The History and Development of Hard Drive Technologies

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  The History and Development of Hard Drive Technologies


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copyright (c) 1992 -1998
Nicholas Majors & ActionFront Data Recovery Labs Inc.
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The original IBM PC, introduced in 1981, did not support any type of hard drive. Program code in the BIOS did not recognize any such device and early versions of DOS precluded mass storage by limiting the maximum number of directory entries. This is not surprising when you consider that the original 4.77 MHz PC came with 16 K of RAM, expandable to 64 K on the motherboard. Even floppy drives and a disk operating system were options to upgrade the built in cassette recorder port and ROM basic.

To utilize a hard drive in a PC requires:

a Hardware IRQ (interrupt request)
I/O port addresses for the controller
a DMA channel (now optional)
low level program code
a physical interface to the bus (expansion slot or built-in)
operating system support
sufficient power and cooling
After DOS 2 introduced the sub-directory system and support for mass storage devices, companies started selling hard drives for the PC. These were external cabinets that contained the disk itself, a controller card, and a dedicated power supply (since the PC's 63.5 watts were insufficient). Migrating from other architectures, these units connected to the PC through cabling to an 8 bit adapter card that plugged into an available expansion slot. These subsystems were configured to use one of the available IRQ's, DMA channels, a range of I/O ports, and program code that was made available through a device driver loaded into memory after startup (booting from a floppy).

In 1983, the IBM XT (eXTended) was unveiled with its built-in 10 MB fixed disk. IBM worked with a company (Xebec, I think) to take the controller components normally located in the external cabinet and build them right onto a bus interface card, creating what we commonly call the "hard disk controller".

Program code was provided by a ROM chip on the controller card which supplemented subroutines in the BIOS, and the internal power supply was upgraded to 135 watts to provide power for the internally mounted drive.

The XT design utilized :

IRQ 5
I/O port addresses 320-32f
DMA channel 3
Program Code from adapter ROM at c8000
DOS 2.0 or higher
Several companies started manufacturing and selling similar drive/controller packages with various improvements including higher capacities, superior performance and built-in floppy controller components on the same card (to save an expansion slot). These third party subsystems could even be added to an original PC, so long as the internal power supply was also upgraded.

In 1984, the IBM AT (Advanced Technology) brought a complete overhaul to hard disk systems. Program code was included in the motherboard ROM BIOS, eliminating the need for a ROM chip on the new 16 bit controller card, and a higher quality drive improved access times. The system included use of newly added higher IRQ's, eliminated the use of DMA for hard drives, and changed the range of I/O addresses.

The AT design utilized :

IRQ 14
I/O port addresses 1f0-1f8
DMA channel - none
Program Code from motherboard ROM BIOS
DOS 2.0 or higher
Hardware configuration details for the AT, including hard disk parameters, were stored in a low power CMOS chip connected to a small battery, eliminating many of the motherboard configuration dip switches and jumpers. The battery allowed information to be maintained while the computer was turned off and information could be changed by running a setup program from disk.

The original AT supported 14 different drive types, recognizing specific hard disks ranging from 10 to 112 MB. Any drive with physical parameters that did not match one of these types needed a ROM extension on the controller card or a device driver loaded during boot-up.

DOS versions prior to 4.0 (or 3.31) did not support partitions larger than 32 MB no matter how big a drive was. This was because of sector numbering that could not exceed 16 bit values (up to 65,536 sectors). To make a larger partition required the use of special software like Ontrack's Disk Manager. This software was so popular that many drive manufacturers shipped it with their product. Unfortunately, while this offered one of the better solutions, it did pose compatibility problems for many disk utilities because, in effect, you had a non-DOS partition.

Many people opted instead to divide their drives into 32 MB partitions creating a C: D: E: etc. up to the physical capacity. Prior to DOS 3.3, even this was not possible, because DOS did not recognize extended partitions!

The number of drive types supported by CMOS has expanded to over 40 and most current BIOS's provide a user definable type to allow parameters to match any drive. Most PC's today still rely on this original CMOS drive type scheme to configure and control hard disks, though many architectures and BIOS's have changed how the information is stored and updated.

This inherently creates certain limiting factors, including a problem with more than two hard drives and BIOS/OS limits to recognizing more than 1024 cylinders, 16 heads and 63 sectors per track. Various technologies must be used to translate non-compliant physical geometries into logical parameters recognized by the system to maintain compatibility with operating system and utility programs that are tied to these limits.

With each sector holding 512 bytes of data this allows for drives no larger than 504 MB.
(1024 X 16 X 63 = 1,032,192 sectors X 512 = 528,482,304 bytes)

To understand this limit, we have to examine how hard drives are accessed by a PC.

Primarily, I/O commands are sent to the controller via a range of reserved port addresses. This is a very complicated and tedious process and the ROM BIOS includes a subroutine (INT 13) to perform these tasks for you. The DOS operating system then has certain functions that further simplify the process. They include INT 25/26 functions to read and write absolute sectors to the drive as well as higher level functions (INT 21) to Open files, Close files, Write data to files, etc. Most programs rely on these DOS functions to control access to hard drives and DOS in turn calls the INT 13 BIOS subroutines which do the actual I/O commands.

All of these subroutines are assembly language code that are loaded into memory from either your BIOS chip(s) or Operating System files at startup.

Assembly language routines store and manipulate values in registers within the CPU. The lowest common denominator for INTEL based PC's is 16 bit registers. Any program or routine that wants to be backward compatible, must use these base registers whether run on an 8088 or a Pentium. This basic INT 13 is the backbone of PC compatibility and uses 16 bit registers in the following way:

DX - 8 bits for head number - 8 bits for drive number
CX - 10 bits for cylinder number and 6 bits for sector number
The largest 10 bit number possible is 1023 hence the limit on cylinder numbers (from 0-1023) and the largest 6 bit number is 63 (from 1-63) allowing 63 sectors per track.

But note - the DX register allows a maximum of 255 heads, not the 16 that original specifications called for. This is what allows various translation schemes to deal with drives up to 8 GB while remaining INT 13 compatible.

If the DX register allows 8 bits for drive number, shouldn't it be able to control 255 drives instead of only two?

When the INT 13 sends commands to your controller, it must know the physical geometry of the drive in question. During initialization of the PC, values for disk types are read from CMOS and stored into an area of RAM called the BIOS DATA AREA. Pointers to those entries are stored in the Interrupt Vector Table (at addresses 0:104h and 0:118h). The table only reserved pointers for two drives, so even if your CMOS held more values, the standard routines wouldn't know how to deal with them.

Why were the INT 13 routines so limited? They were written at a time when 10 and 20 MB drives were the norm, and 120 MB was unbelievable. They were designed to communicate with a specific controller interface, the ST412/506 standard (WD1003 controller), and the specs for passing parameters included only 10 bits for cylinders number, 4 bits for head number and a single bit for drive number.

Some of these limitations have been overcome in the past by replacing the PC's INT 13 sub- routines with code that could recognize and deal with different devices. That's why virtually all SCSI adapter cards include a ROM chip and you must set the drive type to ZERO. In effect, no standard drives installed for the motherboard BIOS to control.

NetWare and other advanced Operating Systems use the drive type only long enough to boot- up and then replace the INT 13 code with their own device drivers. You then need to load a different .DSK file written specifically for each type of controller.

In the past couple of years, many motherboard and controller BIOS's have been enhanced to deal with ever increasing demands, but the process of establishing new standards has led to continuing confusion and compatibility problems.