MobileDyne Systems, Inc.

MBR Data Structure

MBR Data Structure

struct master_boot_record {
  char    bootinst[446];   /* space to hold actual boot code */
  char    parts[4 * sizeof (struct fdisk_partition_table)];
  ushort  signature;       /* set to 0xAA55 to indicate PC MBR format */

A PC disk is layed out in cylinders, heads, and sectors ("CHS"). A sector is alwasy 512 bytes. Since the bootstrap code is using 446 bytes, the partition table and signiture must fit within the remaining 66 bytes. The signature is 16 bits, or two bytes. That leaves 64 bytes for 4 partition entries, each of which must be specified by only 16 bytes.

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make:   09/16/99 at 09:06:57