PC Glossary


This is a discussion of PC terms as related to my web pages discussing the boot process with emphasis on the MBR and boot code for Windows 95 OSR2.

These terms are not meant to be complete, just as a reference for the above web pages, although the definition for MB includes more alternate definitions than most sources. Refer to the more complete glossaries on the web for other terms or more in-depth information.

Definitions

ATA
AT Attachment (Same as IDE interface)
Back-Words Storage
In an 80x86 based PC, values are stored with the least-significant byte of the word in the lower memory location and the most significant byte in the higher memory location, sometimes called "back-words" storage. For example the value A59C is stored as bytes 9C A5 if you are looking at a hex dump as in the Hex views of the MBR and boot sector codes. Same with Dwords, for example the value A59CE2F3 is stored as the bytes F3 E2 9C A5, with F3 being at the lowest memory address and A5 being at the highest memory address. Qwords are stored in the same manner. The op-code establishes whether the value following it is a Byte, Word, Dword or Qword.
Bit
A single unit of information assigned one of two values, 0 or 1.
Byte
A unit of information comprised of 8 bits (on microcomputers).
CHS (Cylinder/Head/Sector)
Original addressing format for the ATA hard drive interface using the standard BIOS INT 13 functions. Newer INT 13 extensions use the LBA form of addressing.
Cylinder
A combination of all the tracks in a vertical column, making a cylinder. Cylinders are numbered starting from 0. Cylinder 0 consists of Track 0, Side 0 then Track 0, Side 1 then Track 0, Side 2, etc.
Drive
See Partition.
DWord (Double Word)
A unit of information comprised of 32 bits or 4 bytes or 2 words (on microcomputers). See Back-Words Storage for how this is stored in memory on a PC and shown in the "Hex Views" of the MBR and Boot sectors.
Extended partition
A combination of all the logical partitions. The extended partition has no separate physical manifestation except for the entry in the MBR. The starting location of the extended partition in the MBR is the starting location of the first partition boot record (EBR) in the extended partition. The ending location of the extended partition in the MBR is the ending location of the last logical drive in the extended partition. The extended partition consists of a linked list of partitions, each corresponding to a logical drive. See the page Extended Partition Tables for an example.
A unit of information comprised of 2 words or 4 bytes (on microcomputers).
GB (gigabyte)
Giga is a prefix indicating 1 billion (10**9) as in 1 GHz = 1,000,000,000 hertz (cycles per second). This is also the case when referring to hard drive sizes, which are usually given in decimal bytes, 1 GB = 1 billion bytes. However see MB (megabyte) for other possibilities. Memory sizes usually use 1024 * 1024 * 1024 (2**30) bytes as a gigabyte.
Head (or Side)
One read/write head or equivalently one side of a hard disk platter. Heads are numbered starting from 0.
Hidden sectors (in BIOS Parameter Block)
Number of sectors preceding the boot sector up to and including the sector containing the partition table defining the partition. For primary partitions, this is the same value as given by "Relative Sectors" in the MBR. For logical partitions in the extended partition, this will be the number of sectors on one track as the partition table defining the partition will be on Head 0, Sector 1 and the boot record will be on Head 1, Sector 1 of the same cylinder and should be the same value as given by "Relative Sectors" in that partition table for the boot sector.
IDE (Integrated Drive Electronics)
Type of hard drive interface.
KB (kilobyte)
Kilo is a prefix indicating 1000 (10**3) as in 1 KHz = 1000 hertz (cycles per second). However when referring to computer memory size it usually means 1024 (2**10) bytes.
LBA (Logical Block Addressing)
LBA is a newer (for ATA it is newer) form of hard disk addressing which is a linear sector addressing scheme i.e., 0,1,2,3,4,5... as compared to CHS.

LBA = (C * Number_of_Heads + H) * Sectors_per_Track) + S - 1 where C, H, and S are the CHS values of the desired address.

