Hard Drive Basics



The first thing you need to understand is the difference between the physical number of bytes and the size of data in bytes. If you think this is double talk, you're right! A byte is 8 bits. One thousand of these is called a kilobyte and it comes out to be 1024 bytes of data, because it's a multiple of the number 8 (and not 10 as we find in decimal numbers). The physical number 1 K, however, is plain old 1000. So an 1 KB hard drive can't hold 1,024 bytes of data because it's only 1,000 bytes in size!

Yes, that is double talk. I will not argue that point with you. But I want you to understand that your new 80 GB hard disk drive will only hold about 78 GB of actual data. 80 GB of actual data space requires roughly an 82 GB hard drive!

Yes, this is apples and oranges! You are storing apples in a box made to the specifications of oranges, so you won't quite fit as many apples as you thought!

Remember this small fact of life, should you be calculating how much space you actually need (and don't forget to subtract about 2 MB for the operating system files and software BIOS that gets installed on the drive)! Thus, if you need exactly 100 GB of data storage you will need to get at least a 104 GB hard drive to be on the safe side.

Next, you need to understand a little about BIOS and the ATA standards, which work hand-in-hand.

BIOS makes your computer a working thing, otherwise it would just be a box full of parts. A computer is a big switch box and BIOS automatically regulates one set of switches called interrupts which channel information to and from devices (BIOS stands for Basic Ins and Outs). Your video card, mouse, modem, printer port, floppy drive, hard drive, audio card, network card and keyboard are all given a time slice each second by the BIOS and during this time slice data is passed to and from the device by the CPU or directly between components by Direct Memory Access (DMA).

There are 16 hardware interrupts and they fire off at regular intervals unless stopped by a software process (which is why your computer sometimes hangs up -- something grabbed an interrupt and won't let go). If something monopolizes an interrupt, then data from some other devices is held in a cache (memory pool) and executed after the interrupt system returns to normal. This explains why, when your system hangs up and you bang on the keyboard, a whole bunch of type appears after the system restores.

Modern BIOS controls many functions including "smart drives" or those ATA protocol drives (which is probably why the term Intelligent Device Electronics or IDE probably caught on -- ATA/IDE drives are designed to be somewhat smart). If your BIOS supports current automatic functions (ATA is such a function) then the BIOS will hook up with the drive and make the right adjustments. If your drive is smarter than your BIOS then it may make wrong choices or no choices and you will have to set items such as PIO (Programmed Ins and Outs) and UDMA (Ultra Direct Memory Access) manually by using the + and - keys (or some other key set -- your BIOS will tell you what keys change settings) and experiment until you find the best setting. You may also have to set physical jumpers on the drive(s) for primary and secondary.

ATA drives are the most commonly found (and mistakenly identified as IDE drives, which is a registered trade mark of a specific hard drive company and not a general type of drive) and the current standard is ATA-5 or ATA-6. The companies that make drives don't identify them this way, instead they use the throughput value, which is AT100 for the current protocol standard. So, virtually every drive you find new on the mass market shelf is an ATA100 drive. If you find an ATA66 or ATA33 drive, it is old, but still usable. Only the maximum, optimal throughput is less and for most general uses your system never needs much more than 2 MB of throughput.

Video work is an exception. It requires 4 MB per second of throughput, which is two or three times the intensity of professional multi-track recording. Most hard drives in standard mode can't reach these levels through the CPU (which is how most applications work), so video makers use what is called Direct Memory Access (DMA) and even an old ATA33 drive supports Ultra DMA. So, generally speaking any drive that spins faster than 4,500 RPM and supports UDMA should be able to handle the 4MB per second data transfer rate required by a modern video card and still have a little headroom.

Speed is the amount of times the disk returns to the same spot in a given minute. Today the average drive is around 4,500 RPM, although the 7,200 RPM drive is becoming far more common in the modern "box" that is sold for even $600. There are now 10,000 RPM drives.

