🚀 go-pugleaf

> Date: Tuesday, 1-Jul-86 9:07 MET
> From: Luc Dekens <FGCBA10@BLEKUL11.BITNET>
> Subject: diskorganization on ST
> To: INFO-ATARI16@SU-SCORE.ARPA
> 
>     Hi, is there anybody out there how could give me some info
> concerning the disk-structure. I know where to find the bootblock
> and how it is organized, but I'm in the dark what concerns directory-
> organization, allocation tables and so on.
> 
>        Luc Dekens                    FGCBA10@BLEKUL11.BITNET
>        Chemistry Department
>        University of LEUVEN
>        Belgium

~~~~~~~~~~~~~~~~~\ lineater, \~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

ask and thou shalt receive...


                              ST Disk Directory

    For those of you that have gone from the 8-bit ATARI computers to
the 16-bit ATARI ST's, there have been many surprises for you in the 
past months.  Among them is probably " What do I do with all of this 
extra memory.", or maybe " The graphics on this system sure are
something." and one of my favorites, " That disk holds a lot of data."
  
    All of these statements are true for obvious reasons.  For the
longest time there was only 48K of ram available in the ATARI Systems,
so jumping to 512K is a fantastic leap.  The same holds true for the
graphics capability of the new ATARI, by going to a monitor and
improving the resolution, another big leap has been made.  But, one of 
the most important improvements to me has been the disk storage.  After
using an ATARI 810 disk drive for several years, going to 360K per 
disk is great.
  
    Now, since I have that off my chest, I would like to go a little
deeper and try to explain some of the differences in the way the
ST disks (single sided) are handled.
 
    The ST disk is divided into 80 tracks (0 thru 79), 9 sectors per
track (1 thru 9), and is written in QUAD density (512 bytes per sector).
When a disk is formatted, a Boot sector (track 0, sector 1), the File 
Allocation Tables (it starts at track 0, sector 2) and the Directory 
(it starts at track 1, sector 3) are established using all 18 sectors
in the first two tracks.  The remaining sectors (tracks 2 thru 79, 
sectors 1 thru 9) are initialized in pairs or by cluster (two sectors 
= one cluster).  As a cluster is initialized the File Allocation Table 
(F.A.T.) is updated to indicate the status of the cluster; this 
continues until all 351 data clusters have been completed.  If a cluster
cannot be formatted or initialized, the corresponding entry in F.A.T.
is marked not-available and will remain that way until the disk is 
re-formatted or thrown-away.  If a cluster is marked as bad, the F.A.T. 
entry will contain a number between $FF0 and $FF7.  That range of 
numbers simply means the cluster is un-usable and will never used to 
store data.

    The ST disk uses track 2, sector 1 thru track 79, sector 9 to store
any files you write to the disk.  All filenames are listed on the disk 
directory in the order they are entered.  The directory is seven (7)
sectors long and has room for 112 entries, with each entry being 32 
bytes in length.  Each entry contain the Filename and Extension, the 
file Attributes, the Time the last change was made to the file, the 
Date the last change was made to the file, the number of the first 
cluster in the file, and the length (in bytes) of the file.  In addition, 
there are 10 bytes that have been reserved for future use (see fig.1).  


      fig.1                ST Directory Fields

         1) Filename                  8 bytes       bytes 0 thru 7
         2) Filename Ext.             3 bytes       bytes 8 thru 10
         3) Attributes                1 byte        byte  11
         4) RESERVED                 10 bytes       bytes 12 thru 21
         5) Time of Last Change       2 bytes       bytes 22 and 23
         6) Date of Last Change       2 bytes       bytes 24 and 25
         7) First Cluster Number      2 bytes       bytes 26 and 27
         8) File Size (in bytes)      4 bytes       bytes 28 thru 31


    The Filename and Extension are the first two fields in each entry
of the directory.  They take up the first 11 bytes of an entry and 
follow the same format as the ATARI 800 or the IBM PC, with one small
exception.  If the first character of an entry is $E5, the file has 
been deleted and is no longer available for your use.  If no changes
have been made to the disk since the file was deleted, it may be possible
to recover it by using one of the many sector editors available.     

    The Attributes field is one byte long and contains a number that 
indicates any special or unique characteristics about this entry.  
There are only five bits of the entry used on the floppies at this time, 
and they are listed in Fig. 2.

