Like the Boot Records of an OS, the first three bytes could be called the Jump Instruction. But only the first two bytes are being used to form the actual JMP (Jump) instruction to the rest of the executable code; the third byte (90h) is just a NOP instruction ('No Op' do nothing). So the execution jumps over the 71 next bytes which can be thought of as a BIOS Parameter Block (or BPB); BIOS parameter block (BPB) is a description of the physical medium (hard disk or floppy) that might be stored in a file system=E2=80=99s Volume Boot Record. File systems with a BIOS parameter block include FAT16, FAT32, HPFS, and NTFS. ECMA-107 or ISO/IEC 9293 (which describes FAT as for flexible/floppy and optic distal disk cartridges) also describes this as an FDC Descriptor or an FDC Extended Descriptor.
00000000 EB48 jmp short 0x4a
00000002 90 nop Jump to the start of the program at 0x7c00 and is jumped to with CS:IP
0:0x7c00. Jump over BPB data area to main body of code. This BPB Data Area (BIOS Parameter Block) is filled with useful data for any program examining it as a normal Volume Boot Record.
The BYTES in the BPB which are referenced in the code below are:
[00000005] -> 8E D0 BC 00 B0 B8 00 00 8E D8 8E C0 FB BE ("Disk Address Packet" for LBA mode.)
[00000040] -> 80 ("Boot Drive") NOTE: For those of you with multi- OS booting systems, if your Linux installation with GRUB's remaining software (stage2, menu file, etc.) is located somewhere other than on the Primary Master drive, this value will be 81, 82, etc. depending upon which drive that Linux OS's /boot/grub directory is located.
[00000041] -> 00 ("Force LBA mode byte") [00000042] -> 00 80 (8000h) Memory location where GRUB stores the next stage of the code to execute. [00000044] -> Note: A very important location for anyone using GRUB! This (4-byte) Quad-Word contains the location of GRUB's stage2 file in sectors! You will always see the bytes 01 00 00 00 in this location whenever GRUB has been installed in the first track (Sectors 1 ff.) of an HDD; immediately following the GRUB MBR in Absolute Sector 0.
[00000048] -> 00 08 (800h) [Don't confuse this with the 8000 at
00000042.]
00000003 10
00000004 8E
00000005 D0
00000006 BC
00000007 00
00000008 B0
00000009 B8
0000000A 00
0000000B 00
0000000C 8E
0000000D D8
0000000E 8E
0000000F C0
00000010 FB
00000011 BE
00000012 00
00000013 7C
00000014 BF
00000015 00
00000016 06
00000017 B9
00000018 00
00000019 02
0000001A F3
0000001B A4
0000001C EA
0000001D 21
0000001E 06
0000001F 00
00000020 00
00000021 BE
00000022 BE
00000023 07
00000024 38
00000025 04
00000026 75
00000027 0B
00000028 83
00000029 C6
0000002A 10
0000002B 81
0000002C FE
0000002D FE
0000002E 07
0000002F 75
00000030 F3
00000031 EB
00000032 16
00000033 B4
00000034 02
00000035 B0
00000036 01
00000037 BB
00000038 00
00000039 7C
0000003A B2
0000003B 80
0000003C 8A
0000003D 74
0000003E 03
0000003F 02
00000040 80
00000041 00
00000042 00
00000043 80
00000044 8B
00000045 82
00000046 00
00000047 00
00000048 00
00000049 08
General setup:
0000004A FA cli This line will Clear Interrupt Flag This is a workaround for buggy BIOSes which don't pass boot drive correctly. If GRUB is installed into a HDD, check if DL is masked correctly. If not, assume that the BIOS passed a bogus value and set DL to 0x80, since this is the only possible boot drive. If GRUB is installed into a floppy, this does nothing (only jump).
0000004B EA507C0000 jmp 0x0:0x7c50 Long Jump to the next instruction because some bogus BIOSes jump to
07C0:0000 instead of 0000:7C00.
00000050 31C0 xor ax,ax
00000052 8ED8 mov ds,ax
00000054 8ED0 mov ss,ax set up %ds and %ss as offset from 0
00000056 BC0020 mov sp,0x2000 set up the REAL stack
00000059 FB sti Set Interrupt Flag This instruction sets the interrupt flag (IF) in the EFLAGS register. After the IF flag is set, the processor begins responding to external, mask able interrupts after the next instruction is executed.
0000005A A0407C mov al,[0x7c40]