IDE/ATA Disk BIOS Settings
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IDE/ATA Disk BIOS Settings
Since the system BIOS on virtually every PC provides native support for IDE/ATA hard
disks, there are a number of parameters that can be set to tell the BIOS what hard disks
are in the system, and how to control them. Each hard disk in the system will have its own
settings, so there is one set for the primary master, one for the primary slave, and so
on. This normally applies only to IDE/ATA SCSI hard disks are configured
through their
and built-in SCSI BIOS.
Most modern BIOSes support hard disk autodetection, which allows the BIOS to
interrogate each hard disk to determine its logical geometry, supported transfer modes and
other information. This can be done either at setup time or dynamically each time the
machine is booted, depending on the BIOS. .
Dynamic autodetection is the standard way of setting up modern drives, especially since
drives over 8 GB in size . In some situations, especially with
older PCs, you may still set some drive parameters manually. The following are the
settings normally found in the BIOS setup program for configuring IDE/ATA hard disks.
Since , I include only a very brief description of each here. Note that on
modern systems some of the oldest compatibility settings may not even be present any more:
Disk Type: Originally used to allow you to pick your hard disk from a predefined
list, this is now used to control automatic or manual parameter setup for the drive. (The
old tables describing ancient drives are now often not even present on many modern BIOSes,
since they no longer serve any purpose.)
Size: This is the size of the drive in . It is calculated from the other parameters.
Cylinders: The number of
on the disk. The value used depends on whether BIOS translation is enabled, in some
BIOSes. For a drive set to &Auto&, no number may be shown.
Heads: The number of logical heads on the disk. The value used depends on whether
BIOS translation is enabled, in some BIOSes. For a drive set to &Auto&, no
number may be shown.
Sectors: The number of logical 512-byte sectors in each logical track on the
disk. This is usually 63 for modern drives. Again, for a drive set to &Auto&,
you may not see a number here.
Write Precompensation: A compatibility setting that specifies at which cylinder
number write adjustments should be made, for very much older drives.
Landing Zone: The cylinder where the heads are parked by the BIOS when the drive
not used on modern drives since they automatically .
Translation Mode: The BIOS translation mode being used, for support of . .
Block Mode: Controls the BIOS's ability to perform disk transfers in blocks.
PIO or DMA Mode: The
used to perform transfers to and from the
hard disk.
32-Bit Transfer Mode: Controls the use of higher-performance .
The PC Guide
Site Version: 2.2.0 - Version Date: April 17, 2001 Charles M. Kozierok.&All Rights
Please read the
before using
this material.
Custom SearchThe Master Boot Record (MBR)
Why it is Necessary?
If you have arrived here through a search engine, and there's no menu to the left !
When you turn on your PC, the processor attempts to begin the process of processing data. But, since the system memory is empty, the processor doesn't really have anything to execute, or even begin to know where to look for it. To
ensure that the PC will always boot regardless of the BIOS code, both chip and BIOS manufacturers developed their code so that the processor, once turned
on, always starts executing at the same place, FFFF0h.
Similarly, every hard disk must have a consistent &starting point& where key information is stored about the disk, such as the number of partitions and what type they are. There also must be someplace where the BIOS can load the
initial boot program that starts the process of loading the operating system. The place where this information is stored is called the master boot record (MBR), also referred to as the master boot sector or even just the
boot sector. Do not confuse the master boot sector with , which are indeed different.
The master boot record is always located at cylinder 0, head 0, and sector 1, the first sector on the disk. This is the consistent starting point that the disk will always use. When a computer starts and the BIOS boots the machine, it
will always look at this first sector for instructions and information on how to proceed with the boot process and load the operating system. The master boot record contains the following structures:
Master Partition Table: This small bit of code that is referred to as a table contains a complete description of the partitions that are contained on the hard disk. When the developers designed the size of this
master partition table, they left just enough room for the description of four partitions, hence the four partition (four physical partitions) limit. For this reason, and no other, a hard disk may only have four true
partitions, also called primary or physical partitions. Any additional partitions must be logical partitions that are linked to (or are part of) one of the primary partitions. One of these partitions is marked as
active, indicating that it is the one that the computer should used to continue the boot process.
Master Boot Code: The master boot record is the small bit of computer code that the BIOS loads and executes to start the boot process.
