Hard Disk Capacity Barriers

Whilst Bill Gates’ assertion that 640KB ought to be enough for anyone is the most famous example of lack of foresight when it comes to predicting capacity requirements, it is merely symptomatic of a trait that has afflicted the PC industry since its beginnings in the early 1980s. In the field of hard disk technology at least 10 different capacity barriers occurred in the space of 15 years. Several have been the result of BIOS or operating system issues, a consequence of either short-sighted design, restrictions imposed by file systems of the day or simply as a result of bugs in hardware or software implementations. Others have been caused by limitations in the associated hard disk drive standards themselves.

IDE hard drives identify themselves to the system BIOS by the number of cylinders, heads and sectors per track. This information is then stored in the CMOS. Sectors are always 512 bytes in size. Therefore, the capacity of a drive can be determined by multiplying the number of cylinders by the number of sectors by 512. The BIOS interface allows for a maximum of 1024 cylinders, 255 heads and 63 sectors. This calculates out at 504 MiB. The IEC’s binary megabyte notation was intended to address the confusion caused by the fact that this capacity is referred to as 528 by drive manufacturers, who consider a megabyte to be 1,000,000 bytes instead of the binary programming standard of 1,048,576 bytes.

The 528MB barrier was the most infamous of all the hard disk capacity restrictions and primarily affected PCs with BIOSes created before mid-1994. It arose because of the restriction of the number of addressable cylinders to 1,024. Its removal – which led to the E (for Enhanced) being added to the IDE specification – by abandoning the cylinders, heads and sectors (CHS) addressing technique in favour of logical block addressing, or LBA. This is also referred to as the BIOS Int13h extensions. With this system the BIOS translates the cylinder, head and sector (CHS) information into a 28-bit logical block address, allowing operating systems and applications to access much larger drives.

Unfortunately, the designers of the system BIOS and the ATA interface did not set up the total bytes used for addressing in the same manner, nor did they define the same number of bytes for the cylinder, head, and sector addressing. The differences in the CHS configurations required that there be a translation of the address when data was sent from the system (using the system BIOS) and the ATA interface. The result was that the introduction of LBA did not immediately solve the problem of the 528MB barrier and also gave rise to a further restriction at 8.4GB.

The 8.4GB barrier involved the total addressing space that was defined for the system BIOS. Prior to 1997 most PC systems were limited to accessing drives with a capacity of 8.4GB or less. The reason for this was that although the ATA interface used 28-bit addressing which supported drive capacities up to 2**28 x 512 bytes or 137GB, the BIOS Int13h standard imposed a restriction of 24-bit addressing, thereby limiting access to a maximum of only 2**24 x 512 bytes or 8.4GB.

The solution to the 8.4GB barrier was an enhancement of the Int13h standard by what is referred to as Int13h extensions. This allows for a quad-word or 64 bits of addressing, which is equal to 2**64 x 512 bytes or 9.4 x 10**21 bytes. That is 9.4 Tera Gigabytes or over a trillion times as large as an 8.4GB drive. It was not until after mid-1998 that systems were being built that properly supported the BIOS Int13h extensions.

By the beginning of the new millennium, and much to the embarrassment of the drive and BIOS manufacturers, the 137GB limit imposed by the ATA interface’s 28-bit addressing was itself beginning to look rather restrictive. However – better late than never – it appears as though the standards bodies may have finally learnt from their previous mistakes. The next version of the EIDE protocol (ATA-6) – being reviewed by the ANSI committee in the autumn of 2001 – allows for 48 bits of address space, giving a maximum addressable limit of 144PB (Petabytes). That’s 100,000 times higher than the current barrier and, on previous form, sufficient for the next 20 years at least!.