APPENDIX ELEVEN



4266           10AA           ....
     MSB of the VTOC sector, normally sector 360. The VTOC is a bit-
     map of the disk contents; after the initial status bytes, each of
     the following bits represents one sector on the disk in sequential
     order, There are 720 sectors, but sector 0 cannot be accessed
     by the OS. Sectors 1-4 are reserved as "boot" sectors on a DOS
     disk, sectors 360-368 are reserved for the VTOC and directory
     leaving 707 free for files, You can move the VTOC the same
     way you move the directory.

     If you change the directory location (make sure there's nothing
     in the new directory location that you don't mind erasing first).
     go into the VTOC and deallocate the original directory sectors
     (write a one into the bits) and write a zero into the bits
     representing the new locationÄthis prevents them from being
     overwritten, You can also lock out sectors by deallocating them
     in the VTOC.

     Volume Table of Contents

     Byte    Use
     0       DOS code (0 = DOS 2.0)
     1-2     Total number of sectors (707; $2C3)
     3-4     Number of currently unused sectors
     5       Reserved (unused at present)
     6-9     Unused
     10-99   Bitmap: one bit for each sector (0=in use--
             locked; 1=unused--free). The leftmost bit of
             byte 10 ($0A) is sector 0 (see above), the next bit
             to the right is sector 1, and so on, until the
             rightmost bit of byte 99 ($63), which is sector 719
             ($2CF).
     100-127 Unused

     There are only 707 sectors counted in bytes 1 and 2 (not 720).
     since the first 4 are "boot" sectors, then the VTOC and directory
     take another 9, for a total of 13.

     A typical DOS 2.0 VTOC with DOS.SYS and DUP.SYS, but nothing
     else except the boot, VTOC. and directory sectors in use; it looks
     like this:

     Byte
       0  02 C3 02 50 02 00 00 00
       8  00 00 00 00 00 00 00 00
      16  00 00 00 00 00 00 00 00
      24  01 FF FF FF FF FF FF FF
      32  FF FF FF FF FF FF FF FF
      40  FF FF FF FF FF FF FF FF