Logical sector 0, known as the boot sector, contains all of the critical information regarding the disk medium's characteristics (Figure 10-2). The first byte in the sector is always an 80x86 jump instruction——either a normal intrasegment JMP (opcode 0E9H) followed by a 16-bit displacement or a "short" JMP (opcode 0EBH) followed by an 8-bit displacement and then by an NOP (opcode 90H). If neither of these two JMP opcodes is present, the disk has not been formatted or was not formatted for use with MS-DOS. (Of course, the presence of the JMP opcode does not in itself ensure that the disk has an MS-DOS format.)
Following the initial JMP instruction is an 8-byte field that is reserved by Microsoft for OEM identification. The disk-formatting program, which is specialized for each brand of computer, disk controller, and medium, fills in this area with the name of the computer manufacturer and the manufacturer's internal MS-DOS version number.
Figure 10-2. Map of the boot sector of an MS-DOS disk. Note the JMP at offset 0, the OEM identification field, the MS-DOS version 2 compatible BIOS parameter block (bytes 0BH—17H), the three additional WORD fields for MS-DOS version 3, the double-word number-of-sectors field and 32-bit binary volume ID for MS-DOS version 4.0, and the bootstrap code.
Please refer to the printed book for this figure.
The third major component of the boot sector is the BIOS parameter block (BPB) in bytes 0BH through 17H. (Additional fields are present in MS-DOS versions 3.0 and later.) This data structure describes the physical disk characteristics and allows the device driver to calculate the proper physical disk address for a given logical-sector number; it also contains information that is used by MS-DOS and various system utilities to calculate the address and size of each of the disk control areas (file allocation tables and root directory).
The final element of the boot sector is the disk bootstrap routine. The disk bootstrap is usually read into memory by the ROM bootstrap, which is executed automatically when the computer is turned on. The ROM bootstrap is usually just smart enough to home the head of the disk drive (move it to track 0), read the first physical sector into RAM at a predetermined location, and jump to it. The disk bootstrap is more sophisticated. It calculates the physical disk address of the beginning of the files area, reads the files containing the operating system into memory, and transfers control to the BIOS module at location 0070:0000H. (See Chapter 2.)
Figures 10-3 and 10-4 show a partial hex dump and disassembly of a PC-DOS 3.3 floppy-disk boot sector.
0 1 2 3 4 5 6 7 8 9 A B C D E F
0000 EB 34 90 49 42 4D 20 20 33 2E 33 00 02 02 01 00 .4.IBM 3.3.....
0010 02 70 00 D0 02 FD 02 00 09 00 02 00 00 00 00 00 .p..............
0020 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 12 ................
0030 00 00 00 00 01 00 FA 33 C0 8E D0 BC 00 7C 16 07 .......3.....|..
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.
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01C0 0D 0A 44 69 73 6B 20 42 6F 6F 74 20 66 61 69 6C ..Disk Boot fail
01D0 75 72 65 0D 0A 00 49 42 4D 42 49 4F 20 20 43 4F ure...IBMBIO CO
01E0 4D 49 42 4D 44 4F 53 20 20 43 4F 4D 00 00 00 00 MIBMDOS COM....
01F0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 55 AA ..............U.
Figure 10-3. Partial hex dump of the boot sector (track 0, head 0, sector 1) of a PC-DOS version 3.3 floppy disk. This sector contains the OEM identification, a copy of the BIOS parameter block describing the medium, and the bootstrap routine that reads the BIOS into memory and transfers control to it. See also Figures 10-2 and 10-4.
jmp $+54 ; jump to bootstrap
nop
db 'IBM 3.3' ; OEM identification
; BIOS parameter block
dw 512 ; bytes per sector
db 2 ; sectors per cluster
dw 1 ; reserved sectors
db 2 ; number of FATs
dw 112 ; root directory entries
dw 720 ; total sectors
db 0fdh ; media descriptor byte
dw 2 ; sectors per FAT
dw 9 ; sectors per track
dw 2 ; number of heads
dd 0 ; hidden sectors
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.
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Figure 10-4. Partial disassembly of the boot sector shown in Figure 10-3.