This code, when fully executed, transfers control to the boot program stored on the boot (active) partition to load the operating system.
Obviously, due to the great importance of the information stored in the master boot record, should it become damaged or corrupted in some way, serious data loss often occurs. The master boot code is the first program executed when you turn on your PC, and is often the target of a virus.
In order to understand the Master Boot Record and the Master Boot Code buried
within it, it might be helpful to understand the sequence of events that occur
when your computer starts.
Although often overlooked as a diagnostic tool, the Boot Sequence performed by your computer can often tell you what may be wrong with the hardware itself should you be experiencing problems, even moderate ones. The following will take you through the System Boot sequence step by step. If you notice any thing unusual through each sequence of events, then you only need to look at the step where the delay appears to occur or back one step.
What occurs during the System Boot Sequence?
The system BIOS, a small bit of code inserted in the BIOS prom chip located on your computers motherboard, is what starts the computer running when you turn it on. Below we have outlined the typical sequence of events that occur during your computers startup process, although they will vary by the manufacturer of your hardware, BIOS, and the peripherals you have in the PC. Again, this is a typical sequence of events that occur when you turn on your computers power switch:
The internal power supply turns on, initializes and then takes a few moments to generate reliable power for the rest of the computer. If the power received by the motherboard's chipset, and subsequently the processor, is not within expected parameters, the chipset will generate a reset signal to the processor in the same fashion as if you were to touch the reset button. This will continue until the motherboard receives a Power Good signal from the power supply or you turn the system off because of a failed power supply.
After a Power Good signal is received, or after the reset button is released and there is confirmation of reliable power, the processor will be ready to start executing. When the processor first starts, it really has no idea what to do next as there is nothing at all in the memory to execute. Of course the processor designers are aware of this, so they pre-program the processor to always look at the same place in the system, the BIOS ROM, for the small bit of startup code to begin the boot process. This is typically located at memory location FFFF0h, or right at the end of the system memory. Developers locate it there in the event the size of the ROM has to be changed so as to prevent compatibility problems. Since there are only 16 bytes from there to the end of conventional memory area, this location contains just a &jump& instruction telling the processor where to go to find the real BIOS startup program.
The BIOS performs the . If there are any fatal errors, the boot process stops. If the POST is successful, the BIOS calls INT 19 (Interrupt 19) and then proceeds to look for devices attached to the motherboard.
The BIOS code begins its search by looking for a video card, more particularly, its looking for the video card's built in BIOS program, (normally found at location C000h in memory) and if found, runs it. The system BIOS executes the video card BIOS, which in turn initializes the video card. Most modern video cards will display information on the screen about the video card, which is why on some modern PC's you usually see something on the screen about the video card before you see the messages from the system BIOS itself.
Once video has been enabled, the BIOS begins searching for other devices that may have their own ROM and whether that ROM has its own BIOS code. Normally, the floppy drive is located at , and the IDE/ATA hard disk BIOS will be found at C8000h. If a floppy and/or hard drive is found, their codes are executed. If, during this INT 19 process, any other device BIOS's are found, they are executed as well.
The BIOS displays its , which provides some key information about the BIOS as well as other system information.
As the boot sequence continues, the BIOS continues to perform additional tests on the system. Depending upon the system manufacturer, this will usually include a memory count. The BIOS will generally display an error message on the screen if it encounters an error when it counts installed memory. You will find these error messages and their explanations in our
in the Motherboard and BIOS section or you can . (Clicking here will open a new window)
During the next phase of the BIOS startup process, it performs somewhat of an inventory of the hardware installed in the system, and then communicates or interrogates it to ensure that the hardware is functioning. Most modern BIOS's have automatic settings to collect information such as memory timing, based on what kind of memory it finds. Today's BIOS's dynamically set hard drive parameters and access modes, and will display a message on the screen for each drive they detect and configure in this way. It will also search for and label logical devices such as COM and LPT ports. Note: If the BIOS supports the Plug and Play standard, and the feature is enabled, this is the point at which it will detect and configure Plug and Play devices and display a message on the screen for each one found.
During the final phase of the POST and BIOS boot process, the BIOS will display a summary screen with your system's configuration. While early machines, 486 through Pentium II, were fairly accurate about the system configuration information, later machines using the later Pentium III and AMD processors can have some inaccurate information. This is usually related to BIOS setup issues involving processor information. Checking this data can be helpful in diagnosing setup problems, although it can be hard to see because sometimes it flashes on the screen and then scrolls off the top.
