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An Introduction To Hard Disk Geometry
Introduction
This page describes the typical layout of a modern hard drive. You may have heard of file systems such as NTFS, FAT32 or EXT3, which are used by your operating system. Concepts like files and directories are contained in these file systems, so obviously they are very important. But a single physical hard disk can contain multiple filesystems – each on a separate partition. And a hard disk that contains a operating system must contain some elements that play a role in the boot sequence.
Obviously a lot more is going on under the hood. Let’s take a look.
Anatomy of Hard Disk
Hard disks have been around since the 1950′s, but the design has not changed much. The general hard disk design is quite simple, consisting of only a few moving parts. In the picture above you can see:
- Platters: Solid disks with a magnetic coating that contains the data. The platters spin at a constant rate when the hard disk is in operation, typically at 3600, 5200 or 7200 rounds per minute (rpm).
- Arms: The head stack assembly holds the arms that hold the read/write heads. The stack is rotated by an actuator which is not displayed in the image, causing the arms to position the heads between the hub and the edge of the platter. To achieve great speed and accuracy, the arm and its movement mechanism need to be extremely light and fast. The arm on a typical hard-disk drive can move from hub to edge and back up to 50 times per second
Every platter contains many concentric circles – called tracks – that are used to store the data. This radically differs from a CD or DVD, where a single track of data is used, laid out as a spiral. A modern hard disk has tens of thousands of tracks on a platter. The tracks on a hard disk are divided up into smaller segments called sectors. Each sector usually holds 512 bytes of user data, plus as many as a few dozen additional bytes used for internal drive control and for error detection and correction.
The heads that access the platters are locked together on an assembly of head arms. This means that all the heads move in and out together, so each head is always physically located at the same track number. It is not possible to have one head at track 0 and another at track 1,000. Because of this arrangement, often the track location of the heads is not referred to as a track number but rather as a cylinder number. What you should take away from this is that they are essentially the same.
Logical Geometry
The logical geometry of a hard disk is the logical structure that programs see when communicating with the hard disk’s controller – which is located on a logical board inside the hard disk. In all but the earliest hard disks the physical geometry is a lot more complicated than the logical geometry. Luckily, only the hard disk’s engineers have to deal with the complicated nature of the physical geometry; they are hidden from the operating system and the user.
Addressing Modes
CHS Adressing Modes
In the case of early IDE/ATA hard disks the BIOS provides access to the hard disk through an addressing mode called CHS, where “CHS” stands for “cylinder, head, sector”. CHS addressing starts at (0, 0, 1). In old computer system the maximum amount of addressable data was very limited – due to limitations in both the BIOS and the hard disk interface. Some well-known resulting limits are the 502 MB and the 8.4 GB barriers. The CHS addressing mode was declared obsolete in the ATA-5 standard, replacing it with LBA addressing.
LBA Adressing Modes
Modern hard disks use a recent version of the ATA standard, such as ATA-7. These disks are accessed using a different addressing mode called: logical block addressing or LBA involves a totally new way of addressing sectors. Instead of referring to a cylinder, head and sector number, each sector is instead assigned a unique “sector number”. In essence, the sectors are numbered 0, 1, 2, etc. up to (N-1), where N is the number of sectors on the disk. In order for LBA to work, it must be supported by the disk, the BIOS and operating system. The current 48-bit LBA scheme, introduced in 2003 with the ATA-6 standard, allows addressing up to 144 petabytes (144,000,000 gigabytes)..
The BIOS, working with the system chipset on the motherboard and the system I/O bus, controls which types of modes can be used with the hard disk to actually transfer the data. Originally, systems used the BIOS as an intermediary for every byte of transferred information. Modern operating systems implement direct disk access (Direct Memory Access), and do not use the BIOS subsystems, except at boot load time. A detailed description of the modes of transfer between the hard disk and volatile memory is out of the scope of this page, but the prevalent mode in 2011, UDMA, is a good place to start reading.
Master Boot Record
When an x86 PC is powered on, the BIOS will select a storage device from which to boot. From this device, the BIOS reads a boot sector - called the Master Boot Record (MBR) – which contains the primary boot loader. The MBR is a 512-byte sector, located in the first sector on the disk (sector 1 of cylinder 0, head 0). This is equal to LBA 0. After the entire MBR is loaded into RAM, the BIOS yields control to it.
MBR Elements
- The first 446 bytes of the MBR contain executable code – the primary boot loader.
- The next 64 bytes hold a Partition Table which describes the partitions – or volumes – of a storage device. In this context the boot sector may also be known as a partition sector.
- The last two bytes of the MBR contain a magic number. If the value is equal to 0xAA55 then the BIOS will assume it is dealing with a valid MBR. If it is not 0xAA55 the BIOS will produce an error message.
Partition Table Entry
The Partition Table contains a maximum of four entries. Each entry specifies a primary partition, but one of the entries can specify an extended patition. Extended partitions are discussed later. The entries have the following format:
The contents of a typical master boot record are displayed below. Note that the code section also contains some human-readable strings – these strings are used by the machine instructions to display messages depending on the code flow at runtime.
