Its function is to initiate the operating system and load it from the disk. The boot sector and the MBR are two different things.Ī boot sector is yet another structure, 512 bytes in size, that is file-system specific. In the previous section we introduced the term “boot sector” which is not to be confused with MBR’s “boot loader”
#Powerquest partition table editor tool code#
Once an active partition is found and its corresponding boot sector is loaded, then MBR executes the MBR code.If no active partitions are found then we halt.Once an active partition is found, we load the correspondance boot sector of that partition.Iterate through its entries looking for a partition entry marked as active.Here are the usual steps that are done by the boot loader: The boot loader can be infected with a boot-sector virus that runs even before the operating system boots
The boot loader is a program whose sole purpose is to interpret the partition table and decide which partition should be booted from. The MBR loader can be infected with an MBR virus (sometimes refered to as bootkits) that runs even before the operating system boots (or the boot sector executes) Boot loader Usually, the role of the MBR is to find which parition table is active, load its boot sector and execute it The length of the MBR structure is 512 bytes. The MBR is always located at the very start of the disk, at LBA 0 or 0,0,1 in CHS addressing. The master boot record (MBR) is a structure that holds the boot loader program and the partition table. In this section we will explain about the MBR (Master Boot record) and the format of the partition table and where it is located on the physical disk. LBA is linear addressing mode, starting from address 0 up to the maximum addressable location in the disk.Īs opposed to CHS addressing, LBA is much simpler and easier to understand and is used in modern operating systems. Those three component together designates the target disk address. Cylinder/Head/Sector (CHS) addressingĬHS stands for (C)ylinder/(H)ead/(S)ector addressing mode. In this section we will try to explain the disk addressing concepts. To finalize the disk partitioning and file system relationship, you can imagine that a raw non-partitioned disk can be compared to a big room with no divisions where every employee scatters his stuff here and there, but when we divide (partitioning) that space with cubicals we have organized offices and rooms (File system). Each file system has its cons and pros, which are not discussed in this document. For example, you can create a 10GB partition but you cannot attach to it the FAT16 file system, since FAT16 can only manage up to 2GB of disk space. That is why disk-partitioning has rules and limits (discussed later). You may have heard of FAT and NTFS file systems (supported by Microsoft Operating systems).įile systems are a way to allow you to store your data in an organized manner (managable by the host operating system). Those structures are named “File systems”. Practically, disk partitioning is more than just dividing the disk’s space into smaller partitions, but also attaching a given structure to each partition. For example a 10GB can be divided (theoratically) into as many partitions as one desires, say 3GB, 4GB and 3GB partitions. When we partition a disk, we are simply dividing the disk’s space into partitions (logical boundaries). The data you read/write have no standard structure but the structure you devise. You can achieve that with special tools that can read an unformatted/unpartitioned disk. When you purchase a new hard-disk with a given capacity, all you can do with it is read/write raw data from first address up to last address. In this section I will introduce some concepts that are needed to understand the article. Please note that this article does NOT cover the GPT (GUID Partition table). You are not required to have prior knowledge about disk partitioning The goal of this article is to provide just the needed knowledge to understand disk partitioning, hence many details are left out for the readers to investigate by themselves. The information in this article apply for x86 systems, I don’t know about disk partitioning on other systems. This article will explain the format of the partition table and will provide a small tool (Partition Viewer) to demonstrate what we explained. I’ve always been curious about how disks are partitioned, and how the partition table looks like.
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