It's a lot of fun to get inside a machine like the 520ST, and 
if you have the right tools you can exert a lot of control over 
what the computer and its peripherals are doing.  The disk drive 
is a perfect example.
     You already know that you can store all of your data on 
disks, but Dr. Flop E. Disk goes one step further and puts you in 
command of your disk drive.   This utility allows you to view and 
manipulate your data -- something all programmers find useful and 
often, indispensable.
     Written entirely in machine language, the program runs under 
TOS, and accesses GEMDOS and extended BIOS routines.  It may sound 
complicated, but it's very easy to use.
     In fact, it performs some tasks automatically.  For instance, 
it  'diagnoses' the disk's drivemap, which holds information about 
the disk.  Every formatted disk has a drivemap on Track 0 Sector 
1.  It specifies how many sectors there are on each track, and 
from this the Doctor knows that 0-9 sectors per track denotes a 
single-sided disk, and 0-18 sectors per track is a double sided 
disk.
     When you're ready to begin, just double click the left mouse 
on DRFLOPPY.PRG.  All commands are executed using the Function 
Keys, and are displayed  at the bottom of the screen.  The screen 
shows hexadecimal digits with their ASCII equivalents in column 
form at the right.  Track and sector indicators at the top of the 
screen display what area of disk you're working with.
     It's easiest to think of the screen as a window into Dr. Flop 
E. Disk's buffer.  The buffer holds 512 bytes of data from your 
disk, and the screen displays the first 256 bytes of it:  offsets 
$00 through $FF.  You can page (F8) to view the remaining 256 
bytes:  offsets $100 through $1FF.  Use F9 and F10 to to specify 
track and sector locations (shifted moves in reverse).  Once the 
indicators are set to the area of disk you want, F1 gets all 512 
bytes of data stored there.  You can now view and manipulate this 
location of the disk.
     You may wonder why you would want to manipulate any of your 
data, and this is where Dr. Flop E. Disk will prove to be a 
powerful tool.  You're no longer limited to copying an entire file 
or disk, but instead can copy only that portion of data you really 
need, regardless of what file it's in.  The Doctor also allows you 
to change the data on the disk, and can help in recovering 'lost' 
files.
     When doing copies and changes, remember to think buffer!  
Until the put command has been executed, no changes will be 
written to disk, but will remain in the buffer.  The put command 
was purposefully set up to require that you shift the F2 key 
because whatever is stored on the disk at the location shown by 
the track and sector indicators will be written over -- 
permanently.  It is important to verify that the indicators 
correctly show the area of disk you want, especially if you're 
doing a copy.
     Copying data requires you to move to a 'new' track and sector 
with 'old' information in the buffer.  Example:  You want to take 
the data from Track 2 Sector 9 and copy it to Track 30 Sector 1.  
Set your indicators to Track 2 Sector 9 and get (F1) the data 
stored there.  All 512 bytes will now be held in the buffer until 
you put your change.  Set the indicators to Track 30 Sector 1, and 
verify that they're correct.  Use shift F2 to put the copy.  The 
data is now in both  the old and the new locations on disk.  If 
you want to copy to a different disk, and have only one drive, 
just change disks in the drive after your initial get -- the rest  
of the procedure is the same.  If you have two disk drives, it's 
not even necessary to swap disks, simply change drives with F7.
     Another feature of Dr. Flop E. Disk you'll find very useful 
is the find command.  F5 will produce a prompt at the command line 
asking you to type in the string you want to find.  You can search 
for both character and hexadecimal data.  Character strings can be 
up to 70 characters in length, and allow both upper and lower case 
characters and spaces.  It is not necessary to enclose them in 
quotes.  Hex strings can be up to 68 digits in length and must be 
preceded by a capital X.  Valid hex strings must contain only the 
numbers 0-9 and the capital letters A-F, with no spaces allowed.  
Just type in the string you want to find and hit return, and Dr. 
Flop E. Disk will make quick work of it.
     The screen will display the track and sector where the string 
was found, and the cursor will be positioned at the first byte in 
the string.  If you want to search for more occurrences of the same 
string, just hit F6.  The search can be aborted at any time by 
pressing any key.  You can continue to execute the next command 
until the command line tell you that end of disk has been reached.
     Any time you want a hard copy of what you're working on, F7 
will produce a fast screen print of all 512 bytes in the buffer.  
This is an especially helpful alternative to the alt Help screen 
print, which only prints the portin of the buffer that is shown on 
the screen.  F7 will give you a complete listing in only a 
fraction of the time.  You may edit any of the information in the 
buffer by moving the cursor with the arrow keys to the area you 
wish to change, then typing in the new information.  To write the 
changes to the disk, press SHIFT F2.  Always remember to use a 
backup copy of the disk you want to work on.