A device is "Bootable" if it carries a
boot sector with the byte sequence 0x55, 0xAA in bytes 511 and 512
respectively. When the BIOS finds such a boot sector, it is loaded into
memory at a specific location; this is usually 0x0000:0x7c00 (segment 0,
address 0x7c00). However, some BIOS' load to 0x7c0:0x0000 (segment
0x07c0, offset 0), which resolves to the same physical address, but can
be surprising.
When the wrong CS:IP pair is assumed, absolute near jumps will not work properly, and any code like mov ax,cs; mov ds,ax will result in unexpected variable locations. A good practice is to enforce CS:IP at the very start of your boot sector.
When the wrong CS:IP pair is assumed, absolute near jumps will not work properly, and any code like mov ax,cs; mov ds,ax will result in unexpected variable locations. A good practice is to enforce CS:IP at the very start of your boot sector.
On a hard drive, the so-called Master Boot Record (MBR) holds executable code at offset
0x0000 - 0x01bd, followed by table entries for the four primary partitions, using sixteen bytes
per entry (0x01be - 0x01fd), and the two-byte signature (0x01fe - 0x01ff).
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