Computer Boot Sequence

Version: 3.2
Last updated: 28 July 2003

Please note that:

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• If you do pilfer this web site and present it as your own then at the end of the day, you're only deceiving yourself. Also, although I'm not some huge multinational corporation that has a legal department with nothing better to do than generate work for itself, I might nevertheless "visit you in the small hours and put a bat up your nightdress!". [Basil Fawlty]

Mail me with complaints

Introduction

This page gives an overview of the sequence a computer goes through when it's switched on. It includes some detail on such elements as BIOS, the layout of a hard disk and the boot loading programs.

The information is based on Intel (including AMD and Cyrix) computers with a single disk (although I've started to add some information about more than one disk). The process is only slightly different with more than one disk, and quite a bit different on non-Intel platforms. Where there is information that is specific to more than one hard disk, I've said so explicitly.

Much of the information here has been gleaned from experimentation, and may therefore be an over-simplification or possibly inaccurate. If you find any such inaccuracies, please let me know.

I've used the phrase "GOTCHA!" when describing something that in some way or another is not how one would imagine things might work. (I'm sure you know what I mean :)   )

I've used greyscale to make identifying various parts of the hard disk easier. (Many people didn't like the colours.)

Finally, I'd like to thank the many people that have emailed me with corrections, encouragement or just popped in for a chat! So, if you've got any opinions, or suggestions, please let me know.

Version Control

Definitions

These definitions are not CCITT, RFC or any other internationally recognised definitions: I just made them up. However, their purpose is to ensure that we're all talking the same language.

The term "physical sector" actually means the physical sector as presented by the BIOS to the CPU. It is not the "real" physical sector as seen by the disk controller. (That is, it does not take into account translation methods such as Large Block Addressing.)

Hard Disk Geometry

Disk Geometry Overview

Let's start by getting an understanding of the way that a harddisk is laid out and the way that a computer accesses a hard disk.

GOTCHA! Many authors use 1 GB = 1000 MB (decimal gigabyte), others use 1 GB = 1024 MB (binary gigabyte). On this web site, I use the binary values:

• 1 MB = 1024 KB
• 1 GB = 1024 MB

Firstly, there is the real disk geometry and then there is the “real real” disk geometry! What does this mean? Basically, a hard disk contains a set of electromagnetic platters stacked on top of one another (a bit like many CDs in a stack) with a narrow gap between each platter. Each platter is usually double-sided, although for the purposes of this explanation, this makes no difference. Unlike an old vinyl record or CD, each platter has a set of concentric rings that contain the data. (Old vinyl records and CDs actually have very long spiral grooves. This makes for an excellent quiz question: how many grooves on a vinyl LP? Answer: 2; one on each side!) Each ring is then split into segments.

Remember that each platter is double sided, and each side has it's own read/write head (a bit like a vinyl record being read simultaneously on both sides by a needle). So, if you've got 4 platters, you've probably got 8 heads (beating Zaphod Beeblebrox by some considerable margin). Now, for the purposes of getting the terminology right, it's the number of heads which is important, rather than the number of platters. (After all, who cares if we've got 8 platters each with one head, or 4 platters each with 2 heads?)

So, each head reads from one of the concentric rings (technically called a "track") on the cylinder. Interestingly, all the heads move at the same time and are positioned to read or write to the same track on their respective platter. So, if head 3 is positioned to read from track 77, head 7 will also be positioned to read from track 77. Now, what do you get if you stack a load of tracks on top of one another? A cylinder! Therefore, we might say that head 3 is positioned to read from cylinder 77 (which implies that head 7 is also positioned to read from cylinder 77).

Finally, each track is split into small segments. Each segment is called a "sector".

Now that we have the description, let’s start to use the correct terminology.

If we wanted to access one particular sector, we could reference it by specifying which head it was on, which cylinder it was on and finally which sector it was on. That would then uniquely identify the sector that we wanted to access.

How the physical disk is accessed

Knowing which sector we want to access is only part of the story. How do we actually get the disk reader to the right position physically on the disk? Somewhere in the picture, there must be some software that “knows” that if, for example, you want to access cylinder 23 then you have to move the head reader 2.5434567cm from the edge of the platter. Fortunately, such code comes with the hard disk itself. It is called the “disk controller” and it allows us to specify only the Cylinder, Head and Sector that we want to access. The disk controller calculates where the data is physically on the disk and hands the data back to us (or more accurately a pointer in memory to the data).

If that were the end of the story, we’d have a recipe for disaster. For example, one manufacturer could produce a hard disk with 125336566 cylinders, 2 heads and 198 sectors per cylinder. Another manufacturer could produce a disk with 1 cylinder, 2624626 heads and 1 sector per head (though a strange disk indeed that would be!). Therefore, a set of standards called the ATA (AT-Attachment) came into being in 1989 that effectively put a boundary on what was possible in terms of cylinder, head and sector numbers. The disk manufacturers agreed to the standards, but continued to produce disks that didn’t necessarily meet those specifications! (This isn’t actually surprising or bad because the IDE specification allows hard disks to have more sectors on the outer tracks than the inner tracks. This is called “Zone Bit Recording” or “Zone Density Recording”.)  However, they compensated for the fact by getting the disk controller to “advertise” a set of parameters that met the standards, and then translated the sector requests into ones that were appropriate for the physical disk.

