /v c:test.dat a:".

Which is fine when writing.

But I believe the answer to the query is that each sector on the floppy disk
has a 16 bit CRC field for doing error detection when reading the sector.
This allows read errors to be detected.

It is vaguely possible to have an error such that the CRC field does not
detect the error, but on a sector size used on a floppy this is highly
unlikely.

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2002\11\14@052857 by Quentin

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> The floopy only knows if a disk is inserted and where its head is.
Sometimes I also wonder. Like today. Anybody who can-CAN?
:)
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2002\11\14@054314 by cdb

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Some idiot insisted in placing the 'O' on the keyboard next to the
'P', I saw the error, but was feeling slightly flippant at that point
in time so left it in.

Now who is going to buy me a Dvorack Keyboard no dyslexia of the
keyboard there!

Colin
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2002\11\14@063347 by Russell McMahon

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> Some idiot insisted in placing the 'O' on the keyboard next to the
> 'P', I saw the error, but was feeling slightly flippant at that point
> in time so left it in.

> Now who is going to buy me a Dvorack Keyboard no dyslexia of the
> keyboard there!

Oh, you mean like:

   Qw EW RGW sTKWZU id Viefm dyauarBXW UA EYRUKW, ....

E&OE :-)

Which my son came up with some years ago and I scratched my head over for
some while, but the redundancy of the English language was his downfall (and
it's really obvious after the event) :-)

       EN

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2002\11\14@073947 by Olin Lathrop

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> I'm wondering about the data integrity of sensitive data files I'm
moving
> using floppies.

Floppies aren't a good choice for long term storage.  CR-R is much better.

> If there is an error due to a bad sector, physical problem
> with the disk, or any other factors, will the error/lost data be
detected?

Probably.  There are checksums.  Don't expect to recover the data though.

> If so, how does the drive accomplish this detection?

It doesn't.  Floppy drives are very dumb.  All the smarts are in the
controller.


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Embed Inc, embedded system specialists in Littleton Massachusetts
(978) 742-9014, http://www.embedinc.com

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2002\11\14@183435 by Steve Russell

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Each sector (typically 512 bytes) of a floppy disk has a CRC (Cyclic Redundancy Check) code stored with it. CRC is a fancy checksum - an algorithm calculates a 16-bit (IIRC) number that is based on the contents of the 512 bytes of data and writes it to the disk at the same time as the data is written. When the drive reads data from a sector, the *drive controller* (no CPU involved) calculates the CRC from the data read and compares it with the CRC that is stored on the disk. I believe the algorithm used is designed to be such that typical errors in the data (or the CRC itself) will mean that the two CRCs will be different.

An algorithm that ensures a statistically fair spread of calculated CRCs will read a faulty disk and calculate a CRC equal to the stored value only 1 in 2^16 times - on which occasion, the error will go undetected. The use a of an algorithm that anticipates the typical type of error will do better than that.

Steve.


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2002\11\15@133746 by Barry Gershenfeld

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There were some rumors about that the verify didn't really verify,
rather it checked for write errors, or it verified against a
cache rather than the disk, or some such.

What I do when I'm concerned about integrity is to use zip files,
then after it's all done, use the -t (test) option in pkzip to
force it to read the file back and verify it (uses a 32 bit crc).

The nice thing about a floppy is you can easily tell if the disk
is being read (rather than a cache buffer).

Also, you can use SCANDISK to do a surface scan of a floppy.  If
you see any errors, don't trust that disk.

Barry


At 05:18 PM 11/13/02 -0600, you wrote:
>I'm wondering about the data integrity of sensitive data files I'm moving
>using floppies. If there is an error due to a bad sector, physical problem
>with the disk, or any other factors, will the error/lost data be detected?
>If so, how does the drive accomplish this detection?
>
>Thanks!
>
>Steve Ruse

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2002\11\16@055033 by Steve Russell

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Each sector (typically 512 bytes) of a floppy disk has a CRC (Cyclic
Redundancy Check) code stored with it. CRC is a fancy checksum - an
algorithm calculates a 16-bit (IIRC) number that is based on the contents of
the 512 bytes of data and writes it to the disk at the same time as the data
is written. When the drive reads data from a sector, the *drive controller*
(no CPU involved) calculates the CRC from the data read and compares it with
the CRC that is stored on the disk. I believe the algorithm used is designed
to be such that typical errors in the data (or the CRC itself) will mean
that the two CRCs will be different.

An algorithm that ensures a statistically fair spread of calculated CRCs
will read a faulty disk and calculate a CRC equal to the stored value only 1
in 2^16 times - on which occasion, the error will go undetected. The use a
of an algorithm that anticipates the typical type of error will do better
than that.

Steve.


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2002\11\16@130531 by Wagner Lipnharski

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Steve Russell wrote:
{Quote hidden}

... more info:


A typical media failure will not create a single bit error, the media spot
deffect sometimes hit several tracks and many bytes, this will create a
multiplication of the error correction effectivenes, but ends up always in
1/65536*8 error bit undetection. No matter how many bits are involved in
the failure spot, the 1/65536*8 flaw still always the same.

The CRC-16 will always ends up in zero when processing its own final
numbers.
For example, using a program I develop years ago to calculate CRC in DOS
command line, http://www.ustr.net/files/crc16.exe you can see this example
when calculating CRC16 for hex bytes 01-02-03-04-05;

C:\> crc16 01 02 03 04 05 0e
+--------- Wagner Software - 1987 ---------+
+--------3270-CRC=x16+x14+x1--[A001]-------+

01 02 03 04 05
Final CRC Transmited: 0EBB


Then, when inserting the CRC calculated (0EBB) in the calculation, the
result goes to zero.

C:\> crc16 01 02 03 04 05 0e bb
+--------- Wagner Software - 1987 ---------+
+--------3270-CRC=x16+x14+x1--[A001]-------+

01 02 03 04 05 0E BB
Final CRC Transmited: 0000

This is how the majority of CRC checkings are made, the data is read
including the CRC check bytes, if the actual generated CRC goes to zero,
then both crc's were equal and data is valid.

The CRC-X25 (CCITT), always end up with a fixed value of "FFFF" when
including an exclusive or FF from the recorded CRC into the calculation,
examples:

C:\> crcx25 01 02 03
+--------- Wagner Software - 1997 ---------+
+      CCITT-CRC=x16+x12+x5+x1--[8408]     +
+------INVERTS (NOT) ALL BITS BEFORE TX----+

01 02 03
Final CRC Transmited: 3B9D


C:\> crcx25 01 02 03 C4 62
+--------- Wagner Software - 1997 ---------+
+      CCITT-CRC=x16+x12+x5+x1--[8408]     +
+------INVERTS (NOT) ALL BITS BEFORE TX----+

01 02 03 C4 62
Final CRC Transmited: FFFF


C4 = XOR FF of 3B
62 = XOR FF of 9D

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