Distribution floppies (Was: Microsoft OSs (was: Install Floppies)

Fred Cisin cisin at xenosoft.com
Sat Jul 24 17:27:10 CDT 2021

> On Sat, Jul 24, 2021, 10:41 AM Grant Taylor via cctalk <
> cctalk at classiccmp.org> wrote:
>> Maybe someone who is more versed in the possible disk sizes and more
>> accurate (non-rounded) count.  I'm extremely foggy when it comes to
>> 5-1/4 inch disk capacities.  720 kB and 1.44 MB disks I can deal with.
>> But the minutia between 320 kB and 360 kB, combined with rounded
>> measurements, things get foggy for me.

OK, I am going to take a chance that you, or others here, want to 
know more about it.

The sizes that would be used were:
DMF  1.64M
XDF  1.8M

Other PC formats:

"160K"  5.25":
August 1981, the 5150 ("PC") was released with 5.25" "industry 
standard" Tandon TM100-1 drives.  IBM was not ready to use double sided.
DOS 1.00
300 RPM, 48 Tracks Per inch, 250,000 bits per second
1 side,  40 tracks,  8 sectors per track, 512 bytes per sector
163,840   160K
Please note: This is not "rounded", it is based on K meaning Kibibyte of 
1024 bytes.

DOS 1.10
Same, but double sided.
2 sides * 40 cylinders (trackse per side) * 8 Sectors per track * 512 BPS
327,680   320K

DOS 2.00
Change from 8 sectors pr track to 9 sectors per track.
Single sided version:
1 side * 40 tracks * 9 SPT * 512 BPS
184,320  180K

Same but double sided.
2 sides * 40 cylinders * 9 SPT * 512 BPS
368,640   360K

Some software distribution continued to be on 180K, to allow for customers 
who still used single sided drives.

"720K" 5.25":
IBM PC-JX (not sold in USA) had an 80 track per side 5.25" drive (what 
some CP/M companies called "Quad Density")

MS-DOS 2.11
OEM versions of MS-DOS were sometimes provided with 720K support, 
primarily companies that had implemented 3.5" drives, such as Gavilan, 
Zenith, Data General, etc.  They did not all use the same numbers of 
sectors in the file system overhead.  For example, Gavilan, when switching 
from 2.11J to 2.11K changed to be compatible with the later IBM 3.5" 

"1.2M"/"HD 5.25":
PC-DOS 3.00
IBM PC/AT (5170) supported a "High Density" 5.25"
96 tracks per inch, 360 RPM 500,000; bits per second.
2 sides * 80 cylinders * 15 SPT * 512 BPS
1,228,800   1,200K   1.17MiB    1.2 MARKETING-MEGABYTES (1,000 * 1,024)
The AT controller ALSO supported
250,000 bits per second, for 300 RPM 360K drives   and
300,000 bits per second, for 360K disks in the 360RPM 1.2M drive.
(Many companies then came out with two speed (300RPM and 360RPM) drives, 
300RPM in the 1.2M drive permitted 250,000 bits per second for 360K disks)
Weltek developed a kludge o 180RPM at 250,000 bits per second.

MSCDEX   (drive related, but not floppy)
The "network redirector" (undocumented for a while) permitted MSCDEX to 
fool DOS into thinking that a CD-ROM was NOT A DRIVE ON THE MACHINE, but 
was, instead a remote service on a network.   Attempting to CHKDSK a 
CD-ROM gave "Can not CHKDSK a network drive".

"720K 3.5":
PC-DOS 3.20
135 tracks per inch, 250,000 bits per second
2 sides * 80 cylinders * 9 SPT * 512 BPS
737,280   720K
(could also be added to PC (5150), XT (5160) machines)
DOS 3.20 added DRIVER.SYS that added 720K support to DOS, and DRIVPARM 
that changed the drive parameters without an additional driver.
I can not fully explain why DRIVPARM worked with PC-DOS and MS-DOS on 
aftermarket ATs, but FAILED on ATs with IBM AT BIOS, with either DOS.