    fig.2            Attributes

         Bit 0  =  Read Only  (not set if the file is Read-Write)
         Bit 1  =  Hidden
         Bit 2  =  System
         Bit 3  =  Volume Label  (Name assigned to the disk)
         Bit 4  =  Sub-Directory  (Folder Name)
         Bit 5  =  Archive  (This will be used on the Hard Disks)
         Bit 6 & 7  are not used at this time

    The next field is marked RESERVED and is 10 bytes long.  This field 
will be filled with 00's on all disk entries and no plans for its use 
are known at this time.

    The Time of Last Change field is 2 bytes long and is updated each 
time you write to a file.  This field contains the HOURS, MINUTES, and 
SECONDS(/2) of the last change to the file.  This field is in the 
low-byte, high-byte format and uses all 16 bits.  Starting with the 
highest bit, it uses 5 bits for the Hour, 6 bits for the Minutes and the
last 5 bits for the Seconds.  (The value in the seconds portion of the
field must be multiplied by 2 to get the correct seconds count.)

    The Date of Last Change field is handled almost the same as the 
previous field.  This field is changed along with the Time of Last 
Change, and is also in the low-byte, high-byte format.  Starting with 
the highest bit, use 7 bits for the Year, 4 bits for the Month, and
the last 5 bits for the Day.  Don't be to surprised when the year equals
a 5 (as most files will) because the year stored has 1980 subtracted 
from it.

     The First Cluster field contains the number of the first cluster
used for the file.  The cluster information is stored in high-byte, 
low-byte order and should never go above $15F (351), since there are
only 351 clusters.

     The File Size field is a value equal to the number of bytes used
in the file.  This number divided by 1024 (1k) will tell you how many
clusters are being used for the file (cluster * 2 = sectors).

     
     The File Allocation Table (F.A.T.) starts on track 0, sector 2,
and may be the most important table on the disk.  F.A.T. is used to 
keep track of the sector linkage for all data sectors.  It is also
used for files listed in Folders (sub-directories, see Attributes).
The F.A.T. is five sectors long and is valuable enough to be have a
duplicate table on the same disk.  The location of the second F.A.T.
is currently track 0, sector 7, but that is subject to change at any
time.

     The best way to think of the F.A.T. is like a puzzle.  Each 
entry of the F.A.T. is 12 bits long (not bytes, bits), and the values
range from $000 to $FFF (see fig.3).  Byte 0 of F.A.T. tells what 
density the disk is formatted in ($F7 is normal for byte $00), and 
bytes 1 and 2 will $FF as they are not used.  The rest of the table
is used as follows.  (I'll be using the F.A.T. from the disk I have 
been working on for my examples.  see fig.3)  Before I begin I should 
point out that 12 bits per entry means that 2 entries equal 3 hex
characters (bytes).  We will start with bytes 03, 04, and 05 for our
first 2 entries (see fig.3).  Byte 03 will be bits 0 thru 7 of the
first 12 bit entry, and bits 0 to 3 of byte 04 will be bits 8 thru 11
of the first entry.  As you can see in the first example, the number 
is $FFF.  In F.A.T., if an entry contains $FF8 thru $FFF it means the 
cluster is the last cluster of the file (EOF).  Also, if an entry
contains $FF0 thru $FF7 it means the cluster is not usable for some
reason.  Now, if you will return to byte 04, you will remember that 
we have only used 4 bits (0 to 3).  Bits 4 thru 7 of byte 04 will 
become bits 0 thru 3 of the second entry and byte 05 will be used as 
bits 4 thru 11.  This entry indicates that cluster 4 will be the next
cluster for this file and it continues in that manner for the rest of 
the table.

    Now you know what cluster to look at but maybe you want to know 
which track and sector that is.  The quickest way I've come with is
one of the following formulas,

  a)  To convert from Cluster to Track and Sector;
    
     ( ( ( cluster #  +  9) * 2 ) - 1 ) / 9  = track
     ( ( ( cluster #  + 9 ) * 2 ) - ( track # * 9 )  = sector

  b)  To convert from Track and Sector to Cluster;

     ( ( ( track #  * 9) + sector # ) - 17 ) / 2  = cluster #


     A brief explanation.

     Data sectors start at Track 2, Sector 1.  That means 18 sectors
(or 9 clusters) have to be accounted for in the formula.  Also there
are 2 sectors per cluster so you must multiply by 2 at some point.
