
4

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:::                                 :::
:::   DISK TIPS & TRICKS TUTORIAL   :::
:::             PART 4              :::
:::   REPAIRING VTOC, UNDELETING    :::
:::         PROGRAMS, ETC.          :::
:::                                 :::
:::         BY CANDY APPLE          :::
:::                                 :::
::*:::::::::::::::::::::::::::::::::*::
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   *:::::::::::::::::::::::::::::::*
   ::       DISTRIBUTED BY        ::
   :: CRYSTAL CASTLE 313/856-3804 ::
   *:::::::::::::::::::::::::::::::*

THUS FAR, YOU HAVE HOPEFULLY GAINED SOM
E INSIGHT INTO DOS AND
ITS INNER WORKINGS.  THE LAST TUTORIAL 
COVERED SOME BASIC
KNOWLEDGE ABOUT THE VTOC, ITS BIT MAP, 
AND HOW TO GAIN UP TO
53 USABLE SECTORS ON YOUR DISK.

ARMED WITH THIS KNOWLEDGE, YOU CAN SEE 
HOW EASY IT WOULD BE TO
RESCUE CLOBBERED FILES.  BUT LETS TRY A
N EXAMPLE TO ENFORCE
THIS KNOWLEDGE.


REPAIRING A CLOBBERED VTOC
--------- - --------- ----

LET'S START BY BLOWING AWAY A VTOC.  GE
T OUT YOUR PRACTICE
COPY OF THE SYSTEM MASTER DISK.  WITH Y
OUR DISK EDITOR PROGRAM,
READ IN ITS VTOC (TRK$11, SCT$00, IN CA
SE YOU FORGOT).  
STARTING WITH BYTE $00 (WHICH IS THE FI
RST BYTE AND IS PROBABLY
AN "04"), WRITE AN "0F" TO EACH BYTE IN
 THIS SECTOR.  THIS
SECTOR IS NOW FILLED WITH "0F"S'.  WHEN
 YOU'RE FINISHED, WRITE
THIS SECTOR BACK TO THE DISK.  NOW IF Y
OU TRY TO BOOT OR LOAD
A FILE, YOU'LL GET A "FILE NOT FOUND" E
RROR BECAUSE DOS THINKS
THE CATALOG BEGINS WITH SECTOR $0F ON T
RACK $0F.

TO FIX IT, READ IN THE VTOC FROM A GOOD
 DISK.  STARTING WITH
BYTE $38, FILL THE REST OF THE SECTOR W
ITH "00"S'.  THIS ZERO'S
OUT THE ENTIRE BIT MAP WHICH INSURES US
 THAT NO FILES ON THE
DISK ARE OVERWRITTEN DURING FUTURE USE.
  NOW WRITE THIS SECTOR
ONTO TRK$11, SCT$00 OF YOUR CLOBBERED D
ISK.  IT'S NOW USABLE
AGAIN EXCEPT THAT NO MORE DATA CAN BE S
AVED ON IT.  THE 
SOLUTION WOULD BE TO FID (NOT COPYA, AS
 IT TRANSFERS THE 
ZEROED-OUT VTOC) YOUR FILES TO A NEWLY 
INITIALIZED DISK AND
THEN REINITIALIZE THE CLOBBERED DISK.  
SINCE THIS IS JUST A
DEMONSTRATION, DON'T BOTHER DOING THIS 
TO YOUR PRACTICE COPY.


UNDELETING A FILE
---------- - ----

IT'S INTERESTING TO KNOW THAT WHEN A FI
LE IS "DELETED", IT IS
NOT REMOVED FROM THE DISK.  THE ONLY TH
ING THAT HAPPENS IS THAT
A FEW BYTES ON THE CATALOG TRACK GET CH
ANGED AND THE BIT MAPS
ARE ADJUSTED.  ACTUALLY THE FILE'S NAME
 THATS IN THE DIRECTORY
AND THE PROGRAM CODE REMAINS INTACT UNL
ESS YOU SAVED OTHER
FILES TO THE DISK SINCE YOU DELETED THE
 FILE IN QUESTION.  IN
THIS CASE, DOS MAY HAVE OVERWRITTEN IT.

AGAIN USING YOUR PRACTICE COPY OF THE S
YSTEM MASTER AND YOUR
DISK EDITOR, READ IN TRK$11, SCT$0F.  S
TARTING FROM BYTE $08
ON DOWN TO BYTE $28, YOU SHOULD SEE THE
 FOLLOWING:
              ! 
BYTE    FIRST V BYTE        ASCII CHAR
$08: 00 00 00 13 0F 82 C8 C5:@@@SOBHE:
$10: CC CC CF A0 A0 A0 A0 A0:LLO     :
 :    (2 MORE ROWS OF "A0"S' HERE)
 :
$28: A0 A0 A0 A0 06 00 14 0F:    F@TO:
               ^
          LAST ! BYTE

THE CHARACTERS ON THE RIGHT ARE THE ASC
II TRANSLATION.  YOUR
DISK EDITOR PROGRAM MAY GIVE YOU THE AB
ILITY TO CONVERT THE
HEX CODE TO ASCII CODE OR EVEN HAVE IT 
OFF TO THE SIDE LIKE
THE ABOVE.  USE IT AS NECESSARY.

