Disk Density Differences
========================
File: DOCS.Disks.DiskDen - Update: 1.20
Author: J.G.Harston - Date: 31-08-1998
There is a bit of confusion around about 3.5" disks concerning how
much they can store, and what one should be used. This short article aims
to clarify some of these areas.
3.5" disks with only a single read/write hole are double density
(DD) disks, and are designed to store up to 1Mb of data. Disks with two
holes are high density, have HD marked on them, and are designed to store
1.5Mb to 2.5Mb of data. Most drives detect the hole and refuse to read HD
disks if the computer accesses them for DD. They can be fooled by covering
the hole with a bit of sticky paper, but this is not a good idea. Because
they store twice as much data the magnetic granularity is twice as high,
and the recording flux permeability is half. Now again, in English:
HD disks need to store twice as much data in a single track as DD
disks, so each bit takes half the space on the disk surface. So, the
grains of magnetic particles must be smaller to fit more particles per
track. Also, when writing, HD disks are 'recorded' onto at a lower
'volume'. This may seem contradictory, but the way it works is as
follows: The magnetic grains are smaller, so if the same recording
strength is used, the recording will 'stray' into neighbouring areas. So,
when writing to HD disks, the recording flux is lower to prevent it
straying into adjacent areas.
You can try to store HD data onto a DD disk, but it is unlikely to
succeed as the grains are not large enough to store the amount of data.
Errors will occur very soon after formatting, if not at formatting itself.
What about storing DD data on a HD disk? The grains are small
enough for the data, in fact smaller than they need to be. However, DD
recording strength is twice HD recording strength. Recording DD data on a
HD disk will force the data into the grains, like using a pencil with too
much pressure. It will be difficult for the recording head to erase old
data to write new data over it. The data on the disk will deteriorate
every time something is written to it.
Also, as HD disks are more expensive than DD disks, it is bad value
for money to use 2.5Mb HD disks to store only 400k of data. A similar
argument applies to 5.25" disks. BBC series computers should only use DD or
DS/DD 5.25" disks.
Therefore,
3.5" DD 3.5" HD
400k HADFS
720k DOS 1.44M DOS
640k ADFS
800k ADFS 1.6M ADFS
Disk Formats
============
Type Tracks Sect/ Bytes/ Size Bytes/ Recording
Track Sector Track Method
In order of size:
ADFS S 40*1 16 256 160k 4k MFM DD
DFS/HADFS 40tk 40*2 10 256 200k 2.5k FM SD
ADFS M 80*1 16 256 320k 4k MFM DD
DOS 360k 40*2 9 512 360k 4.5k MFM DD
DFS/HADFS 80tk 80*2 10 256 400k 2.5k FM SD
ADFS L 80*2 16 256 640k 4k MFM DD
HADFS B 80*2 16 256 640k 4k MFM DD
DOS 720k 80*2 9 512 720k 4.5k MFM DD
ADFS D,E 80*2 5 1024 800k 5k MFM DD
DOS 1200k 80*2 15 512 1200k 7.5k MFM HD
DOS 1440k 80*2 18 512 1440k 9k MFM HD
ADFS F 80*2 10 1024 1600k 10k MFM HD
In order of track data density:
DFS/HADFS 40tk 40*2 10 256 200k 2.5k FM SD
DFS/HADFS 80tk 80*2 10 256 400k 2.5k FM SD
ADFS S 40*1 16 256 160k 4k MFM DD
ADFS M 80*1 16 256 320k 4k MFM DD
ADFS L 80*2 16 256 640k 4k MFM DD
HADFS B 80*2 16 256 640k 4k MFM DD
DOS 360k 40*2 9 512 360k 4.5k MFM DD
DOS 720k 80*2 9 512 720k 4.5k MFM DD
ADFS D,E 80*2 5 1024 800k 5k MFM DD
DOS 1200k 80*2 15 512 1200k 7.5k MFM HD
DOS 1440k 80*2 18 512 1440k 9k MFM HD
ADFS F 80*2 10 1024 1600k 10k MFM HD
Single density: using standard drive, recording using FM
Double density: using standard drive, recording using MFM
High density: using high density drive, recording using MFM
Single/Double density uses the same drive. The recording method is
different. High density uses a different drive.
Disk drives
5.25" 40*2 SD/DD 360k drive.
5.25" 80*2 SD/DD not used by PC systems.
3.5" 80*2 SD/DD 720k drive.
5.25" 40*2 HD does not exist.
5.25" 80*2 HD 1.2M drive. Can do double step/DD to read DOS 360k.
3.5" 80*2 HD 1.44M drive.
A high density drive is capable of recording up to 10k per track. Almost
all HD drives default to double density unless the computer explicitly
tells it otherwise. Some drives have problems with DFS 1.20 as it times
out too quickly; DFS 1.21 fixes this problem. With the correct HD drive
and driving firmware, they will correctly access both HD and DD disks.
A double density drive is capable of recording up to 5k per track with MFM
and up to about 3k per track with FM. There are very few drives that only
do DD as almost all HD drives default to DD anyway.
