DEC RX50 mysterious 81st cylinder

Bjoren Davis bdweb at mindspring.com
Sat Apr 23 23:11:28 CDT 2022 

Hello Retroovers,

Here's something interesting that Oleksii in Ukraine discovered.

While playing with a Fluxengine, he found that some RX50s have data, 
stored in FM format, in a single sector the 81st track of the diskette.

For fun I went through my collection of original DEC distribution 
diskettes for the DEC Professional, read the single sector and compared 
them.

Here is what I found:

Common data (locations with variations marked with __):
00:   46 4d 54 20 4d 46 4d 20 4e 4f 2f 46 43 20 31 2a   |FMT MFM NO/FC 1*|
10:   31 30 2a 35 31 32 20 38 30 54 20 2d 44 45 43 20   |10*512 80T -DEC |
20:   52 58 35 30 00 20 20 20 32 32 32 36 34 2d 32 33   |RX50. 22264-23|
30:   20 20 __ __ 84 __ __ __ 20 46 01 f5 30 30 __ 30   | __.___ F..00_0|
40:   __ __ __ 20 20 20 20 20 20 20 20 20 20 20 20 20 |___             |
50:   20 20 20 20 82 f1 c8 54 9a fd ce 57 9b 7d 0e b7   | ...T...W.}..|
60:   6b 05 20 20 20 20 20 20 20 20 20 20 20 20 20 20 |k.              |
70:   20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 |                |

Variations:
                              offset
                32 33    35 36 37    3e    40 41 42
file           -----------------------------------
BL-AI19C-BH    04 02    10 34 19    30    31 35 30
BL-CF74C-BH    03 26    00 01 36    30    30 39 30
BL-CF74C-BH    03 26    00 03 45    30    30 39 30
BL-CL73B-BH    11 10    00 53 44    30    30 37 31
BL-HC42A-BH    04 09    19 59 54    30    31 33 30
BL-HD04A-BH    01 21    17 26 09    30    31 34 30
BL-HD04B-BH    01 07    11 34 01    30    30 f3 31
BL-HD04B-BH    06 18    02 24 01    30    30 31 30
BL-HD05A-BH    01 22    02 33 40    30    32 30 31
BL-HD05B-BH    02 11    19 59 51    30    30 30 31
BL-HD05B-BH    02 11    20 00 55    30    30 30 31
BL-HD06A-BH    01 03    13 31 10    30    30 34 30
BL-HD06B-BH    06 18    05 08 58    30    30 35 31
BL-HD06B-BH    06 18    05 07 54    30    30 35 31
BL-HD07B-BH    06 18    01 42 23    30    31 38 30
BL-HD07B-BH    06 18    01 43 28    30    31 38 30
BL-HD07B-BH    01 07    12 22 22    30    30 f3 31
BL-HD08B-BH    01 07    13 51 38    30    30 f3 30
BL-HD08B-BH    01 19    12 45 32    30    31 38 30
BL-HD08B-BH    01 07    13 52 42    30    30 f3 30
BL-HD09B-BH    01 07    13 05 38    30    30 f3 31
BL-HD09B-BH    01 17    05 34 48    30    31 39 30
BL-HD09B-BH    01 07    13 04 33    30    30 f3 31
BL-HD10B-BH    06 18    09 43 45    30    30 35 30
BL-HD10B-BH    06 18    05 45 16    30    31 39 30
BL-HD10B-BH    01 20    19 22 06    30    30 30 31
BL-HD11B-BH    01 17    06 05 36    30    30 31 31
BL-HD11B-BH    01 17    06 06 41    30    30 31 31
BL-JB90B-BH    06 17    14 45 00    30    31 31 30
BL-JB90B-BH    06 18    12 29 42    30    32 30 30
BL-JB90B-BH    06 17    14 46 03    30    31 31 30
BL-JB91B-BH    06 18    12 43 59    30    30 39 30
BL-JB91B-BH    01 13    12 01 21    30    30 30 30
BL-JB92B-BH    01 15    16 03 31    30    30 36 30
BL-KS73A-BH    06 04    09 36 40    30    30 30 30
BL-N596F-BH    01 03    10 09 00    30    31 31 30
BL-N596G-BH    07 03    18 02 08    34    39 36 31
BL-N605G-BH    02 13    01 12 58    30    32 30 31
BL-N631H-BH    12 16    15 41 36    30    31 33 30
BL-N631I-BH    01 19    10 28 32    30    30 35 30
BL-N633G-BH    12 11    03 48 40    30    30 32 30
BL-N633H-BH    03 05    14 21 50    30    30 33 30
BL-N634G-BH    07 03    17 00 26    30    30 31 30
BL-N638F-BH    12 05    19 13 38    30    31 32 31
BL-N639H-BH    03 26    02 32 50    30    30 39 30
BL-N640G-BH    03 26    00 59 57    30    30 31 30
BL-N640G-BH    03 26    04 41 36    30    30 34 30
BL-V444B-BH    12 13    02 51 00    30    31 34 30
BL-Y472B-BH    03 20    08 21 38    30    31 32 31
BL-Y982D-BH    03 05    20 02 57    30    31 39 30
BL-Y982D-BH    07 03    15 48 31    30    30 37 31
BL-Z934D-BH    04 10    04 16 13    30    30 33 30
-----------
         MIN    01 02    00 00 00    30    30 30 30
         MAX    12 26    20 59 59    34    39 f3 31
       NVALS    08 12    14 22 21    02    04 0b 02
          OR    17 3f    3f 7f 7f    34    3b ff 31
         AND    00 00    00 00 00    30    30 30 30


Key
MIN   is the minimum value seen
MAX   is the maximum value seen
NVALS is the count of unique values seen
OR    values have a 1 for every bit that is ever 1
AND   values have a 1 for every bit that is always 1

----

Some interesting things to note:

  * only some diskettes have 81st cylinder data.  They tend to be the
    later releases (1985 and later).  (My table simply omits those
    diskettes I have without 81st cylinder data).
  * different copies of the same diskette (e.g., BL-HD11B-BH) have
    slightly different values, so it doesn't appear that these value
    encode some kind of master ID.
  * the values at offsets 0x32, 0x33, 0x35, 0x36, 0x37 appear to be
    BCD-encoded month, day-of-month, hour (0..23), minute, and second. 
    Logically, this would mean that the value at 0x34 should be the
    BCD-encoded year, but it's always 0x84, and most of these diskettes
    were released well after 1984.
  * the values at 0x30, 0x40, 0x41, 0x42 are ASCII decimal digits to
    complete the string "00_0___", except the value at 0x41 is sometimes
    0xf3.
  * the textual data at the beginning seem obvious enough: "FMT MFM"
    means MFM-encoded data; "1*10*512" is 1 head, 10 sectors/track, 512
    bytes/sector; "80T" means 80 tracks; "DEC RX50" means what it says. 
    But what does "NO/FC" mean?  And what about "22264-23"?

Does anyone know anything about this?  Is it also present on 
VAX/PDP-11/Rainbow RX50 diskettes?  I'd be very curious to know.

Oh, and before anyone asks: yes, I'm planning on putting up images of 
these diskettes onto archive.org.  An interesting corollary question: 
should those images somehow include this information, and, if so, how 
(because as far as I know the DEC Professional floppy controller is 
incapable of reading these data)?

Thanks.

--Bjoren

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