Leccion

00 word sector size in bytes
02 byte sectors per cluster (allocation unit size)
03 word number of reserved sectors
05 byte number of FATs on disk
06 word number of root directory entries (directory size)
08 word number of total sectors; if partition > 32Mb then set
to zero and dword at 15h contains the actual count
0A byte media descriptor byte (see ~MEDIADESCRIPTOR~)
0B word sectors per ~FAT~

% Additional/different fields for DOS 3.0+
0D word sectors per track
0F word number of heads
11 word number of hidden sectors
15 11bytes reserved

% Additional/different fields for DOS 4.0+
15 dword number of total sectors if offset 8 is zero
19 6bytes reserved
1F word number of cylinders
21 byte device type
22 word deviceattributes

- located in the boot sector at offset 0Bh

- see ~BOOT SECTOR~
:Batch Control Block:BCB
^BCB - Batch Control Block (undocumented)

^DOS 2.x thru DOS 3.2 BCB Format

% Offset Size Description

00 byte unknown
01 word if non-zero; segment of control block for active FOR
03 byte type of batch command
0 - normal batch command
1 - FOR-loopactive
04 dword offset of next command to execute in batch file
07 word offset of variable %0 (batch file name)
09 9 words offset of %N batch file parameters, 0FFFFh indicates
parameter is null
1C nbytes null terminated path and filename of the current
batch file immediately followed by command line
parameters. Each parameter %0-%9 plus a CR is
appended and resultingstring is null terminated.

^DOS 3.3 BCB Format

% Offset Size Description

00 byte unknown
01 byte global echo switch, if exec'd by batch CALL-command
1 - turn ECHO ON on return to calling batch file
0 - turn ECHO OFF on return to calling batch file
02 word batch file BCD segment if executed via CALL
if zero; batch file was called from command lineif non-zero; batch file executed via CALL-command
04 word if non-zero; segment of control block for active FOR
06 byte type of batch command
0 - normal batch command
1 - FOR-loop active
07 dword offset of next command to execute in batch file
0B word offset of variable %0 (batch file name)
0D 9words offset of %N batch file parameters, 0FFFFh indicatesparameter is null
1F nbytes null terminated path and filename of the current
batch file immediately followed by command line
parameters. Each parameter %0-%9 plus a CR is
appended and resulting string is null terminated.

- BCB length is variable and depends on the size and count of the
parameters and fully qualified batch file name
- the MCB for a BCB has a process Idof the transient portion of
the latest COMMAND.COM
- offsets displayed are relative to the BCB segment
- SHIFT command changes the offsets of the parameters in the table
at the offsets 0B0h thru 1Ch
- BCB of DOS 3.3 is the same as earlier versions except 3 bytes
were added after offset 0
- to find a BCB, locate the first block in the MCB chain belonging
to COMMAND.COM (thesecond allocated block always belongs to
COMMAND.COM). Then scan the ~MCB~ chain for a 64 byte block with
the same owner ID as COMMAND.COM). This will be the BCB.
:BIOS Data Area:BDA:BIOS memory:memory map
^BDA - BIOS Data Area - PC Memory Map

% Address Size Description

00:00 256dwords Interrupt vector table
30:00 256bytes Stack area used during post and bootstrap
40:00word COM1 port address
40:02 word COM2 port address
40:04 word COM3 port address
40:06 word COM4 port address
40:08 word LPT1 port address
40:0A word LPT2 port address
40:0C word LPT3 port address
40:0E word LPT4 port address (except PS/2)
Extended BIOS Data Area segment (PS/2, see ~EBDA~)
40:10 2 bytes Equipment list flags (see ~INT 11~)

³7³6³5³4³3³2³1³0³ 40:10 (value...