IN THE EXAMPLE ABOVE STARTING IN ROW $0
8, YOU'LL SEE THAT THE
"130F" WILL INDICATE WHERE THAT HELLO'S
 TRACK AND SECTOR LIST
(MORE ON THAT LATER) CAN BE FOUND WHICH
 IS ON TRK$13, SCT$0F.
ACTUALLY THE "13" IS THE FIRST BYTE OF 
THE FILE NAME SPACE. 
THE "82" TELLS US THAT THE HELLO PROGRA
M IS A LOCKED APPLESOFT
FILE.  THE NEXT 5 SYMBOLS ARE THE SCREE
N CODES (OR ASCII WITH
THE HIGH BIT SET) FOR THE LETTERS H-E-L
-L-O.  THE 25 TRAILING
"A0"S' ARE SCREEN CODES FOR BLANKS THAT
 WILL FILL THE UNUSED
PORTION OF DOS'S ALLOWABLE 30 CHARACTER
S FOR A FILE NAME. DOWN
IN LINE $28, THE FOURTH "A0" IS THE LAS
T BYTE OF THE FILE NAME
SPACE.  THE "06" INDICATES THE LENGTH O
F THE HELLO PROGRAM, IN
THIS CASE 6 SECTORS.

NOTE: YOUR SYSTEM MASTER COPY MAY BE DI
FFERENT THEN THE ABOVE 
DUE TO THE WAY FID ALLOCATED SPACE ON T
HE DISK.  THIS IS NO
PROBLEM, BECAUSE THE POSITION OF THE TH
INGS WE'VE TALKED ABOUT
WILL STILL BE THE SAME ON THE CATALOG T
RACK.  LATER ON, WHEN
WE START TRACKING DOWN FILES FROM ADDRE
SSES IN VARIOUS PLACES
ON YOUR DISK, JUST SUBSTITUTE THE INFOR
MATION FROM YOUR DISK
IN PLACE OF THE TUTORIAL'S INSTRUCTIONS
.

NOW BREAK OUT OF THE EDITOR PROGRAM SO 
YOU CAN "UNLOCK" AND
"DELETE" THE HELLO PROGRAM ON YOUR PRAC
TICE DISKETTE.  GO
BACK TO YOUR EDITOR PROGRAM AND REREAD 
TRK$11, SCT$0F OF THE
PRACTICE DISKETTE.  YOU'LL NOTICE THAT 
THE DISPLAY OF THE HELLO
CATALOG ENTRY IS STILL THERE BUT THERE 
IS A CHANGE IN SEVERAL
OF THE BYTES.  YOU SHOULD NOW SEE THE F
OLLOWING:
              !
BYTE    FIRST V BYTE
$08: 00 00 00 FF 0F 02 C8 C5:@@@SOBHE:
$10: 0C CC CF A0 A0 A0 A0 A0:LLO     :
 :
$28: A0 A0 A0 13 02 00 14 0F:   SB@TO:
              ^
        LAST  !  BYTE

YOU SHOULD NOTICE THAT THE FIRST BYTE W
HICH WAS A "13" HAS NOW
JUMPED DOWN AND REPLACED THE LAST BYTE 
(A0) AND AN "FF" APPEARS
WHERE THE FIRST BYTE WAS.  THIS IS ALL 
THAT HAPPENS WHEN A FILE
IS DELETED.

TO UNDELETE THE HELLO FILE TYPE AN "A0"
 TO REPLACE THE "13"
(LAST BYTE) AND PUT THE "13" WHERE THE 
FIRST BYTE IS (THE "FF")
LINES $08 AND $28 SHOULD LOOK LIKE THIS
:

$08: 00 00 00 13 0F 02 C8 C5
 :
$28: A0 A0 A0 A0 06 00 14 0F

NOW WRITE THIS SECTOR BACK TO THE PRACT
ICE DISKETTE.  HELLO
SHOULD NOW LOAD, RUN AND APPEAR IN THE 
CATALOG JUST LIKE 
BEFORE.

HOWEVER THERE IS ONE MORE DETAIL THAT N
EEDS TO BE TENDED TO.
READ IN THE VTOC WITH THE DISK EDITOR. 
 START LOOKING FOR THE
LINE BEGINNING WITH BYTE $80.  IF YOU Z
EROED OUT THE VTOC
DURING THE FIRST PART OF THIS TUTORIAL,
 YOU'LL PROBABLY SEE AN
"FC" IN THE LINE INSTEAD OF "00".  DOS 
HAS ALTERED THE TRACK
$13 BIT MAP TO REFLECT THE FACT THAT SE
CTORS $0F AND $0E ARE
NOW AVAILABLE.

ONE WAY TO FIX YOUR VTOC WOULD BE TO WR
ITE A "00" OVER THE 
"FC".  HOWEVER, CHANGING BIT MAPS ISN'T
 ADVISABLE WITH  MOST
FILES SINCE YOU PROBABLY DON'T KNOW WHI
CH SECTORS THEY OCCUPIED
BEFORE THEY WERE DELETED.  THE BEST THI
NG TO DO IS TO LOAD THE
FILE INTO MEMORY AND THEN RESAVE IT BAC
K TO YOUR DISK AGAIN.
DOS WILL ADJUST THE BIT MAP FOR YOU.

THIS TUTORIAL SHOULD KEEP YOU BUSY REPA
IRING ALL THOSE 
CLOBBERED VTOC'S.  NEXT TIME WE'LL TALK
 ABOUT THE FILE TYPE
FLAGS AND SHOW YOU HOW TO ELIMINATE HID
DEN CONTROL CHARACTERS
IN THE CATALOG.