Therefore, only the disk controller “knows” the real real disk geometry. What most people call the real disk geometry is actually the geometry that the disk controller “pretends” to exist.

Real Geometry --> Disk Controller --> “Real real” geometry --> Physical disk

Because we don’t care what the “real real” geometry is, we can concentrate on the real geometry from now on!

The Standards

There are many standards for disk geometry, but the one pertinent to this discussion is the ATA standard. (Note: For details on individual variants of the ATA standard, such as ATA-5, see http://www.ncits.org/.) This says (amongst other things) that:

1. The cylinder number must be representable by 16 binary digits.
2. The head number must be representable by 4 binary digits.
3. The sector number must be representable by 8 binary digits.
4. There are 512 bytes per sector.

This means that the maximum specification of an IDE disk that conforms to the ATA standard is as follows.

Here's an example of a disk that meets the ATA specified limits:

Cylinder 0

Cylinder 1

Cylinders 2 to 6779

Cylinder 6780

How we used to access the disk: CHS

The original method for accessing a disk is based on asking the disk controller for the data in the appropriate Cylinder, Head and Sector. This is called the “CHS” method.

The software that we use to access the disk is called the BIOS. The BIOS is discussed in more detail later (The BIOS). However, for now, all that we need to know is that the authors of the BIOS very kindly wrote the software to communicate with the disk for us. All we have to do is tell it that, for example, we’re after Cylinder 12345, Head 3 and Sector 243 and then go off and fetch it. This piece of software within the BIOS that does the work is called an “interrupt service routine” (or “INT” as most people say). The BIOS actually contains a fair number of INTs, but the one that we’re interested in is interrupt number 19 (or 13 in hexadecimal and usually written as INT13h).

So, in assembler code, we load the CH register with the 8 low-order bit of the cylinder number, the top 2 MSB of the CL register with the remaining 2 high-order bits of the cylinder number (to give us 10 bits for the cylinder), the 6 LSB of CL is loaded with the sector number (to give us 6 bits for the sector) and DH is loaded with the head number (to give us 8 bits for the head). We load AH with the function (such as read or write) that we want to perform and then simply “run” (or call) INT13h. (In the section on the Master Boot Record, you’ll see that the Partition Table layout exactly matches this register-loading scheme. This saves time when making calls to INT13h.)

In summary, therefore, this is what happens:

O/S --> INT13h --> CHS call --> Disk Controller

Consequently, the requirements for this type of disk access are as follows:

• An operating system that can make INT13h calls.
• A BIOS that understands INT13h calls.
• A BIOS that can make CHS calls.
• A disk controller that understands the CHS values referenced.

To ensure that one INT13h was the same as any other INT13h, a set of standards were defined to ensure that the INT13h call always did the same thing independently of who wrote the BIOS. The standards are based on IBM’s BIOS for PCs written in 1981. You might think that the ATA standard (1989) matches the INT13h standard (1981). Wrong! The INT13h standard says that:

1. The cylinder number must be representable by 10 binary digits.
2. The head number must be representable by 8 binary digits.
3. The sector number must be representable by 6 binary digits staring at 1, not 0.
4. There are 512 bytes per sector.

As mentioned earlier, the Partition Table has the same set of restrictions because its layout is based on INT13h.

This means that the maximum call that can be made using INT13h is as follows.

The lowest common denominator (LCD) between INT13h and the ATA standard represents the actual calls that we can make using CHS.

So, using CHS, only 504 MB can be accessed using this method. Until 1994, this was indeed the case: any space beyond 504 MB was lost because it simply could not be referenced. In 1994, the idea of “translation” was invented (or more accurately, put into use) whereby the real disk geometry is not that which is presented to the operating system.

Note that:

• Physical sector numbering starts with 1 whereas the other values start with 0.
• The disk fills up in the following order: Sectors, Heads, and Cylinders.
• Each physical sector has 512 bytes.

How we used to access the disk (and some still do): ECHS

The ATA standards are here to stay. Fortunately, the BIOS coders agreed on a way round the 504 MB limitation. They came up with a system called “Enhanced CHS”, or ”ECHS”.

ECHS is a simple translation method written on the end of the interrupt 19.

In summary, this is what happens:

O/S --> INT13h --> ECHS Translation --> CHS call --> Disk Controller

Consequently, the requirements for this type of disk access are as follows:

• An operating system that can make INT13h calls.
• A BIOS that understands INT13h calls.
• A BIOS that can perform the ECHS translations.
• A BIOS that can make CHS calls.
• A disk controller that understands the (translated) CHS values referenced.

As far as we’re concerned, the ECHS translation is hidden from us, so, like the disk controller, we don’t really care what it actually does to help. However, an explanation follows on what ECHS actually does.