PC-DOS 3.30
"High Density" 3.5"
135 tracks per inch, 500,000 bits per second
2 sides * 80 cylinders * 18 SPT * 512 BPS
1,474,560  1,440K  1.40625 MiB;     1.44 MARKETING-MEGABYTES
Note that the only way that you can get 1.44 is if you define "Megabyte" 
to be 1,024,000   1000 * 1024
(So, yes, "1.4M" is ROUNDED)

HDD >32M  (not floppy)
MS-DOS 3.31, PC-DOS 4.00
Note that IBM did not warn Norton in advance, so the Norton fUtilities had 
difficulties on PC-DOS 4.00, which was usually reported as "DOS 4 is not 
compatible and buggy".  Most of those "bugs" went away with the next 
release of Norton.

PC-DOS 4.00
1,000,000 bits per second (or could be kludged with 500,000 bits per 
second at 150RPM)
2 sides * 80 cylinders * 36 SPT * 512 BPS
2,949,120;  2880 K;  2.8125 MiB;  2.88 MARKETING MEGABYTES 
It never really caught on.

Microsoft Distribution Media Format
By cheating on the intersector-gaps, Microsoft managed to get 21 sectors 
per track, instead of 18 on "1.4M" disks!
2 sides * 80 cylinders * 21 SPT * 512
1,720,320;  1680 K;   1.640625MiB;   1.68 MARKETING MEGABYTES
Floptical, USB, and other drives that have their own firmware can't handle 
them.  Older versions of DOS need a device driver.

IBM chose a different approach.  Because the inter-sector gaps are 
relatively standard, you can fit more on a disk using larger sectors.
Eight 1024 byte sectors will fit on a HD 3.5" track.  There isn't enough 
room for a ninth one.
But, what about bigger than 1,024?
An 8K sector will fit on a track; certainly not enough room for a second 
one.  BUT, there is enough room left for a 2K! and then enough for a 1K, 
and then enough for a 512 byte sector.
Their driver software fooled DOS into thinking that it was seeing 
Twenty-three 512 byte sectors!
2 sides * 80 cylinders * 23 "virtual sectors" * 512 BPS
1,884,160;  1840K;  1.796875 MiB;  1.84 MARKETING-MEGABYTES

1) All sizes are the total size on the disk; I did not deal with 
[here] the space taken up by fle directory and other file system overhead.
I also didn't mention that some systems used 35 of the 40 tracks, or 75 or 
77 of the 80 tracks  (77 tracks was the standard for 8")

2) There were other choices besides what IBM picked for bytes per sector 
and sectors per track.  Some systems ued TEN 512 byte sectors per track. 
That required minimizing the gaps between sectors, which sometimes create 
read problems with NEC 765 type FDCs.  (temporarily running the drive 
slightly slower than the standard 300 RPM could sometimes get a successful 
read).  Therefore, 40 track double sided double density could be anywhere 
from 300K to 400K, depending on layout, NOT just 360K.
Different density calls for different sensitivity or coercivity of the 
media.  5.25" is available in 300 and 600 Oersted.  They do NOT 
interhange reliably!  The color of the cookie is different.  Some say to 
look for a hub ring.  EARLY 5.25" did not have a hub ring, and sometimes 
the drive mangled the hub, so they started reinfocing the hub.  By the 
time of 1.2M, the drives had been improved, and it was decided that hub 
rings weren't necessary any more.  hence, a disk with a hub ring is 
probably a late 360K; a disk without a hub ring is either EARLY 360K, aor 
3.5" is available with 600 or 720-750 Oersted.  The difference is close 
enough that one can often get away with the wrong one.  Both types have a 
write protect hole, but the 1.4M has an additional hole to indicate "high 
density". 2.8M also has a hole, but it is in a slightly different place.

3) MS-DOS/PC-DOS version numbers are ALWAYS a major version number, a 
period, then a TWO DIGIT minor version number.   DOS 2.10 sees itself as 
DOS 2, 0Ah.  YES, it is a TEN, not a ONE!  3.31 intenally is 3,1Fh.
There is no version 3.3; it is internally 3.THIRTY

4) There is some confusion regarding number of tracks on drives with 
multiple heads, does a 720K disk have 80 tracks or 160 tracks?  A 
work-around for that confusion is to use "CYLINDERS", or "TRACKS PER SIDE".

