Classic Computer Magazine Archive COMPUTE! ISSUE 34 / MARCH 1983 / PAGE 42


Richard Mansfield, Senior Editor

If you have a question that's been bothering you, send it in to the address below. Many of these columns have been written in response to an issue raised in one of your letters. This month, a question from Todd Oldham reminded me how I felt when I was using a tape drive and was trying to justify switching to a disk drive.

For the first year I owned a computer I never used a disk. In fact, there were few computer stores then and I had only seen disk drives in photos. I was happy enough with tapes. On the other hand, reading about disks made me think that they must be in some way fairly desirable if people were spending about as much money to buy a disk drive as they had spent on their computer itself. But just exactly what could a disk do for my system? It seemed to promise to transform it into a far more powerful machine. But in what ways?

Todd writes: "Why don't you run an article on this device, including information on what it does, how it can benefit someone like me, and how well this product performs?" Fair enough.

An Athletic Memory

A disk makes your computer's memory much larger and faster. It doesn't make the computer smarter by adding new commands to BASIC or anything, but it does make it easier, far easier, for the computer to access large amounts of information quickly. In a way, your RAM memory becomes bigger, stronger, and more agile.

Adding a disk does not give you more internal RAM in a literal sense. However, the great speed with which you can switch programs and data in and out of RAM from a disk makes it appear that you have limitless internal memory. It can seem as if a disk is an extension of RAM. Sometimes this effect is called virtual memory because a disk can be virtually as good as adding hundreds of thousands of RAM memory cells.

There are currently three places that a computer typically puts its programs and information (data, such as a list of addresses): RAM, tape, and disk. RAM memory is inside the computer and is limited in size. Usually RAM can only hold one program and some associated data at one time. What's more, RAM goes blank when the power is turned off. It is, however, the fastest way to make information available to the computer. It's inside the computer to start with.

Tape is inexpensive because cassette tape drives are common and relatively easy to manufacture. The manufacturing problems are fairly simple to solve for a machine which slowly pulls a ribbon of magnetic material past a "head" that picks patterns off, or puts them on, the tape. The head never moves, just the tape.

A floppy disk looks like a limp 45-rpm record enclosed within a protective black cardboard envelope. When inserted into the disk drive, the disk whirls around a spindle at about 300 revolutions per minute. Equally important, the head can quickly move to any of about 35 positions along a line from the spindle to the edge. In this way, any piece of information, anywhere on the disk, can be located and sent to the computer at lightning speed. It can be stored (sent from the computer to be memorized on the disk) just as fast.

To give you an idea of this speed, a 4,000 byte program takes about ninety seconds to come in from, or go out to, a tape. A disk loads or saves the same size program in seven seconds. You might not think that this represents a crucial difference, but in many ways it makes all the difference in the world.

Here's one example. A normal video screen will hold about 1000 bytes. Suppose that you wanted to have dozens of different screenfulls of instructions. You could have them each on screen in a second from a disk. They wouldn't all fit at once in RAM memory, but they could be brought in almost instantly from disk and flipped like the pages of a notebook. It's possible, but slow and awkward, to bring in such screens from tape. You can only load things off tape in the order in which they were saved.

This (plus the great increase in speed) is perhaps the main advantage that disk has over tape. Disk heads are able to leap to any position on the surface of a disk. Tape items are all lined up in order. That's fine if you want the first item on the tape. If you want the last item, you have to pull off all the others first. You can't just drop down instantly to any desired location on a tape.

Changes Of Behavior

As might be expected, switching to disk memory has an impact on the way that you program. Programmers are often cautioned to make a copy, called a backup, every 20 minutes or so to prevent losing everything if there is a power outage or something else causes RAM memory to fail and destroy your work. This backup task becomes a snap with disks: a matter of waiting a few seconds. You're more likely to make frequent backups when it's this easy to do.

There are also several new ways that you can program when you have a disk drive. You can bring programs into the computer quickly and "overlay" the program that's in memory. In this way, programs can be made quite large, and divided into segments which call each other in from disk at appropriate times. This technique is called chaining.

An additional advantage is the fact that a typical disk (Commodore) can hold 176,640 bytes on one side. If you have a double-drive (two drives in one housing), this means that you've got 345K of memory available to your computer.

Why Not Disks?

Asking "why not get a disk drive?" is like asking "why not trade in the car on a helicopter, it's faster and more efficient?" True, but more expensive. Prices are falling, however. A year ago, a disk drive cost roughly ten times the price of a tape drive. Now you can buy single-drive disk units for under $400. Rumor has it that the drive assemblies will soon cost OEM's (Original Equipment Manufacturers) about $50. This could well mean that the drives' retail price will be going down further before bottoming out.

The Beginner's Page
COMPUTE! Magazine
P.O. Box 5406
Greensboro, NC 27403