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Virtual Worlds #01
31 декабря 1999 |
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Technical assistance - Dos Review: material on the format of the disk operating system IS-DOS.
DOS REVIEW # 00
(C) Nuts
So said a series of articles,
written Nuts'om in which
describes the different Farmat
DOS (disk operating system). Here all is
you wanted to know about disk
formats, but did not know who
ask.
Now NUTS incur any
nonsense about it as well and much
another.
Long or a short is, but because of
that is very far away if, as
does not take a computer and a
him every time a new disk
format. MICROSOFT can only
praise for what she
unify disk format
uymy for such computers. She
also does not stint on the information, and if zhmotnichala, it
quickly split the brave hackers. And do not whine that there,
in the "Microsoft country", the poor
nick die without life-giving
information flow. Well, if anyone should all chew and
put your mouth, please.
Much more complicated to deal with
other foreign, as well, and
domestic systems. If
say about the SPECA already spoken a lot, and described a lot,
and He left a bunch of foreign
periphery, as software nemeryannom
under different operating systems, and that much
only now comes to remote areas, well and some of which
there are only names: any
there OPUS, D80, etc., etc.
What can we say about the other machines on which someone had
never heard, and someone they are lying bestolku. And then, you
see, and the them and the software been written by WHO and
small truck, and the people of the heap
Profile Views and soldered. In short, there chtoto ALIVE
FOREVER. And you can long breeding pleasant tarybary that
RULES, and that - MUST DIE. All this because,
that netushki reliable spravochki that there is and how it is
faring.
We start all the same with MS-DOS ...
Attempts to talk about this OS
were, and software to work with this
thing written a lot, since more
with god know how long. But
say, I personally do not pinned
none of these tricks. All
it is somehow specialized for
certain area, often
flexibility in working with different
varieties of discs, as a rule, only MS -> TR & Design
at 1985 There is, of course,
and a very pleasant exception, and
much is being done, but the fact remains.
In general, I personally decided to delve into the
literature, scout situation and provide all
polzitelnoe Fiction. Decided it
was still a long long time, and the case
was almost done TIME
KEEPER'om, so, too, because people do not
sit still, they write a word, copying.
To begin with, that all the disk parameters are stored in the
0th sector 0th track. Only thing that the amount of information
built up with the versions
MS-DOS, and only the fourth version
You can learn all about the drive. But
because the formatter FLOPPY
FORMAT Ivan Roshchina formats
CDs and records at this, so
called BOOT-sector is the same,
that itself MS-DOS, then forget about
this problem and get back to the contents of this sector are
help table:
OFFSET LENGTH OF APPOINTMENT
0 3 team moving to bootloader code.
March 8 program name-formatter.
0b 2 Length in bytes of the sector.
0d a number of sectors per cluster.
0e 2 Number of reserved sectors (from
since the beginning of the disk)
October 1 Number of FAT.
February 11 Maximum number of files in
root directory.
February 13 Total number of sectors on the disk.
Jan. 15-byte descriptor type of disc.
February 16 number of sectors in one FAT.
February 18 number of sectors per track.
> DOS 3.0
1a 2 Number of heads on the disk.
1c 2 Number of hidden sectors.
April 20 If the drive> 32MB, then its size
sectors are already here.
> DOS 4.0
24 January Number disc (HDD only) - number in the
BIOS.
January 25 Reserve.
January 26 signature expansion of the sector, ie only
wrote that DOS 4.0 and above, contains this # 29
April 27 Serial number - written during formatting
disc contains a unique number and date
November 26 Name the disk.
August 26 Contains "FAT12" or "FAT16".
What do these numbers mean ...
In the beginning I will say that I will
studiously silent about the work
with the hard disk. This is not because
I, for whatever reasons, I want to hide. On the contrary,
The situation with the hard drive is very
instructive to install it
under other systems, but much
more difficult to
describe it in the review of the diskette
formats - there because you and the problem of increasing the
volume, and the main boot partition with the possibility of
placing several operating systems, and a bunch of formats when
working with packed Disk ...
For a start - a little theory.
Operating systems for the most part do not operate with sectors
and tracks, and with shifts in absolute sectors. T. is, the
concept of "track" that is practically not used, but there are
a lot of sectors, ranging from zero and to a certain end, maybe
one hundred thousandth. If, say, 10 sectors on the disk on the
same logical path, starting from zero, then zero
logical path is to be the sector with the 0-th to ninth on the
First logical path from the 10 th
on the 19th and so on.