Remember that when we make our INT13h calls, we must stick within the C:1024, H:256, S:63 boundary. So, all that ECHS does is to divide the head call that we make by either 2, 4, 8 or 16 and multiply the cylinder call that we make by the same amount. The actual multiplier (2,4, 8 or 16) that we use is chosen by the BIOS when we first install the hard disk and tell the BIOS about it. (In fact we don’t directly get to see the multiplier, but you can calculate it manually.) From there on, the multiplier doesn’t change. (Note also that the sectors are never translated and that we can assume for the purposes of the following sections that there are always 63 sectors per cylinder. Although the ATA standard allows for less (whilst still staying within the constraints of INT13h), in practice all disks present 63 sectors, and anyway if it’s never translated, it’s not very interesting to discuss!

Mathematicians may note that there is potential for loss of information because when we divide the head number that we’re after by, for example, 8, we lose the remainder of the calculation. Here’s a demonstration of this, using a multiplier of 8 as an example. Remember that we divide the head by 8 and multiply the cylinder also by 8.

As you can see, if we make an INT13h call to C1, H1 we get given back the data in the real C8, H0. If we then want to access C1, H2 using INT13h, we also get given back the real C8, H0! Not only would this be disastrous, also note that the real cylinders 1 to 7 never get used! So, as far as I am aware, what actually happens is that the translation calculates the head first but saves the remainder of the division in the process. It then calculates the cylinder and adds to this the remainder that it saved earlier on. This is what we get.

If you were to make a complete table for a sample hard disk, you would see that:

1. We have still stayed within the INT13h constraints.
2. We are using “all” the space on the hard disk (with an exception noted below).
3. We are not losing data.

It is interesting to note that if I’m right about the division process, the real disk fills up in a rather curious order:

Head 0, cylinders 0 to 7; then head 1, cylinders 0 to 7; then head 2, cylinders 0 to 7 all the way up to head 15, cylinder 0 to 7. Then we go back to head 0, cylinders 8 to 15; head 1, cylinder 7 to 15 and so on.

However, because the disk controller is written to take this in consideration, we do not lose performance as a result of this disk-filling order.

The ECHS translation system still leaves 3 interesting unanswered questions.

1. Is it still possible to lose disk space as a result of ECHS?
2. How does the BIOS decide which multiplier to use?
3. What is the maximum disk size with ECHS taken into consideration?

The first thing to remember is that when the BIOS records the disk geometry in the CMOS, it records the limits that INT13h calls can make and also the translation method to be used. Because fractions aren’t allowed, the INT13h maximum values stored are always rounded down after applying the multiplier to the disk geometry. (If they were rounded up, it would be possible to make INT13h calls that, after translation, referenced hard disk CHS values that didn’t exist on the physical disk.)

Let’s take a sample real hard disk geometry with 63 sectors per cylinder that fits within the ATA standard of:

• Cylinders: 3599 (numbered 0 to 3598)
• Heads: 16 (numbered 0 to 15)
• This gives us a total of 3627792 sectors. (A 1.7 GB disk, approximately.)

The following table shows various multipliers applied to this geometry.

*Because the cylinder value is 1799, which is too large for INT13h, this option is not used.

The 16, 8 and 4 multipliers are all valid, but as can be seen from the table, the 4 multiplier wastes less space on the disk. Fortunately, it also ends up being the chosen multiplier! So, the rule for choosing the multiplier is: make the multiplier is as low as it can be to bring the cylinder and heads within the range permitted by INT13h (that is, 1024 cylinders numbered 0 to 1023). To achieve this, the BIOS does a simple loop (with some added error checking):

• Multiplier = 1
• Cylinder = Cylinder -1
• Is Cylinder < 1024? If not:
• • Do a right bitwise rotation on the cylinder (i.e., divide by 2)
  • Do a left bitwise rotation on the multiplier (i.e., multiply by 2)
• Use the multiplier on the Cylinder and Head values to obtain the translated values.

At the end of this loop, the multiplier will be as small as it can be to bring the cylinder to within the permitted range.

The following table gives other translation examples based on this translation algorithm. I’ve deliberately shown values that occur at “change points” and “end points”. (This is because all other web sites that I have found only demonstrate using cop-out mid range values that don’t identify what happens at the limits.)

As can be seen, the maximum number of cylinders that can be translated is 16384. The translated geometry of C:1024, H:256 gives a maximum translated disk size of 8,455,716,864 bytes.

GOTCHA! The ATA standard actually specifies that cylinder 16384 be reserved “to support the legacy of a maintenance cylinder”. So in practice the maximum translated geometry is: C:1023, H:256, which gives a slightly lower maximum translated disk size of 8,447,459,328 bytes. You may find both values quoted as the famous “8 GB limit”.

This answers the three questions.

So, using ECHS, only roughly 7.8 GB can be accessed using this method. Any space beyond 7.8 GB would be wasted because it could not be referenced. This explains why the largest partition that Windows NT can be installed into is 7.8 GB: Windows NT relies on ECHS and INT13h during installation time to access the disk. (Once the O/S is loaded, other factors come into play. More of this later.)

How did ECHS affect DOS and Windows 95?

