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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