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
More information about the cctech
mailing list