Unfortunately, DOS and Windows 95 cannot address a disk with 256 heads because the total number of heads (NOT the maximum value) is stored in 8 bits, giving a total number of heads of 255, not 256. The maximum translated cylinders value is therefore only 128 heads when ECHS is used. So, when running DOS and Windows 95 (but not Windows NT), the maximum space that can be addressed is 1024 cylinders x 128 heads = 4,227,858,432 bytes. This is described as the “4 GB limit”.

Two alternative methods were devised to resolve this issue:

1. Revised ECHS
2. LBA Assist (or Assisted LBA)

GOTHCHA! The term “LBA Assist” has very little to do with the similar term “LBA”. The former is a method of overcoming the fact that DOS and Windows can’t access a disk with 256 cylinders. The latter is a completely different translation mechanism that is described in detail later. Why use the term “LBA Assist” then? For now, it suffices to say that it’s called that because an LBA Assist translated drive is always accessed using LBA, not using any of the CHS variants. However, not all LBA accessed disks require LBA Assist translation. Mathematically speaking, LBA Assist is a subset of LBA.

How we used to access the disk: Revised ECHS

A simple way around the 256 head problem was to do a small “pre-translation” on the disk geometry when the number of heads exceeds 8192 that converts the number of heads from 16 to 15.

In summary, therefore, this is therefore what happens:

O/S --> INT13h --> ECHS Revision --> ECHS Translation --> CHS call --> Disk Controller

Consequently, the requirements for this type of disk access are as follows:

• An operating system that can make INT13h calls.
• A BIOS that understands INT13h calls.
• A BIOS that can perform the Revised ECHS translations.
• A BIOS that can make CHS calls.
• A disk controller that understands the (translated) CHS values referenced.

How is this done? The process of “pre-translation” is simple.

• If cylinders > 8192 and heads = 16
• • Heads = 15
  • Cylinders = cylinders * 16 / 15 (losing the fraction component)
  • Do a standard ECHS translation

So, going back to the previous table, but this time with Revised ECHS. Note that I’ve changed the last two rows to show the new limits imposed by this scheme.

Therefore, using revised ECHS, we can address a maximum of 1024 cylinders x 240 heads: 7,927,234,560 bytes ~ 7.4 GB. Although this is less than non-revised ECHS, it does allow DOS and Windows 95 to access far more disk space.

How we used to access the disk: Assisted LBA

The other simple way around the problem was to use a modified version of ECHS, whereby we ensure that instead of 256 heads being the largest, use 255 instead. Amazingly simple really! This means that the following translated head values are allowed: 1,2, 4, 8, 16, 32, 64, 128 and 255.

So, this is what happens with Assisted LBA (and note the LBA call):

O/S --> INT13h --> LBA Assist Translation --> LBA call --> Disk Controller

Consequently, the requirements for this type of disk access are as follows:

• An operating system that can make INT13h calls.
• A BIOS that understands INT13h calls.
• A BIOS that can perform the LBA Assist translations.
• A BIOS that can make LBA calls. (Described later.)
• A disk controller that understands the LBA values referenced. (Described later.)

The BIOS achieves assisted LBA using the following algorithm.

• If cylinders > 8192
• • Variable CH = Total Sectors / 63
  • Divide (CH – 1) by 1024 (as an assembler bitwise right shift) and add 1
  • Round the result up to the nearest of 16, 32, 64, 128 and 255. This is the value to be used for the number of heads.
  • Divide CH by the number of heads. This is the value to be used for the number of cylinders.

So, back to the ECHS table above, but this time with LBA Assist. I’ve changed the last two rows again to show the new limits imposed by this scheme.

Therefore, using Assisted LBA, we can address a maximum of 1024 cylinders x 255 heads: 8,422,686,720 bytes, which is 495,452,160 bytes more than Revised ECHS. This is certainly more disk-efficient than Revised ECHS!

How we used to access the disk: LBA

We’re nearly at the part that explains when happens with hard disks these days! So far, I’ve discussed CHS, ECHS, Revised ECHS, LBA Assist and also mentioned in passing LBA where I said that Assisted LBA as a translation mechanism relies on a disk-accessed method called “LBA”.

We’ve hit a number of barriers in the size of hard disks, the last two of which were around the 8 GB mark. Overcoming this limit could, in theory, be achieved by using ECHS multipliers of 32 or 64, which would keep the INT13h calls within C:1024 and H:256 and the ATA calls within C:65536 and H:16. However, a more radical approach was taken which ultimately (thought not initially) changed the way that hard disks were accessed. This radical solution was in two complimentary parts. The first part of the radical solution is Logical Block Addressing (LBA). (The term “block” can be used synonymously with “sector”.) The second part of the radical solution is described later.