Here is all there is to it, what cluster is Track 22, Sector 3 ??
     I'll put the numbers into the formula.

     ( ( ( 22 * 9) + 3 ) - 17) / 2  = cluster #
           ( ( 198 + 3) - 17 ) / 2  = cluster #
                      ( 201 - 17 )  = cluster #
                           184 / 2  = cluster #
                                92  = cluster #

     If there had been a remainder, it would have been the second 
sector in the cluster.  Thats all there is to it, but just so you can
practice a little, I included a chart with some of the Clusters 
marked with the Track and Sector (see fig.4).







  fig.3  (in hex)           --  F.A.T. --


               0  1  2  3  4  5  6  7  8  9  A  B  C  D  E  F    

         0    F7 FF FF FF 4F 00 05 60 00 07 80 00 09 A0 00 0B       
        10    C0 00 0D E0 00 0D 00 01 11 F0 FF 13 40 01 15 60      
        20    01 17 F0 FF 19 A0 01 1B C0 01 1D E0 01 1F 00 02
        30    21 20 02 23 40 02 25 60 02 27 80 02 29 A0 02 2B






     fig.4             Track & Sectors  =  CLUSTERS


         BOOT
         SECT  <           FAT #1          > <            FAT
        +-----+-----+-----+-----+-----+-----+-----+-----+-----+
        | 0.1 | 0.2 | 0.3 | 0.4 | 0.5 | 0.6 | 0.7 | 0.8 | 0.9 |
        +-----+-----+-----+-----+-----+-----+-----+-----+-----+

         #2        > <      Disk Directory   (7 Sectors)     >
        +-----+-----+-----+-----+-----+-----+-----+-----+-----+
        | 1.1 | 1.2 | 1.3 | 1.4 | 1.5 | 1.6 | 1.7 | 1.8 | 1.9 |
        +-----+-----+-----+-----+-----+-----+-----+-----+-----+

         <  CLU #2 > < CLU #3  > < CLU #4  > < CLU #5  > < CLU 
        +-----+-----+-----+-----+-----+-----+-----+-----+-----+
        | 2.1 | 2.2 | 2.3 | 2.4 | 2.5 | 2.6 | 2.7 | 2.8 | 2.9 |
        +-----+-----+-----+-----+-----+-----+-----+-----+-----+

          #6 > < CLU #7  > < CLU #8  > < CLU #9  > < CLU #10 >
        +-----+-----+-----+-----+-----+-----+-----+-----+-----+
        | 3.1 | 3.2 | 3.3 | 3.4 | 3.5 | 3.6 | 3.7 | 3.8 | 3.9 |
        +-----+-----+-----+-----+-----+-----+-----+-----+-----+

                                  ||
                                 \||/

         < CLU #334> < CLU #335> < CLU #336> < CLU #337> < CLU 
        +-----+-----+-----+-----+-----+-----+-----+-----+-----+
        |76.1 |76.2 |76.3 |76.4 |76.5 |76.6 |76.7 |76.8 |76.9 |
        +-----+-----+-----+-----+-----+-----+-----+-----+-----+

         #338> < CLU #339> < CLU #340> < CLU #341> < CLU #342>
        +-----+-----+-----+-----+-----+-----+-----+-----+-----+
        |77.1 |77.2 |77.3 |77.4 |77.5 |77.6 |77.7 |77.8 |77.9 |
        +-----+-----+-----+-----+-----+-----+-----+-----+-----+

         < CLU #343> < CLU #344> < CLU #345> < CLU #346> < CLU 
        +-----+-----+-----+-----+-----+-----+-----+-----+-----+
        |78.1 |78.2 |78.3 |78.4 |78.5 |78.6 |78.7 |78.8 |78.9 |
        +-----+-----+-----+-----+-----+-----+-----+-----+-----+

         #347> < CLU #348> < CLU #349> < CLU #350> < CLU #351>
        +-----+-----+-----+-----+-----+-----+-----+-----+-----+
        |79.1 |79.2 |79.3 |79.4 |79.5 |79.6 |79.7 |79.8 |79.9 |
        +-----+-----+-----+-----+-----+-----+-----+-----+-----+





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		that i wasn't straight"

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