Seek time is rated in milliseconds (1/1000 th of a second) and this is the time it takes for the heads to reach a point on the disk where required data is located (according to the contents or File Allocation Table). Most drives today are rated between 8 and 15 ms.

Access time is the Seek Time plus the time it takes for data to electronically be returned to the originating program, which can be up to twice as long on an old computer. So access time on a given system can vary from as little a 9ms to as much as 30 ms.

Cache memory is used for some intensive operations as RAM memory is faster than any hard drive. However in our tests with the Western Digital 2 MB cache 60 GB, 7,200 RPM hard drive and their 8 MB cache, 80 GB, 7,200 RPM hard drive on video work showed no difference in test processes in UDMA mode. Both rated out around 22,000 MB per second read/write times. There was a difference in direct mode, but not as much as one would think! It was only about 40% more, so that 8MB cache didn't do four times better like one would think might happen!

It is possible that in an ultra intense situation the 8 GB cache drive may sustain slightly higher rates or in CPU based situations it may also hold an edge, but this is unclear at this point in time.

Most computers allow you to have up to four total drives connected via the Primary and Secondary IDE (ATA) controller. Starting around 1987 with ATA-2 each of these can accommodate two drives (0 and 1).

Drives, controllers and BIOS that are all compatible with at least ATA-4 (around 1996 or 7) using the color coded connector cord (blue, gray and black) can automatically sense PIO, UDMA and multiple drives, including ATAPI CD drives provided you have the jumper on the drives set for the automatic (plug and play) sensing (generally the far right pin group). Otherwise you must place the jumpers over the correct pin pairs for master and slave (0 and 1). Generally the second set of pins from the right is master and the third set is slave, but check your specification sheet with any given hard drive or CD player to be sure!

Chaining two drives on the same controller will slow both DMA and direct read slightly (my tests show this to be about a 10-15% drop in performance, with DMA dropping from 24,000 to 21,000 and direct read dropping from 3,100 to 2,900 on a 2 MB cache 7,200 RPM disk). Chaining a hard drive with a CD may drop slightly more than chaining two hard drives together, but sometimes you have to chain CD and hard drive. I was forced to do this. I have both a CD-RW and a DVD and these would not chain together, so I had to put each CD unit as master and make each hard drive a slave.

Position of the hard drive can also affect performance and I found default horizontal placement reduces throughput on both my Western Digital drives. Not by a lot -- in direct read mode (not DMA) I gained about 50 KB per second placing the drive in a vertical position. I attribute this to gravity aiding the movement of the arms which hold the read/write recording heads or friction and drag being lessened to some degree.

If you really need to squeak out extra performance putting your drive as the master with nothing else attached, adjusting the jumpers until the best performance is obtained, playing with PIO and UDMA in BIOS and standing the drive up and down may actually help you get another 200 - 400 KB per second of direct read and as much as 5 MB per second DMA.

Also make sure that the DMA box is checked in Control Panel, System, Device Manager, Hard Drive Settings form (see picture). This allows any supported program to use Direct Memory Access, thus bypassing the Central Processing Unit which can bottleneck speed.



Most applications only require between 500 KB and 2 MB per second throughput, but video capture and CD-R writing will use 4 MB per second or more of intensive throughput. In the future, when computers deal with digital HDTV, they will be required to read and write at over 29 MB per second (which even the 8MB Western Digital 7200 drive will not do at the current time on my machine), but Digital Television is not being done on home computers as of this point in time -- only on dedicated digital work stations (such as the AVID) costing a small fortune, but by 2005 we will start seeing this technology creep into our domain and our current technology is almost good enough to handle it, so by the time AT-7 hits we may see totally complaint A/V systems.

I have found that in an older computer adding a newer hard drive can double the performance speed while adding 10 times more storage. Disk drives seem to double in speed every 18 months. While a very old computer (say one built from components in 1992) may not use all the speed of a modern ATA100 hard drive, you will often see at least a 50% boost in boot up and program loading times.

 






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