The five most important things to note about LBA are:

• LBA is not the same as LBA Assist. The latter is a subset of the former and is a translation method based on the CHS addressing scheme.
• LBA was not (as many web sites claim) invented to overcome any disk size limits. In fact, there is nothing intrinsic to LBA that allows size limits to be overcome. It is simply a different way of accessing the hard disk.
• Unlike CHS and ECHS, both of which require the BIOS to know the disk geometry (the cylinder, head and sector values) in order to access the disk, LBA does not require the BIOS to know anything about the disk geometry to access the disk.
• The operating system still has to make INT13h calls and is therefore still bounded by the old INT13h limits.
• LBA has been around since around 1981. It is the method used to access SCSI drives. However, in the mid-1990s IDE disks finally caught up with SCSI and started to use LBA as well.

Basically, LBA numbers the sectors from 0 upwards in a linear sequence (e.g. from 0 to 876458). Simple really! It “sits” between the INT13h and the disk controller.

The LBA standard says (amongst other things) that:

1. The logical block address must be representable by 64 binary digits.
2. (Nothing else relavant to this explantion!)

Without doing the sums, it should be clear that LBA has a fair amount of longevity built into it. E.g. my 60 GB disk would use up only 27 of the 64 digits available!

So, this is what happens with LBA:

O/S --> INT13h --> LBA Assist Translation --> LBA call --> Disk Controller

Notice that this sequence of events is the same as in the previous section and is just repeated here for completeness.

Consequently, the requirements for LBA disk access are as follows:

• An operating system that can make INT13h calls.
• A BIOS that understands INT13h calls.
• A BIOS that can perform the LBA Assist translations.
• A BIOS that can make LBA calls.
• A disk controller that understands the LBA values referenced.

GOTCHA!: Note that the operating system is still making INT13h calls and therefore the extra capacity built into LBA cannot be referenced using this scheme. This is why LBA per se does not overcome any disk size limits.

Before looking at the formula, and its implications, for LBA, it is worth answering the question of what happens when the BIOS and hard disk support both ECHS and LBA and the hard disk is less than 8 GB? In theory, either system could be chosen. The answer is that it depends on the BIOS. Some BIOSes will always select LBA if it can (for reasons that will be explained later). Other (particularly older BIOSes) will only select LBA if you ask them to. It is often said that LBA is faster than ECHS. This is not necessarily true. Both translation systems have to perform calculations based on the requested CHS values. However, due to certain O/S features, LBA has the potential to be faster, but this is more a function of the O/S than LBA itself.

The formula used to translate the CHS value into an LBA value is as follows. Firstly, the total disk size.

• LBA(Total) = C x H x S (as expected)

Next, the referenced sector:

LBA(Referenced) = (s – 1) + (h x S) + (c x S x H)

Where:

There are 2 important implications of LBA:

1. The order that the disk is filled is: Sectors, then Heads then Cylinders. This is what you’d probably expect to happen.
2. No (or very little) space is wasted in LBA translation: the total number of blocks is simply the product of the cylinder, head and sector values.

The disk size restrictions are identical to the previous section on LBA Assist, but are repeated here for completeness.

Therefore, because we’re still using Assisted LBA, we can address a maximum of 1024 cylinders x 255 heads: 8,422,686,720 bytes.

LBA Example

We can now complete the picture of LBA access using INT13h calls. We’ll use a disk with the following real geometry as an example:

• Cylinders: 16400
• Heads: 16
• Sectors: 63

We’ve told our BIOS to use LBA Assist as the translation method (which implies that we’ll also use LBA as the disk access method, of course).

The LBA Assist will present a disk geometry of the following to the operating system.

• Cylinders: 1024
• Heads: 255
• Sectors: 63

Clearly, we’ve lost the last 80 real cylinders because the number of cylinders exceeded the maximum of 16320. This is an example of why disk space is lost with operating systems that rely on INT13h calls. Anyway, let’s continue.

The operating system requires, for example, to read from the following location.

• Cylinder: 512
• Heads: 200
• Sectors: 44

It loads the appropriate CPU registers (described here) and makes the INT13h call.

The BIOS receives the calls and performs the LBA address calculation:

LBA = (44 – 1) + (200 x 63) + (512 x 63 x 255) = 8,237,923

The BIOS makes an LBA call of 8,237,923 to the disk controller and retrieves the data.

How we now access the disk: LBA and Extended INT13h

At last we’ve got to the section that explains what happens in modern PCs, which is the second part of the radical solution referenced above. In every situation described above, there has been an 8 GB (roughly) limit on the hard disk size. The limit is due to the limitations of INT13h. With the best translation systems available, the 8 GB limit still exists. Therefore, a fundamental change was required: the BIOS Extensions. In combination with LBA, the BIOS Extensions allows disks greater than 8 GB to be accessed.

The BIOS extensions are a series of additions to the “old” Interrupt Service Routines which allows wider functionality. In the case of hard disk access, we’re interested in the “INT13h Extensions”.

The INT13h extensions allow the full 64 bit LBA address to be accessed directly rather than having to mess around with CHS addresses. Because CHS values are no longer being used, the INT13h limits are no longer an issue.

So, in assembler code, we load the DS:SI pointer register with the address of a 16 byte “address packet”. The address packet contains a few things, but most importantly, the last 8 bytes specify the LBA sector that we wish to access. (Remember that an LBA address is 64 bits, or 8 bytes.) We load AH with the function (such as read or write) that we want to perform and then simply “run” (or call) INT13h. The AH that we specified in this call differs from the AH value that we would use in a standard CHS call.