EXTENDED PARTITION
Due to the size of the MBR’s partiton table, there is a limit of 4 primary partitions. To get around this issue, engineers came up with a special partition type: the extended partition. A hard disk may contain a maximum of one extended partition. The extended partition can be subdivided into multiple logical partitions. In DOS/Windows systems, each logical partition may then be assigned an additional drive letter. In the MBR’s Partition Table, extended partition entries usually have a Partition Type of either 0x05 or 0x0F; depending upon the size of the disk.
When the operating system encounters an extended partition type in the Partition Table entry, it will use the Starting Sector field to locate the first sector of the extended partition. In that sector it will look for a structure called the Extended Boot Record (EBR). This is a descriptor for a logical partition.
EBRs have exactly the same structure as the MBR; the only difference is that a some fields in the EBR go unused. The only used fields are the first and second Partion Table entries, along with the mandatory boot record signature (or magic number) of 0xAA55 at the end of the sector. This results in the following layout:
So each logical partition in the extended partition is preceded by an EBR, and the EBR’s are chained together. The operating system will follow the chain until it reaches the end. The complete hard disk layout might look something like this:
Note that every filesystem contains a Boot Sector of its own. This is called the Volume Boot Record (VBR). In the boot sequence the MBR is processed first, and eventually the VBR of the partition that contains the OS is processed.
Sources
Keyboard error at boot.
Keyboard error at boot.
Cause
An error with the keyboard can be caused by any of the below possibilities.
- Keyboard is not connected properly.
- Stuck key
- Bad keyboard
- Bad PS/2 or AT port
Solution
Keyboard is not connected properly
Verify that the keyboard is connected properly to the computer by turning off your computer and then disconnecting and reconnecting the keyboard to the computer.
Stuck key
Ensure that there are no stuck keys on the keyboard. If all keys appear to be ok and you have a standard desktop computer with keyboard, turn the keyboard over and gently hit the back of the keyboard to loosen any dirt or hair that may be stuck in the keyboard.
Bad keyboard
Try another keyboard on the computer to verify that the keyboard has not gone bad.
Bad PS/2 or AT port
If all of the above solutions are not able to resolve your issue it is likely that the port on the back of the computer may be bad and the motherboard or I/O board may need to be replaced.
Motherboard issue.
Additional information
This document is intended to help users who are experiencing issues with the POST and may have any of the below symptoms.
- Computer beeps irregularly when the computer is turned on.
- Computer turns on but does not boot or do anything.
- Computer reboots every few seconds.
Answer
Tip: Make sure the computer turns on, if nothing happens (no lights, no sound, no fans, etc.) the computer has a power related issue.
Caution: Some of the below steps recommend removing physical parts within the computer. While in the computer it is highly recommend that you be aware of ElectroStatic Discharge (ESD)and its potential hazards.
Remove new hardware
If any new hardware has been recently added to the computer, remove that hardware to make sure it is not the cause of your issue. If after removing the new hardware your computer works it's likely the computer is either not compatible with the new hardware or a system setting needs to be changed to work with the new hardware device.
Remove any disks or USB devices
Remove any disks, CD's, DVD's that are in the computer and if any USB devices (iPods, drives, phones, etc) are connected disconnect all of them as well. Reboot the computer and see if anything changes.
Disconnect external devices
Remove everything from the back of the computer except the power cable. Turn on the computer and see if it beeps normally. If the computer has never beeped keep the monitor or display connected to see if any change occurs.
Identify beep code
If you are receiving a sequence of beeps see the beep code page for a listing of different beep codes and their explanation or your motherboard or computer documentation. These beep codes are meant to help identify what computer component is failing or bad. If your beep code is not listed, continue troubleshooting.
Check all fans
Make sure all fans are running in the computer. If a fan has failed (especially the heat sink fan for the CPU) your computer could be overheating or detecting the fan failure causing the computer not to boot.
Check all cables
Verify that all the cables are properly connected at that there are no loose cables by firmly pressing in each cable.
- All disk drives should have a data cable and power cable connected to them.
- Your power supply should have at least one cable going to the motherboard. Many motherboards may also have additional cables connected to them to supply power to the fans.
Disconnect all expansion cards
If the above recommendations still have not resolved the irregular POST, disconnect the riser board (if applicable) and each of the expansion cards. If this resolves the issue or allows the computer to POST connect one card at a time until you determine what card is causing the issue.
Disconnect all drives
If you were unable to determine by the beep code what is failing or do not have a beep code disconnect the IDE, SATA, SCSI, or other data cables from the CD-ROM, hard drive, and floppydrive from the Motherboard.
If this resolves your irregular POST or generates error messages re-connect each device until you determine what device or cable is causing the issue. In some situations it can also be a loose cable connection that causes the issue.