Once the BIOS finishes what it needs to do, it begins searching for a drive to boot an operating system. All BIOS's contain a setting that controls this search sequence for a boot drive. Most are set to first look for a bootable floppy disk, and if one is not found then proceed to a hard disk, which is usually the C: drive. Some BIOS's permit you to boot from your CD-ROM drive or other devices such as a SCSI (Small Computer System Interface), depending on the boot sequence selected. Once the BIOS identifies its target boot drive, the it looks for boot information to start the operating system boot process. If it is searching a hard disk, it looks for a master boot record at cylinder 0, head 0, sector 1, the first sector on the disk. If it is searching a floppy disk, it looks at the same address on the floppy disk for a volume boot sector.
Once the boot sector is found and its contents or data verified, the BIOS starts the process of booting the operating system by using the information in the boot sector. If this is a floppy disk boot sector, the information is read into memory at location
. INT 19 goes to memory location
to continue the process. If no boot sector is found on the floppy drive, INT 19 moves to the next bootable drive in the list provided by the motherboard BIOS, usually a hard drive, and then attempts to read the MBR. If a Master Boot Record is found, it is read into memory at location
and INT 19 jumps to memory location
the same as was the case with the floppy. At this point, the BIOS attempts to move control of the computer from the BIOS to the actual operating system.
Next, the small program in the Master Boot Record will attempt to locate an active (bootable) partition in the hard drives partition table. If such a partition is found, the boot sector of that partition is also read into memory at location
and then MBR program itself jumps to memory location . Keep in mind that each operating system has its own boot sector format. The next step involves the small program in the boot sector locating the first part of the operating system's kernel loader program, or in some cases the kernel itself or perhaps a boot manager program, and read then that into memory. For you Windows NT and Windows 2000 fans, this kernel loader is referred to as NTLDR. You will find a .
If no boot device of any type can be found, the system will display an error message and stop. The specific error message is depends on the BIOS developer and/or the computer's manufacturer, and can be anything from a rather clear &No boot device& to the very cryptic &NO ROM BASIC - SYSTEM HALTED&. This will also happen if you have a bootable hard disk partition but forget to set it active. Believe it or not, you can partition a drive, format it and install the operating system and never realize that there is problem until the first start of that operating system occurs.
This entire process is referred to as a &cold boot& (since the machine was off, or cold, when it started). A &warm boot& also known as a &soft boot& is the same thing except it occurs when the machine is rebooted using the Ctrl + Alt + Del keys. In this case the POST is skipped and the boot process continues at roughly step 8 above. As a side note, INT 19 is also called when the CTRL-ALT-DEL keys are used. On most systems, Ctrl + Alt + Del causes a soft-boot or shorten version of the POST to be executed before INT 19 is called.
Many things can damage the Master Boot Record, therefore it might be helpful for you to know where certain portions of the code reside in the MBR, should you have to recover the MBR manually.
You will find that:
The MBR program code starts at offset 0000.
The MBR messages start at offset 008b.
The partition table starts at offset 01be.
The signature is at offset 01fe.
If an active partition is found, regardless of where it is found, that partition's boot record is read into memory at
and then the MBR code is added to
pointing to the partition table entry that describes the partition being booted. The boot record program uses this data to determine the drive being booted from and the exact location of the partition on the disk. If no active partition table entry can be found, the boot process enters ROM BASIC via INT 18. All other errors will cause the system to hang or stop.
Notes (Extremely Important)
The first byte of an active partition table entry is 80.
This byte is loaded into the DL register before INT 13 is called to read the boot sector. When INT 13 is called, DL is the BIOS device number. Because of this, the boot sector read by this MBR program can only be read from BIOS device number 80 (the first hard disk). This is one of the reasons why it is usually not possible to boot from any other hard disk. Of course, there are exceptions to every rule, but those are beyond the scope of this exercise.
The MBR program uses the CHS based INT 13H AH=02H call to read the boot sector of the active partition. The location of the active partition's boot sector is in the partition table entry in CHS format. If the drive is 528MB, this CHS must be a translated CHS. No addresses in LBA form are used (another reason why LBA does not solve the 528MB problem).