MB (megabyte)
Mega is a prefix indicating 1 million (10**6) as in 1 MHz = 1,000,000 hertz (cycles per second). This is also the case when referring to hard drive sizes, which are usually given in decimal bytes, 1 MB = 1 million bytes. Note however that FDISK displays hard drive sizes in binary megabytes, using 1024 * 1024 (2**20) bytes for a megabyte. Floppy drive sizes and some versions of the Norton Utilities, when referring to hard disks, use 1000 * 1024 bytes for a megabyte. For example a 1.44 MB floppy is actually 1440 KB. Memory sizes usually use 1024 * 1024 bytes as a megabyte. Thus there are three existing definitions of MB and unfortunately many products do not clearly state which one is being used. This leads to several possibilities for the size of a gigabyte. Some organizations are trying to bring some order to the chaos, see Definitions of the SI units: The binary prefixes.
Number of Sectors (in partition tables)
See Total Sectors.
Partition
A physical division of the disk usually associated with a drive letter. Since each partition corresponds to a drive letter, partitions can also be referred to as drives. The term logical volume or logical partition as used in this document refers to a partition within the extended partition. The active primary partition has drive letter C: while other primary partitions cannot be seen and do not have drive letters (in DOS/Windows). Primary partitions are listed in the MBR while partitions within the extended partition each have their own EBR.
Partition Boot Record (MBR and EBR)
The MBR (Master Boot Record) is an example of a Partition Boot Record. Each logical volume (or drive) in the extended partition also has a Partition Boot Record. These are referred to by Microsoft as Extended Boot Records (EBRs). These have the same format as an MBR but contain no program code, just a partition table and the ending 2-byte signature. See the Microsoft document FNCB_DIS.DOC under On Line References at the end of this page for more details on EBRs.
QWord (Quad Word)
A unit of information comprised of 4 words or 8 bytes (on microcomputers). See Back-Words Storage for how this is stored in memory on a PC and shown in the "Hex Views" of the MBR and Boot sectors.
Relative Sectors (in partition tables)
See Starting Sector.
Reserved sectors (in BIOS Parameter Block)
Refers to sectors preceding the start of the first FAT, up to and including the Boot Sector(s). This usually just refers to the boot sector(s) as FAT1 directly follows the boot sector(s). Previous to Windows 95, there was only one boot sector and thus only one reserved sector.
SCSI (Small Computer System Interface)
Type of peripheral interface, including hard drives.
Sector
Smallest packet of information that is read from or written to a hard drive (usually 512 bytes). Sectors are numbered starting from 1 (in CHS format).
Starting Sector (in partition tables)
In an MBR, this is the number of sectors from the MBR to a primary partition boot sector or to an extended partition (EBR). In an EBR this is the number of sectors from the EBR to the boot sector for the partition (usually the first entry in the partition table) or the number of sectors from the beginning of the extended partition (first EBR in the extended partition) to the next EBR (usually the second entry in the partition table). For partitions defined in the MBR, Relative Sectors is equivalent to the starting sector number for the partition in LBA format. For partition entries in EBRs, the starting sector of the partition in LBA format is derived by adding the Relative Sectors in the EBR to the Relative Sectors in the MBR for the extended partition. However as pointed out in PARTTAB1.TXT (found in Hale's ALLHIW.ZIP) the Relative Sectors in the EBR was originally the absolute starting sector in LBA format. Also see "Hidden Sectors".
TOS (Top Of Stack)
The stack is an area of memory set aside for storage of temporary data. The base address of the stack is pointed to by the SS (Stack Segment) register and the offset (or Top Of Stack) is pointed to by the SP (Stack Pointer) register.
Total Sectors (in partition tables)
Number of sectors in a partition or logical volume counting from the boot sector to the end of the partition or logical volume. The sectors on the track which contain the partition table are not included in this number.
Track
All the sectors (usually 63) on one side of a hard disk platter that would pass under a non-moving read/write head as the platter rotates on its spindle, forming a circle. Tracks are numbered starting from 0.
Volume
See Partition.
Word
A unit of information comprised of 16 bits or 2 bytes (on microcomputers). See Back-Words Storage for how this is stored in memory on a PC and shown in the "Hex Views" of the MBR and Boot sectors.

On Line References

A nice glossary of computer terms by Erik van Straten can be found at Harddrive Related Terms and Tricks. His Harddrive Related Tools and Information contains some hard drive programs and drivers and includes some links for additional hard drive info.

The Microsoft article "Disk Sectors Critical to Startup" contains information on both the MBR and the boot sector including details on the BPB for both FAT16 and FAT32 volumes. This article is part of Chapter 32 "Disk Concepts and Troubleshooting" in the Microsoft Windows 2000 Professional Resource Kit. Thanks to the Starman and Fons Van Assche for keeping me updated on Microsoft's changing links. (updated 7/2/03)


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This page was created on 18 December 2000, last updated on 2 July 2003.