![]() ![]() The first sector of the active partion is actually sector 63 of the partition as the first sector contains a redundant copy of the MBR and sectors 2 through 62 are normally empty (the remainder of the first cylinder). The FA instruction is disable interrupts, 33 C0 is XOR AX,AX The first two bytes usually indicate the MBR version:ģ3 C0. It closely resembles the above except that an extended instruction is used to copy the first sector of the active partion to avoid the cylinder 1024 limitation. The most common is the MBR used by Windows 98 and ME. It requires that the first sector of the active partition must be at or under cylinder 1024 as the standard INT 13h Read function is used to copy that sector to location 7C00h (the INT 13h BIOS interrupt limits cylinders to 0 to 1023, sectors to 1 to 63 and heads 1 to 15, 8455716864 bytes, 8.4 GB). The above description is for the MBR of a hard drive initialized with the DOS and Windows 95(A) FDISK. The MBR may be of one of the several standard versions or a customization. If no bootable partition is found or the partition marked active does not contain an operating system, an error message is displayed using INT 10. The relocation is necessary as the code next checks the partition table (64 bytes near the end of the MBR) for the first partition that is marked active, the first sector of which is then copied to location 7C00h and executed. The MBR code then first copies itself to location 0600h (1536 decimal) and continues execution with a jump to location 061Dh. It is then executed by a jump to it with the CPU instruction: JMP 0000:7C00 The motherboard completes a POST (power on self test) then the BIOS copies the MBR of the drive to memory location 7C00h (31744 decimal). If formatted as a hard drive they contain both an MBR and a boot record, if as a floppy, just a boot record. USB flash drives may also feature an MBR. It contains the boot loader code and the MPT (Master Partition Table). The MBR (Master Boot Record) is the first sector of a hard drive. ![]()
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