Moreover, to limit
the number of bits that specify the number
absolute sector, the entire disc
divided into blocks of several sectors. Moreover, the zero
unit can be located and not in the beginning of the disk and
start with kakogolibo absolute sector. The first sector are
precisely loading and information.
In MS-DOS, such blocks are called clusters. Number of sectors
per one cluster, you can learn from the boot sector. Number of
logical sector, which is zero cluster, we have
calculated based on data from
boot sector, but this
later.
Such a breakdown is tricky
to always be aware of
free disk space, and various pieces of one file to record in
different areas of the disk, wherever possible, and collect
it, even if it is not
in consecutive sectors.
Therefore, it may not particularly care about cleaning the disc
from the remote files.
To know the location of
each piece file, the table is FAT. Each element, starting with
the first, contains a reference on the element of this table,
where extends the file number
cluster which corresponds to
that element table, which contains a link to this file. Then
Thus, if we have learned that the beginning
file is, say, the fifth cluster, we load
it and look at the fifth element
Table FAT. And there, it means that
Suppose the number 25. Well, then we
quietly read the 25th cluster and
Consider the 25th entry in the table. A
He points to the number of the next
cluster (he number of the next element in the table), or
that this cluster is the last for this file, and more
did not read right. It can
also indicate that this sector is free or marked
as defective.
The only problem is that the
disks, the size of one element
is 12 bits (half byte)
and half of each 2-nd byte
refer to the previous one
byte, and another - to the next.
To determine the content
of the table must be multiplied
This cluster number by 1.5,
we shift to a couple of bytes
in the table, which contains the next cluster. If
previous cluster had the odd
number (determined by zero
bat in the room of the cluster), we have to use 12 bits
this pair, or - 12 seniors.
If the resulting value is
0, then this cluster is free. If # FF8 - # FFF, then this is
the last cluster of the file. # FF0 - # FF7 - the reserved
sector, including # FF7 - the failed cluster.
Only the 0th element of this table is no exception. He
contains a copy of the descriptor byte
this disc. For example, for
disks, which can be read
on the Spectrum:
Byte: Type of drive:
# F9 3'5 720 kb 2SD 9_SEC 80_TRKS
# Fa 5'25 320 kb 1SD 8_SEC 80_TRKS
# Fb 5'25 640 kb 2SD 8_SEC 80_TRKS
# Fc 5'25 180 kb 1SD 9_SEC 40_TRKS
# Fd 5'25 360 kb 2SD 9_SEC 40_TRKS
# Fe 5'25 160 kb 1SD 8_SEC 40_TRKS
# Ff 5'25 320 kb 2SD 8_SEC 40_TRKS
But these bytes are many
other discs, including those not
with a standard breakdown.
In fact, in these baytahopisatelyah type used
only the low 3 bits, and senior
bits are always zero.
In this byte:
0th bit: 1 - two
0 - unilateral
First bit: 1 - 8 of sects. at Dor.
0 - not 8
Second bit: 1 - constant,
0 - Removable Disk
To determine the start of FAT, in
boot sector indicated
number of reserved and
boot sectors. Ibid
You can find number of tables
FAT - for reliability make
multiple copies. It is easy to be
calculate the total number of employees at the beginning of the
disk sectors and determine the one on which
begins 0th cluster.
The first two clusters always takes root, and
maximum number of files in
It is determined by the size of the clusters. But this number
is contained and in the boot sector, and to determine the size
of the catalog in sectors have to divide the
it to 16.
The root directory is always present on the disk. It
contains information about files and
first level of subdirectories.
Recent represent a
ordinary files, but with zero
length, and their size is determined
on the table FAT, and they can be
segmented, ie, the scattered
across the disk. At the beginning of each
subdirectory is the descriptors 2
Files. " and "..". The first repeats the description of this
directory catalog at a lower level and
duplicated information on this directory. The second contains a
description of the earliest katalogapraroditelya. Both of them
are used for easy navigation through a disc. If
catalog-grandparent is in the zero cluster, then it
root directory. In this directory of links available, but if
boot disk, there are descriptors of system files.
Descriptor of any file or
directory occupies 32 bytes of data:
Displacement: FUNCTION:
0-7 file or directory name, big letters, the remainder
fill the gap. If the 0th byte is equal to the file #
E5
removed if the # 05 first letter of the code file
indeed # E5. If 0 - a place of never
used - the end of the catalog.