So, this is what happens with LBA and INT13h Extensions in combination:

O/S --> INT13h Extended call --> LBA call --> Disk Controller

Note the absence of CHS values.

Consequently, the requirements for LBA/INT13h Extended disk access are as follows:

• An operating system that can make Extended INT13h (LBA) calls.
• A BIOS that understands Extended INT13h (LBA) calls.
• A BIOS that can make LBA calls.
• A disk controller that understands the LBA values referenced.

Therefore, in order for a disk of more than 8 GB to be recognised, all three of the operating system, the BIOS and the disk itself must all understand LBA addressing. Also, the operating system and the BIOS must also understand INT13h Extensions.

Does this means that the operating system doesn’t even get to see and CHS values? Actually, no, it doesn’t mean that. The disk controller will always present a set of CHS values to the BIOS even when they don’t actually get used. The reason is that the controller cannot make any assumptions on the capabilities of the operating system (though it does find out what the BIOS is capable of), which may not have LBA/INT13h Extensions capabilities.

The CHS values used follow a set of rules defined in the more recent ATA standards. (In fact, the set of rules have changed through the various revisions of the ATA standards. The most recent is presented here.)

1. Any attempt to access beyond 15,481,935 sectors must be done using LBA. (Interestingly, 15,481,935 = 63 x 15 x 16383.) This rule keeps us well clear of any Revised ECHS or LBA Assist limits.
2. If 1,032,192 (504 MB) < Total Sectors <= 16,514,064 (~7.8 GB), the maximum value for the Cylinders is 16383 (assuming that the Heads is 16). (Interestingly, 16,514,064 = 63 x 16 x 16383.) This rule basically says that if the disk is within the 7.8 GB limit, the number of cylinders can be calculated accordingly to minimise lost space.
3. If Total Sectors > 15,481,935 and the device supports CHS (which it invariably will), then Cylinders = 16383. This rule sets a safe upper limit on the number of cylinders for CHS access.
4. If Total Sectors <= 8,257,536, then 3 <= Heads <= 16. This rule in effect says that it’s OK to use 128 heads and 1024 cylinders on disks that are under the 4 GB limit.
5. If Total Sectors > 16,514,064, then 3 <= Heads <= 15. This rule says that for disks above the 7.8 GB limit, only advertise a maximum of 15 heads. Combining this rule with the 3rd rule above: if the disk is more than 7.8 GB, then 15 heads and 16383 cylinders will be advertised.
6. If Total Sectors > 1,032,192, then Sectors = 63. Combining this rule with the 3rd and 5th rules above, if the disk is more than 7.8 GB, 15 heads, 16383 cylinders and 63 sectors will be advertised. This sets an upper limit of 7,926,750,720 bytes (~7.38 GB) when the disk is more than 7.8 GB but the operating system doesn’t support LBA. Many questions have been asked about this limit on the news groups.

Once the disk controller has “decided” which CHS values to advertise, it hands them to the BIOS that then does an LBA Assisted translation on them for un-extended INT13h backwards compatibility. Despite this set of convoluted rules, an operating system that does understand LBA (e.g. Windows 2000) is not “fooled” by these limits.

For example, if we take my own IBM 75GXP 45 GB disk as an example.

If I had an O/S that didn’t understand LBA, the O/S would see the values in the “BIOS: LBA Assist” column. However, because it does understand LBA, it can calculate the “correct” cylinder value from the Total Sectors.

In summary, because we’re still can address 2^64 sectors using LBA and the Int 13h Extensions, we can address a maximum of 2^64 sectors: 9,444,732,965,739,290,427,392 bytes (given a 512 byte sector)!

Better than LBA and Extended INT13h: Direct disk access

Finally, a brief word on bypassing the BIOS altogether. Firstly, recapping the previous section, this is what happens with LBA and INT13h Extensions in combination:

O/S --> INT13h Extended call --> LBA call --> Disk Controller

Why bother with the BIOS at all if the LBA value is send straight to the BIOS and out the other side unchanged? The only reason is if the operating system doesn’t have the code within it to talk directly to the disk controller. These days, this is actually quite unusual. For example, Windows 2000 loads the drivers necessary to communicate with the disk controller directly. Consequently, the operating system can bypass the BIOS altogether (only once it has loaded to disk drivers, of course). Before the O/S is loaded, disk access is still done through the BIOS INT routines.

So with direct disk access, we have:

O/S --> LBA call --> Disk Controller

This is the fastest way to access the hard disk. (OK, it is possible to use Direct Memory Access, DMA, which is even faster, but this is really more to do with the CPU and memory than hard disk access methods and is therefore outside the scope of this document.)

Final Words on Disk Access

When going into the BIOS setup, the following disk access options are seen: NORMAL, LARGE and LBA. The following table shows what each option does.

Note the following:

1. It is not possible to differentiate between ECHS or Revised ECHS in the BIOS setup. All modern BIOSes will do “Revised ECHS”.
2. LARGE can access the disk using either CHS or LBA. Generally, if the hard disk supports LBA, the BIOS will use LBA to communicate with the disk.

