What Is a Logical Disk?

Logical disks are also called logical hard disks, which are hard disks (physical hard disks) that actually exist in a PC and are divided into several logical hard disks. A logical hard disk does not really exist. It is a logical drive letter that represents each partition after the partition is created. ^[1]

A logical disk is a storage abstraction that the kernel uses as a fixed-size, random-access, and chunked

Logical network disk structure

Due to the large capacity of the hard disk, for management and use reasons, it is necessary to divide it into several areas, that is, to partition the hard disk. The standard division method is to divide a hard disk into a maximum of 4 partitions, numbered 1,2. , 3, 4. The actual number of partitions is determined by the user. The DDS system software FDISK and COM programs can be used to complete the partition. The first partition is the primary partition, and the remaining partitions constitute the logical hard disk of the hard disk. The partition table is registered in the partition. If the data in the partition table is damaged, the logical disk will be lost or the entire hard disk will be lost.

Each partition has basically the same information storage structure, in this order: the main boot area MBOOT, the boot area B00T, the file allocation table FAT (FATI, FAT2), the root file directory table FDT, and the data area DATA Area.

Only the main boot area in the main partition contains the main boot program and the hard disk partition table. There are main boot areas in other partitions, but there is no main boot program in the main boot area. There is only a logical partition table, so other partitions are called pseudo-primary boot Area.

Each partition can be loaded with a different operating system, such as: DDS system, UNIX system, network operating system, etc.

Logical Network Disk Partition Table

The main boot area consists of three parts: the main boot program, the partition table, and the main boot area valid flag.

The task of the master boot program is to check the boot program of the partition and load the boot program into memory and execute it. The valid flag of the main boot area is placed in the last two bytes of the main boot area, and the content is: 55H, AAH. Not the mark, it means that the hard disk has an error and the hard disk is rejected.

The partition table is composed of 4 records (16 bytes each), which records the start and end information and capacity of the 4 partitions. This table is located at 1BEHIFDH in the main boot area. The data structure of each record is the same.

Logical network disk boot sector

The boot sector includes four aspects: the name and version number of the initial equipment manufacturer, the disk I / O parameter table (BPB), the boot program, and the prompt information area.

Initial equipment manufacturer name and version number (3 + 8 bytes), register operating system name and version number.

BPB is the basis for DOS to access disks. Each disk (including logical disks) must have a correct BPB table, which immediately follows the version number. Its contents include: the number of bytes (words) per sector, each cluster The number of sectors (bytes), the number of reserved sectors (words), the number of copies of the file allocation table FAT (bytes), the number of directory entries (words) under the root directory, the total number of sectors of the disk or logical disk ( Words), disk media descriptor (bytes), each FAT, number of sectors occupied (words), number of sectors on each track (words), number of heads (words), number of hidden sectors (words) Double word).

The task of the bootloader is to check the integrity and order of the system file names and to load the system core files into memory, that is, to boot the operating system.

The prompt information area stores the prompt information when a boot error occurs, the two system core file names, and the boot area valid flag (55H, AAH). The core file names of the system must be correct and complete, otherwise the boot will not succeed. ^[3]