5) 67.5 tracks per inch 3.5 inch drives, with 40 tracks per side, EXISTED.
Epson Geneve PX-8 is the only system that I am aware of that used them.

6) "Kilobyte" is widely accepted as meaning 1,024 bytes, although the 'K' 
prefix means 1,000 in most other contexts.  "KibiByte" is one attempt to 
solve that.
BUT, 'M' means 1,000,000 in most contexts, and [rarely] 1,000.
MebiByte is the unambiguous term for 2 to the tenth power, 1,048,576
"MegaByte" has at least three definitions.
1,048,576   ("MebiByte"   2^20)
1,000,000   (which lets barketing claim more "megabytes")
1,024,000   ("MARKETING MEGABYTE" of 1,000 * 1,024 (10^3 * 2^10)

7) What does "HIGH density" mean?  Was that the mental state of those that 
named it?
In the beginning, there was FM "Frequency Modulated" - clock pulses with a 
'1' pulse or '0' no pulse in between.
When there were multiple '1' pulses with their clock pulses in a row, 
there were limits to how close they could be, and therefore how fast, or 
how many per inch.
But, SOME bit patterns didn't have any tight spaces.  By finding 32 or 64 
different patterns that COULD be squeezed, it was possible to choose 
ones to represent 5 or 6 bit patterns very tightly.  With a translation 
table between the 8 bit values and multiple "sueezable" patterns, one 
could get about 40% more data in a given space; hence GCR "Group Coded 
Record" coding, as used on Apple and Commodore, and a few others.
Going back to FM, . . .
Clock pulses weren't really needed between adjacent '1' pulses.  If you 
simply left out THOSE clock pulses, then the resulting pattern was less 
dense, and the stream could be squeezed (higher data transfer rate) to 
almost twice as much data per time or space.  Hence MFM "Modified 
Frequency Modulation".  The marketing people called that "DOUBLE DENSITY", 
But, it oversimplified enough that it 
caught on.  Now that "DOUBLE DENSITY" existed, it was inevitable and soon 
that FM became known as "SINGLE DENSITY", even though some engineers said 
that FM was "half" density (one data bit for every two pulse spaces), and 
that MFM ought be called "single density".
THUS, if you look at historical records, the term "DOUBLE DENSITY" came 
into being BEFORE the term "SINGLE DENSITY"!  (Cf. "World War II" V "World 
War I" (previously "The Great War"))

8) SOME/MANY companies referred to double density with twice as many 
tracks (such as the 720K format) as "QUAD DENSITY". 
Intertec/Superbrain) used "QUAD DENSITY" to refer to double density 40 
tracks per side with 2 sides.  THAT certainly created confusion.  But, 
there's MORE! When they came out with an 80 track double sided double 
density format, they called that "SUPER Density".  Thet abbreviated that 
"SD", which just about everybody else used as abbreviation for "SINGLE 

9) In addition to squeezing ten 512 byte sectors on a track, some people 
even tried using additional tracks.  Since there usually isn't a hard stop 
after track 39 (or 79), most drives could handle 41 or 42 (81 or 82) 
tracks.  John Henderson (March/9/1944 - January/31/2017) of Tall Tree 
Systems created a popular program called JFORMAT to squeeze ten sectors 
and more tracks.  He is also remembered for JRAM, some of the first 
expanded memory boards.  "Extended" used OS modes that understood more 
memory; "Expanded" worked in 1M/640K by moving a block of high memory at a 
time into a space in the 1M address range.  He was then shut out of the 
negotiations when Lotus/Intel/Microsoft developed their LIM/EMS standard.
I remember when he came out with the JRAM boards, he asked everybody at 
the West Coast Computer Faire, "DO you do any software that uses lots of 
memory?"  Not looking for a contractor for a specific project, he was 
trying to create a market for big memory boards.

Grumpy Ol' Fred     		cisin at xenosoft.com

More information about the cctech mailing list