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Lecture # 27
Extended Read
• Service used for extended read is int 13h/42h
On Entry
AH=42H
DL=drive
DS:SI= far address of Disk address packet
On Exit
If CF=
AH=0= Success
If CF=
AH= Error code
Interrupt 13H/42H can be used to read a LBA addressed block whose LBA address is
placed in the Disk Address packet as described in the slide above.
Extended Write
• Service used for extended write is int 13h/43h
On Entry
AH=43H
AL=0,1 write with verify off
2 write with verify on
DL=drive
DS:SI= far address of Disk address packet
On Exit
If CF=
AH=0= Success
If CF=
AH= Error code
Similarly int 13H / 43H can be used to write onto to LBA addressed block as described in
the slide above.
Reading a LBA block
**#include <dos.h> #include <bios.h> struct DAP { unsigned char size; unsigned char reserved1; unsigned char blocks; unsigned char reserved2; unsigned char far buffer; unsigned long int lbalod; unsigned long int lbahid; } dap; char st[80]; unsigned char buf[512]; FILE fptr;
*void main (void) { puts ("enter the lba low double word: "); gets (st); dap.lbalod=atol(st); puts ("enter the lba high double word: "); gets (st); dap.lbahid=atol(st); dap.size=16; dap.reserved1=0; dap.blocks=1; dap.reserved2=0; dap.buffer = (unsigned char far )MK_FP(_DS,buf);
Structure of Partitioning Table
- Total size of Partition Table is 512 bytes.
- First 446 bytes contains code which loads the
boot block of active partition and is executed at
Boot Time.
- Rest of the 66 bytes is the Data part.
- Last two bytes of the Data part is the Partition
table signature.
File System for Each O.S.
- On a single disk there can be 4 different file
systems and hence 4 different O.S.
- Each O.S. will have its individual partition on
disk.
- Data related to each partition is stored in a 16-
bytes chunk within the Data Part of Partition
Table.
Structure of Data Part of P.T.
02 Bytes Signature
16 Bytes Partition into of 4th^ partition.
16 Bytes Partition into of 3rd^ partition.
16 Bytes Partition into of 2nd^ partition.
16 Bytes Partition into of 1st^ partition.
Size Description
The data part can contain information about four different partitions for different
Operating systems. Each partition information chunk is 16 bytes long and the last two
bytes at the end of the partition table data part is the partition table signature whose value
should be AA55 indicating that the code part contains valid executable code.
The structure of the information stored in each 16 byte for partition is shown in the slides
below
Byte File System ID
Low 8-bits of cylinder # for last block within the partition..
Byte
0 – 5 bits are sector # for first block within the partition and bits 6 -7 are higher bits of cylinder #
Byte
Byte Head # for first block in the partition
Byte 80H if Bootable, 0 if Not
Size Description
0F Extended partitions LBA Mapped
0E Win 95 FAT 16 LBA Mapped
0B Win 95 FAT 32
0C Win 95 FAT 32 LBA Mapped
0A OS/2 Boot Manager
09 AIX Data partitions
08 AIX Boot partitions
OS/2, 1FS = Installable file system Advanced Unix Windows NT NTFS
06 MS-DOS 16-bits FAT >= 32MB
Primary Partition
- Partition defined in the MBR (Master Boot
Record) are primary partition.
- Each Primary Partition contains information
about its respective O.S.
- However if only one O.S. is to be installed then
extended partitions.
Extended Partitions MBR
Primary Extended Partition Partition
However if a single operating system is to be kept for instance, then the disk can be
divided into primary and extended partitions. Information about primary and extended
partition is kept in the first physical block. The extended partition may again be divided
into a number of partitions, information about further partitions will be kept in extended
partition table which will be the first physical block within extended partition (i.e. it will
not the first block of primary partition.). Moreover there can be extended partitions
within extended partitions and such that in then end there are number of logical partitions
this can go on till the last drive number in DOS.
