DEC RX50 mysterious 81st cylinder

Warner Losh imp at bsdimp.com
Sat Apr 23 23:18:59 CDT 2022


On Sat, Apr 23, 2022, 10:11 PM Bjoren Davis via cctalk <
cctalk at classiccmp.org> wrote:

> 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"?
>

This doesn't match the Rainbow's hard disk partition tables, fwiw.

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

I'll have to check next time I image my Rainbow disks.. I don't recall
seeing this in previous runs though...

Warner

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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