8.10 Expansion.
11 File attributes. Bits: 0 - Read-only 1 -
hidden file 2 - file system, and 3 - the name of the
disk
first 11-bytes, only the root directory,
primary sector and the length is not 4 - subdirectory
does not use a length field, 5 - archive file
archived it for safety or not.
12-21 are reserved.
22-23 Time of creation or last correction file
contains both 16-bit word: bits 11/15 - Hours
Bits 10-5 - minutes, bits 4-0 seconds / 2. 24-25 Date
file creation or last correction
contains both 16-bit word: bits 9.15 - Year
since 1980, and ending with 2099 bits (!!!); 5.8 -
month, bits 4-0 - day.
26-27 Number of the first cluster file, starting with the
youngest
bytes. The first two clusters takes root
directory and all files beginning with the second
cluster.
28-31 File size in bytes. The first pair of bytes contains
lower level values, both pairs of bytes starting with
Low Byte. Total length may reach up to 4
GB, well used to the format 830KB 20-bit
- Max.
That's all we need to work
with the disk. To move from room
cluster to the absolute sector
must be added to the cluster number, multiplied by two, shift
to no cluster in the absolute sector. Then obtained
number needed to divide the number of sectors per track. In
result is logical
track, where the cluster
and balance - a sector. I note
In most I've seen
literature, it is proposed that
root directory (ie, the very first few clusters)
not seem to be and not clusters, and
introduce the concept of early data
- Following the cluster catalog is kind of like a zero,
well and, therefore, all rates
clusters are of him at
assistance priplyusoaniya-minusovaniya
directory size. In short, only
extra computation. In my
view, if the directory is in a zero cluster, and it should
considered as the origin of data, not
for something special.
It remains only to say that
sometimes is just not enough to overtake the file on a Spectrum
if it is in the format
HOBETA (extension begins with
"$"), SCL-format (the program
AMD) or the file could contain the image of Spectrum Drive - ".
TRD" and ". FDI". To do this, be aware of Format
these files.
Now we'll discuss the IS-DOS.
As the program
FLOPPY FORMAT Ivan Roshchina, this
DOS supports eight breakdowns
disk, which may very much
differ from each other,
because the sector size can be
as 256 and 1024 bytes (the running drive.) Number
sectors at the same time will be 16 or 5
respectively.
Disk, there may be as
unilateral or bilateral.
Attention should be given the order
sectors per track. Since
disk can be avtozapuskaemym
then, though them and is on 5
sectors per track, the last
of them has the number 9.
Now consider the structure
disc. Her description is
disk assembler IS-DOS. But
It is designed for programmers
working directly in
This DOS. Here I will explain the basic facts of this
description.
"Inside" IS-DOS absolutely EVERYTHING
storage device
represented by a set of blocks
256 bytes of each numbered from 0
up to a certain maximum. The position of any file and generally
any desired location on the disk given offset
beginning of the disc - the 0-th block. AND
measured this shift in these
same blocks of 256 bytes. That is not
so important a sector size - for
determine the track and sector
You can use the following formulas:
DOR. = INT (SMESCH. / 5120)
(Or 4096)
CERT .= INT ((see-DOR .* 5120) / 1024)
(4096 256)
A remnant of the difference between
shift to the sector and offset
to give us the desired block offset within the 1024-byte sector.
0th block - this is the 0th 0th sector of the track.
It contains:
OFFSET LENGTH OF APPOINTMENT
0 2 Reserve
August 2 The device name (disk)
March 10 Symptom IS-DOS: "DSK" As it can be determined
membership drive in the system.
May 13 Reserve
Feb. 18 Volume in drive units
February 20 number of 0-th block of the master directory. For
CDs
80TR/DS usually the third, ie same 0th sector
for 1024 byte sectors. Or third -
for 256 byte.
January 22 The number of cylinders on the device
- Is for the HDD.
January 23 Disc Type - bits (0 / 1):
0th - 40/80 tracks
1st - 1 / 2 part.
January 24 Sector Size:
1/2/4: 256/512/1024 bytes
January 25 The number of sectors per track.
January 26 Reserve
January 27 Checksum file ????_ dos.sys This is something
like MAGICK'a - a copy of a computer's memory
system. Loaded and unloaded it special
programs, and its parameters are stored in a
special place.
?? - Any four characters.
