What Is the 528 Million-Byte HD Problem ?

Newly Revised and Edited for The Web (HTML)
�1997 by Daniel B. Sedory

First, I want to make sure that you are not confused by the two numbers,
504 and 528 (This should be useful to everyone with a computer):
• 504 Mega-Bytes ( 528,482,304 Bytes exactly) is how most system utility
  programs, such as FDISK, refer to our first "barrier number."

• 528 MB ("Million-Bytes") (528,000,000 bytes) is an approximation of this number, and is how most hard drive manufacturers refer to this "barrier."

  "So, why the difference ?" :

  All Hard Drive manufacturers refer to their drive sizes using what I prefer they would call "Million Bytes," or now even "Billion Bytes." However, they still wish to use the same words employed by Programmers and the Memory manufacturers: "Megabytes" and "Gigabytes" even though they are using different values!

  On the box of any hard drive you may find the phrase, "A MEGABYTE IS DEFINED AS 1,000,000 BYTES." And the guides printed by WESTERN DIGITAL^� usually contain the following: "Hard drive suppliers define a decimal megabyte as 1,000,000 bytes (10⁶). Alternatively, a binary megabyte is defined as 1,048,576 (2²⁰)." I'm still not sure why hard drive manufacturers were never able to learn how to count in binary along with the rest of us!

  So, be aware, for example, that a hard drive labeled as 850MB (which carries the more detailed number of 853.6MB) is actually 814.1 binary Megabytes. On the other hand, you could say that the HD manufacturer's labels do give you the number of BYTES, as in the DOS "DIR-command" line (such as, "852,365,047 bytes free"), and perhaps there are more people who prefer to have it this way!

I want to correct some things that I've heard over the years about why one might not be able to access everything on a large drive. I once saw the following comment on a BBS:

"You have a large disk (i.e., over 528 MB) and DOS can't see that much disk space without some sort of disk driver. ...." (WRONG!)

Whether it was intended or not, the impression often given by a comment like this is that "DOS" [usually Microsoft^� MS-DOS] itself caused the 528 MB problem. This is just NOT true! As usual, the reality of the situation is a bit more complex:

As long ago as the release of MS^�-DOS version 4, it was already possible to create HD "partitions" exceeding TWO GIGA-bytes * using the FDISK program.
*( NOTE: This is now the HD "size barrier" for many new computer users! There will be more about this in the next HTML document, including an explanation and a table which lays out all of the details. Back to the top)

The real REASON that DOS did not work with large drives for some of us [without additional software] is because our BIOS ROM chips were never updated to handle them! Today, new computers are almost always sold with what may be called a "geometry translating" BIOS.

Actually, even this is not completely true! It really comes down to a conflict arising from the way two 'engineering groups' thought, and, what turned out for us, to be some very poor choices! Let's first look at how DOS is related to this.

Microsoft^� and other DOSes, along with a host of application programs, use the code in the BIOS to handle a number of hardware related actions involving your display, keyboard, I/O ports, disk drives, etc. One of these Interrupts, INT 13 (hex), has a slew of functions which include reading and writing sectors on your disks. After many years of use, the exact code embedded within the BIOS chips had become an accepted standard.

So, what's the problem? Well, it first started with the way that IBM^� designed the INT 13 subroutines to access the model XT hard drives. The scheme, called CHS (Cyliner/Head/Sector), was designed with some limitations. However, this alone, was not the culprit. The BIOS code, and DOS along with it, can, in theory, address up to almost 7.9 GIGA Bytes. (At 1 GB = 1,073,741,824 bytes, that's still quite a large number even by today's standards!!)
[ The HD size problem was destined to happen by an all too often unfortunate fact: that people who should really be working together on common projects, simply do not! The reasons are quite varied, and may be due to competition, politics, greed, 'politics' within a company, or simply ignorance and lack of communication! The idea of a solitary genius making breakthrough discoveries runs deep in the American culture; just as the stories we've heard about "commitees." But there have been many positive results, not to mention a greater consistency, from shared efforts. And I believe the Internet will continue to increase the opportunities for collaboration.]

Some time later, as the size of hard drives grew, some person or group within the HD manufacturing industry made another decision on HD parameters. And, even though it meant that it was now theoretically possible to access over 137 Billion bytes (137,000,000,000 !), the fact that their new scheme had to interact with the BIOS, is what caused the problem.

The Conflict Between the INT 13 BIOS Code and a HD Manufacturer's scheme:

The Original IBM� BIOS Code "Maximum" Specs	The IBM� / Western Digital� developed (IDE/AT) HD Interface "Maximum" Specs
___1024 Cylinders (or Tracks)	___65,536 Cylinders (or 'Tracks')
____256 Heads (or 'Sides')	_____,_16 Heads (or 'Sides')
_____63 Sectors/Track	_____,256 Sectors/Track
8,455,716,864 Bytes MAX.	137,438,953,462 Bytes Maximum
With both schemes using the standard 512 Bytes per Sector.

Do you see the problem?

Because the two "schemes" must understand, or interface, with each other in order to locate data on a disk, we can only address HDs which use the SMALLEST number in EACH of the three CHS specs!
So, we arrive at the 528 "Million-Bytes" barrier as follows:

16 Heads(or Sides) x 1024 Cylinders(or Tracks) per Head ... x 63 Sectors/Track x 512 Bytes/Sector = 528,482,304 Bytes. Or, 504 MB (exactly!), using 1MB = 1,048,576 bytes.



Almost every new computer sold today has a number of options in the "CMOS settings" (BIOS) for accessing its hard drives. For compatibility, especially if you want to read data from an old drive formatted with a disk managing program, they should still include the CHS system. However, LBA (Logical Block Addressing) allows one to access the large drives available today using new code in the BIOS. This code "translates" the numbers used by the hard drive itself into an alternate form which can be used by your programs when accessing the drive. (E.g., if a HD has CHS numbers of 3148, 16, and 63, the BIOS may translate this into 787 Cylinders, 64 Heads, and 63 Sectors/Track.)

Daniel, "The Starman."
Revised and Edited, 12 JUL 97, for the Internet.

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The next document in this series is :
What Is this New 2.1 Billion-Byte Limit ?

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Back to "The Starman's Homepage".

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