Entire MBR record in hex and ASCII
OFFSET 0 1 2 3& 4 5 6 7& 8 9 A B& C D E F&
000000 fa33c08e d0bc007c 8bfffbfc *.3.....|..P.P...*
000010 bf1f2a5 ea1d0600 00bebe07 *................*
e80 3c0fe *.....t....u.....*
000030 cb75efcd 188b148b 4c028bee 83c610fe *.u......L.......*
000040 cb741a80 3c0074f4 be8b06ac 3c00740b *.t....t.......t.*
bb0700 b40ecd10 5eebf0eb febf0500 *V.......^.......*
000060 bb007cb8 010257cd 135f730c 33c0cd13 *..|...W.._s.3...*
f75edbe a306ebd3 bec206bf fe7d813d *Ou...........}.=*
aa75c7 8bf5ea00 7ce76616c *U.u.....|..Inval*
69 746162 *id partition tab*
c26f72 206c6f61 64696e67 *le.Error loading*
f9 6e37465 * operating syste*
d004d69 206f70
*m.Missing operat*
e4 656d0 *ing system......*
00 00000 *................*
0000f0 TO 0001af SAME AS ABOVE
00 08001 *................*
0060d fef83e00 d000000 *...........x....*
00 00000 *................*
00 00000 *................*
00 055aa *..............U.*
Disassembly of the MBR
This sector is initially loaded into memory at
but it immediately relocates itself to .
NOW AT , RELOCATE
disable int's
set stack seg to 0000
set stack ptr to 7c00
SI now 7c00
allow int's
clear direction
DI now 0600
move 256 words (512 bytes)
move MBR from
EA1D060000
jmp to NEW_LOCATION
NEW_LOCATION:
point to first table entry
there are 4 table entries
SEARCH_LOOP1:
SEARCH FOR AN ACTIVE ENTRY
BYTE PTR [SI],80
is this the active entry?
FOUND_ACTIVE
BYTE PTR [SI],00
is this an inactive entry?
NOT_ACTIVE
incr table ptr by 16
decr count
SEARCH_LOOP1
jmp if not end of table
GO TO ROM BASIC
FOUND_ACTIVE:
FOUND THE ACTIVE ENTRY
set DH/DL for INT 13 call
CX,[SI+02]
set CH/CL for INT 13 call
save table ptr
SEARCH_LOOP2:
MAKE SURE ONLY ONE ACTIVE ENTRY
incr table ptr by 16
decr count
jmp if end of table
BYTE PTR [SI],00
is this an inactive entry?
SEARCH_LOOP2
NOT_ACTIVE:
MORE THAN ONE ACTIVE ENTRY FOUND
display &Invld prttn tbl&
DISPLAY_MSG:
DISPLAY MESSAGE LOOP
get char of message
end of message
screen attributes
output 1 char of message
to the display
restore SI
DISPLAY_MSG
do it again
HANG THE SYSTEM LOOP
sit and stay!
READ_BOOT:
READ ACTIVE PARITION BOOT RECORD
INT 13 retry count
INT13RTRY:
INT 13 RETRY LOOP
read 1 sector
read sector into
restore DI
jmp if no INT 13
call INT 13 and
do disk reset
if not zero, try again
display &Errr ldng systm&
DISPLAY_MSG
jmp to display loop
INT 13 ERROR
&missing op sys&
point to signature
WORD PTR [DI],AA55
is signature correct?
DISPLAY_MSG
EA007C0000
JUMP TO THE BOOT SECTOR
WITH SI POINTING TO
PART TABLE ENTRY
Messages here.
........ ........ ......49 6e76616c *
*id partition tab*
6c26f72 206c6f61 64696e67 *le.Error loading*
206f9 6e37465 * operating syste*
6d004d69 206f70
*m.Missing operat*
696e4 656d00.. ........ *ing system.
Data not used.
........ ........ ......00
*................*
*................*
*................*
*................*
*................*
*................*
*................*
*................*
*................*
*................*
*................*
*................*
*................*
00 0000.... *............
The partition table starts at 0000:07be.
Each partition table
entry is 16 bytes.
This table defines a single primary partition
which is also an active (bootable) partition.
........ ........ ........ ....8001 *
0100060d fef83e00 d000000 *...........x....*
*................*
*................*
00 0000.... *............
The last two bytes contain a 55AAH signature.
........ ........ ........ ....55aa *..............U.*
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