February 28 Reserve
Feb. 30 date (what??)
32 32 descriptor ????_ dos.sys
See above.
64 16 Table number of sectors.
First block contains a bit-map
device: each bit corresponds to its own block on the
device - 1 bit / block
0-free
1-busy
Such a table - bit
Map drive - often used
in different operating systems.
Now let's talk about the files and
directories.
We already know where
main directory. It is useful to remember that the files in the
IS-DOS are not only segmented as
in MS-DOS, but continuous, as
in TR-DOS. Ie file can be
scattered pieces-on segments
the entire disk for a long time to boot and
restore it after
entry in the directory of it will be
spoiled, extremely difficult. But worries about cleaning the
disc from deleted files is almost gone.
Also, the file may be continuous, loaded quickly, but
problems with command MOVE, or
more precisely, SQUEEZE (so it
it is called in to HOBET'e
HRC), all spektrumschikam (if
they do not work under the emulator). However, according to
some reports, the problem is solvable (See below), but - so far
unknown. Most (all?)
converters IBM <> ZX work on
the principle of a conditional sequence of sectors the file,
and if you write a converter IS <> TR, then You can make this
file and non-segmented, but for the sake of completeness is
necessary to tell how the system IS-DOS collects segments of
its files disc.
So, in a directory handle
this file contains a link not
on the initial block file as
per se, but a reference to a particular
block - Sector File Allocation Table, or, as it is called
in the company description, block descriptor file segments. 0th
to byte contains the number of file segments - how many pieces
he defeated. The remaining 255 bytes contains 85 three-byte
records each of which describes a
file segment. The first two bytes
each contain the number zero
block of the segment, well, the third - the number of blocks in
the segment.
And to read the file you want
must first define the beginning
this descriptor, then consider
him. Then looping through the
number of segments. On the first
two bytes of each three
determined by the beginning of the substantive
segment and the ordinary way
read sequence
blocks containing sectors of the
segment. If necessary, take care to exclude
unwanted residues of the first and
latter sectors. For the last block of this can be done
simple manipulation with the address
loading the next segment. A
Here's the first tricky. Have to
do move the team LDIR
or read the sector in the intermediate buffer. But in any case,
size of 1024 bytes will have to consider.
To save the files have
also disturb the bitmap
disc. We must look for unoccupied
blocks - their corresponding bits
will be cleared. Need to install it, then fill out the
appropriate record in the block descriptor, or add blocks to an
old record. Make these operations better RAM.
Catalogues, oddly enough,
represent a nearly normal files, but they have two descriptors:
the external, in katalogeroditele, and internal, it itself. At
the main directory is only the inner handle -
its 0th file. It is in the inner descriptor contains
information about the number of files and level of nesting. In
addition, record of the file / directory contains the
determining a label showing the
ownership of the file to a particular group (see below). It
should also be aware of the constraints: number of files in a
directory - to 128, the nesting level - to
6. Catalogue can also be segmented or non-segmented.
Now more about the structure
file descriptor. Note that in
describing the structure of records
Internal handle directory
I have not made everything clear.
OFFSET LENGTH DESCRIPTION
0 8 Filename
August 3 File Type
January 11 status register file
bit (0 / 1):
0 - deleted / a
2 - is protected against reading (1)
3 - write-protected (1)
4 - Visible / hidden file
5 - file / directory (root) file
6 - segmented / non-segmented
7 - protected from removal (1)
February 12 address the default boot
March 14 Length
February 17 number Block descriptor segment, or 0-th block
file - if the file is continuous
January 19 "Special"
June 20th Reserve
February 26 checksum of file
February 28th Time
February 30 Date
Date and time are encoded in
MS-DOS. Checksum, possibly, too.
That is what is known about the inner handle (use the 23
bytes):
OFFSET LENGTH DESCRIPTION
0 8 The name of the current directory
March 8 Its type (gaps)
January 11 CSR (???) directory
February 12 CBNN (like the initial block) directory -
progenitor
February 14 Directory size (bytes)
January 16 nesting level directory.
February 17 number block descriptor segment. (It's interesting.
The catalog can be continuous, then there
perhaps number of first block. But then
definition of CSR should be the status bits)
February 19 number of 0-th block directory (This is then what?)
January 21 Total number of files
(Including the directory itself and remote)
January 22 The number of files
(Pure free catalog, well, remote)
Well, with the IS-DOS understood.
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