Having said all of that, it's quite difficult to explain the layout of a hard disk just using the LBA address. Therefore, in explaining disk layout, I'm going to revert back to CHS terminology:

Cylinder 0

Cylinder 1

Cylinders 2  to  845

Cylinder 846

The BIOS

For the purpose of booting up the computer, the BIOS is used for three main functions:

1. Providing a set of machine code subroutines that can be called by the operating system and whose function is to access the hardware components of the computer such as hard disk.
2. Initiating the boot sequence after a hardware Reset.
3. Allowing changes to the low level setup options.

The BIOS code is burned onto a Flash EPROM memory chip which is installed on the motherboard of the computer. It is not possible to modify the BIOS code. However, the BIOS code can usually be upgraded by obtaining the latest BIOS code from the company that wrote the code (such as Award or AMI), or better still from the company that manufactured the motherboard. (It is not recommended to upgrade the BIOS unless a specific problem relating to the BIOS is being encountered.)

The setup options are those that specify such things as the primary boot device, memory speed etc. They are accessed by pressing an appropriate button just after the PC is switched on. (For example, the DEL key.)

The machine code routines are contained within the BIOS. As the size of each routine differs from one BIOS to the other, a method of accessing those routines has been devised that means that the programmer does not need to know the location of the routines in memory. Instead, they are accessed by issuing a numbered software interrupt to the CPU. The number of the software interrupt issued tells the CPU which subroutine to execute. How does the CPU know where the code is in memory? The BIOS very kindly loads an "interrupt vector table" into memory, which is a mapping between the interrupt number and the location of the corresponding routine in memory. An example of such a routine is one that allows access to the hard disk: interrupt routine number 13, usually called INT 13. (More on Interrupts in a later version.)

So what happens when the computer is switched on (or more accurately, when a "Reset" is issued, as might happen when the reset button on the front of a PC is pressed)?

When a Reset is issued, components on the motherboard wait until the voltage is steady. During this time, they hold a Reset signal to the CPU to prevent it executing code. When the power supply indicates that it is steady, the Reset signal on the CPU is turned off, which allows the CPU to begin executing code. As there is nothing in memory, how can it execute code? Intel have designed their CPUs always to begin execution of code at address FFFF0. As there is only a small amount of memory between FFFF0 and FFFFF, the first instruction is a jump instruction to the main part of BIOS code which could be "anywhere" else. As well as testing and initialising the hardware in the Power On Self Test (POST), the BIOS uses INT 13 to initiate the boot sequence from the hard disk (or whatever device is specified in the BIOS setup).

In summary, the following happens when the PC is switched on and the power is steady:

• ROM BIOS (BIOS) initiates Power On Self Test (POST).
• The BIOS determines the "boot device" - normally a hard disk.
• The BIOS loads the contents of the first physical sector of the hard disk into memory (the MBR) to location 7C00 through to 7DFF.
• The BIOS instructs the CPU to execute the MBR code by issuing a jmp to location 7C00.

The Master Boot Record

The MBR is always located at Cylinder 0, Head 0, and Sector 1. Let’s look again at the first cylinder (called cylinder 0).

Cylinder 0

The Cylinder 0, Head 0, Sector 1 light grey box represents the location of the MBR. The "black" boxes in Cylinder 0, Head 0, Sectors 2 to 63 inclusive are unused. (This unused space is peculiar to Cylinder 0 only. Consequently, "data" starts from Cylinder 0, Head 1, Sector 1.

Remembering that the MBR is 512 bytes, let’s expand the light grey box and look inside.

The MBR code can occupy up to 446 bytes. An example of a MBR as written by Microsoft is as follows (and note that in this case, it fits into 440 bytes):

FA 33 C0 BE D0 BC 00 7C 8B F4 50 07 50 1F FB FC BF 00 06 B9 00 01 F2 A5 EA 1D 06 00 00 BE BE 07 B3 04 80 3C 80 74 0E 80 3C 00 75 1C 83 C6 10 FE CB 75 EF CD 18 8B 14 8B 4C 02 8B EE 83 C6 10 FE CB 74 1A 80 3C 00 74 F4 BE 8B 06 AC 3C 00 74 0B 56 BB 07 00 B4 0E CD 10 5E EB F0 EB FE BF 05 00 BB 00 7C B8 01 02 57 CD 13 5F 73 0C 33 C0 CD 13 4F 75 ED BE A3 06 EB D3 BE C2 06 BF FE 7D 81 3D 55 AA 75 C7 8B F5 EA 00 7C 00 00 49 6E 76 61 6C 69 64 20 70 61 72 74 69 74 69 6F 6E 20 74 61 62 6C 65 00 45 72 72 6F 72 20 6C 6F 61 64 69 6E 67 20 6F 70 65 72 61 74 69 6E 67 20 73 79 73 74 65 6D 00 4D 69 73 73 69 6E 67 20 6F 70 65 72 61 74 69 6E 67 20 73 79 73 74 65 6D 00 00 00 00 00 00

For a disassembled version of the MBR code, go to Ray Knights' Windows 95b Boot Sector page.