Remove the RAM
If you continue to to receive the same problem with all the above hardware removed, disconnect the RAM from the Motherboard and turn on the computer. If the computer has a different beep code or if your computer was not beeping and is now beeping turn off your computer and try the below suggestions. Making sure to turn off the computer each time you're adding and removing the memory and then turning the computer back on to see if the suggestion resolves the issue.
- Re-insert the memory into the same slot.
- If you have more than one stick of memory remove all but one stick of memory, try rotating through each stick.
- Try one stick of memory in each slot.
If you're able to get the computer to boot with one or more of the sticks of memory it's likely you're dealing with some bad memory. Try to identify what stick of memory is bad and replace it.
If you're able to get memory to work in one slot but not another slot. You're motherboard is defective you can either workaround the issue by running the memory in a different slot or replace the motherboard.
Power cycle the computer
In some situations a computer may have power related issues often caused by either the power supply or the motherboard. To help determine if this is the cause of your issue try turning the computer on, off, and back on as fast as possible, making sure the computer power light goes on and off each time. In some situations you may be able to temporarily get the computer to boot.
This should only be used as a temporary workaround if you're able to get this to work. Often this is good for users who may have not done a backup and need to get the computer up one more time to copy files before starting to replace hardware.
Disconnect and reconnect the CPU
For users who are more comfortable working with the inside of their computer or who have built their computer one last recommendation before assuming hardware is bad is to reseat the CPUby removing it and putting it back into the computer.
Bad motherboard, CPU, RAM, or power supply
If after doing all of the above recommendations you continue to have the same issue unfortunately it is likely that you have bad Motherboard, PSU, CPU, or RAM. The next step would be either to replace these components or have the computer serviced. If you plan on doing the repairs yourself or you are a repair shop it is suggested that you replace the Motherboard first, RAM, CPU, and then power supply in that order or try swappable parts from other computers.
Keyboard controller Gate A20 error
A20 error
Issue
A20 error.
Cause
Issue related to the keyboard or keyboard controller.
Solution
- Make sure the keyboard is connected to the proper port on the back of the computer.
- Turn off the computer and disconnect the keyboard and verify there are no bent, burnt or broken pins on the keyboard connector or on the keyboard connector on the back of the computer.
- Try a different keyboard on the computer.
- Unfortunately, if all of the above recommendations do not resolve your issue, the motherboardmay have a bad controller and will need to be replaced.
DRAM refresh failure
Computer has three short beeps as it starts
Question
Computer has three short beeps as it starts.
Cause
Computer RAM (memory) related issue.
Answer
Caution: Some of the below steps recommend removing physical parts within the computer. While in the computer it is highly recommend that you be aware of ESD and its potential hazards. Information about ESD and proper grounding techniques are found on our ESD help page.
Remove added memory
If you've recently added or tried to add additional memory to the computer and have started getting these beeps. Remove all new memory you've added to the computer. If the computer works fine after removing the new memory you're encountering either an incompatibility or defective new memory.
Open computer and reseat memory
Sometimes when the computer is moved or over time a memory stick can become loose causing the computer to be unable to read the memory or get errors as it's reading it. Try fixing this issue by opening the computer removing each of the memory sticks you have in the computer and then placing them back into the computer.
Swap memory location
If reseating the memory did not resolve the issue try swapping the location of the memory. If you have only one stick of memory in the computer try moving it to another slot and then boot the computer.
If you have more than one stick of memory try removing all but one stick of memory and boot the computer. If this does not resolve the issue try removing that stick of memory and try one of the other sticks of memory.
Try another computers memory
If you have access to another computer that uses the same memory, try using its known good memory in your computer.
If another computers memory works you know that you have bad memory. If another computers memory does not work and it is compatible with your computer unfortunately your motherboardor the slots on the motherboard are defective causing it to be unable to properly read the memory, which means the motherboard will have to be replaced.
Replace memory
If it is determined the memory in the computer is bad we suggest that you replace it.
VGA
Short for Video Graphics Array, VGA is a popular display standard developed by IBM and introduced in 1987, VGA provides 640 x 480 resolution color display screens with a refresh rate of 60Hz and 16 colors displayed at a time. If the resolution is lowered to 320 x 200, 256 colors can be displayed. VGA capability is built into plug-in video cards, VGA chips, and monitors that can work with the VGA cards. Today, VGA has been replaced by SVGA.
Below is an illustration of the 15-Pin VGA connector, its pin assignments, and size dimensions with a chart describing what each of the pins are for.
| PIN | FUNCTION |
| 1 | Red Video |
| 2 | Green Video |
| 3 | Blue Video |
| 4 | Monitor ID 2 |
| 5 | TTL Ground (monitor self-test) |
| 6 | Red Analog Ground |
| 7 | Green Analog Ground |
| 8 | Blue Analog Ground |
| 9 | Key (Plugged Hole) |
| 10 | Sync Ground |
| 11 | Monitor ID 0 |
| 12 | Monitor ID 1 |
| 13 | Horizontal Sync |
| 14 | Vertical Sync |
| 15 | Monitor ID 3 |