It is important to remember that although Microsoft wrote the above code, the code is not specific to any operating system. That is, it could be used to load Linux for example. So is the code any good? Well, it's very basic as it doesn't interact with the user! There are much better MBR codes freely available on the Internet. Two examples which interact with the user allowing them to select a Boot Sector code to load are:

• The Linux Loader LILO program - Credits to Werner Almesberger
• Ranish Partition Manager - Credits to Mikhail Ranish: Very Highly Recommended (I use it!)

The four Partition Table entries come after the MBR code. Each Partition Table entry is 16 bytes only. Using the second Partition table entry as an example, the layout of a partition table entry is as follows.

The Master Boot Record Code

Now that we know how a hard disk is laid out and what is in the MBR, we can take a look at what happens when the MBR code is executed.

The MBR code makes use of INT 13 to read data from the hard disk when the PC is switched on. (Once the operating system is loaded, the method of accessing the hard disk can change depending on the operating system.)

• The MBR code looks at the "partition table" using INT 13 calls to find the first (there is normally only one) entry that is marked bootable.
• The MBR code identifies the physical location of the File System Boot Sector from the partition table entry.
• The MBR code transfers itself to location 0600 through to 07FF (from location 7C00) in memory and continues execution from there.
• The MBR code transfers the whole File System Boot Sector into memory to location 7C00 through to 7DFF.
• The MBR code instructs the CPU to execute the File System Boot Sector code.
• The File System Boot Sector code executes.

The actions from here on are File System dependent. Firstly, we need to look at the Boot Sector itself in more detail.

The Partition Boot Sector and Clusters

From here on, we are going to assume that the File System is FAT16.

Before looking at how the partition boot sector is laid out, we first have to understand "clusters". This is because clusters are referenced (or more accurately, the number of sectors per cluster is defined) in this part of the hard disk.

A cluster (or "allocation unit") is a contiguous collection of sectors. The number of sectors that make up a cluster is definied in the Partition Boot Sector. Consequently, it is constant within a partition. That is, if the Partition Boot Sector defines that there are 32 sectors per cluster, then there are always 32 sectors within each cluster within the partition. (Purists may note that technically, this is only true of 512 bytes sectors. However, that is almost exclusively the case these days anyway.) Other partitions, may have different numbers of sectors per cluster.

What is the significance of a cluster? A cluster is the smallest amount of disk space that a Microsoft operating system can reference. Put another way, Microsoft operating systems do not access sectors directly. Instead, they access "clusters". The key things to remember here are:

• There are many sectors per cluster. The exact number of sectors that make up a cluster is defined in the Partition Boot Sector.
• The sectors must be contiguous to form a cluster.
• Clusters themselves are adjacent and numbered in sequence, with no wasted space between the clusters. This is covered in more detail in the section The FAT in Detail.
• Microsoft operating systems see clusters, not sectors. This is for a number of reasons. The main one is that the pre-NT operating systems used 16 bits to reference a cluster. This limitation may have been because, as will be discussed later, FAT 16 also uses 16 bits to specify the number of clusters in a partition (hence the name). This meant that the highest numbered cluster that could be referenced was 1111111111111111 (binary) = 65535. Now, if the operating system accessed sectors and not clusters, it would only be able to access 512 * 65536 (as we're starting counting at 0) bytes = 32 MB. That is, the maximum disk size would be a pathetic 32 MB! If, however, sectors are clumped together into groups of 4, for example (4 sectors per cluster), then the operating system would be able to access 128 MB, and so on.
• NT, 2000 and XP use 64 bits to reference a cluster, even if the file system doesn't support this many bits. Thus, even if we specify 1 cluster per sector, the operating system could still reference 2^64 * 512 bytes = 8,192 EB (provided, of course, the file system also supported 64 bits to reference clusters),! It's not as large as that in practice due to other limits. For example, NT also uses 64 bits to store the file size in bytes. This immediately brings down the maximum file size to 2^64 bytes = 16 EB, whatever the file system. Moreover, as mentioned in the previous section, The Master Boot Record, 4 bytes are used to specify the number of sectors in the partition. This factor limits the size to 2^32 * 512 bytes = 2 TB. Windows 2000 and XP have a method called "Dynamic Volumes" to push this limit up to 256 TB. A later version of this document will discuss FAT 32, NTFS and how these affect volume and file size limits in more detail.
• Files are broken up into small pieces, each piece fitting neatly into a cluster, with any unused cluster space going to waste. As is discussed in The FAT in Detail, the clusters that make up a file do not have to be in order or contiguous. This is covered also in the section The FAT in Detail.

Continuing our discussion of the Partition Boot Sector, to find out how the boot sequence continues on a FAT 16 partition, we need to look at how the FAT 16 File System Boot Sector is laid out. The File System Boot Sector, like the MBR, sits within a single Physical Sector and is located at the very start of the partition. Taking a look at the first Cylinder (Cylinder 0) we can see where the File System Boot Sector of the