24 Dec 1995

Introduction to PC Hardware

The newspaper ads scream out prices, MHz, upgradable, and SVGA. Do you know what these buzzwords really mean? Does anyone? The PC may be the single most important tool for researchers and executives, but because it is purchased in a camera store or discount food warehouse it is often treated as a commodity item. It should come as no surprise that most people who use the techno-jargon have no real understanding of any of the terms or issues.

Some figure they are safe if they buy a well known brand name. That certainly addresses some questions of quality. But buying from a well respected auto manufacturer will be of little help if you end up with a pickup truck when you really wanted a four door sedan. There are important differences in design and architecture between different computer systems, even among machines offered by the same vendor.

The Faster Machine May have a Slower CPU

When people buy a new computer, they generally know what type of CPU chip they are getting. Today the lowest priced machines come with a 486 CPU running at 66 Megahertz, though that is being rapidly replaced by all Pentium models. Because the vendors know that a customer will generally judge a computer by its CPU, they may buy a more powerful processor and then cut corners on the other components.

Within the same family of chips (486 or Pentium) doubling the clock nearly doubles the CPU. Since the memory and the I/O bus do not speed up, the CPU does not quite become twice as powerful. For ordinary programs, the 66 MHz chip can be expected to execute at 1.5 to 1.7 times the speed of a 33 Mh chip. However, for a comparison between two systems to be fair, you have to hold the cost constant. If you accept the "slower" CPU, then this money becomes available to buy something else.

Although Windows 95 is supposed to run on a system with as little as 4 megabytes of RAM, a new system should not have less than 8 megs of memory. Adding Office 95 applications, some multimedia, and networking may push requirements up to 12 or 16 megs.

All modern operating systems use "virtual memory." When the applications fill up available RAM, some of the program and data is moved to disk. When you switch from one program to another, the system pauses and the disk becomes active as Excel moves out to disk and Word comes back in. This is not a problem when programs are used alternately, but if more programs are running simultaneously than can fit in memory, then the system begins to "thrash." The disk light comes on all the time, and the system slows to a crawl.

It takes about 100 nanoseconds (billionths of a second) to fetch some data from RAM. If the data is on disk, then it takes about 10 milliseconds (thousandths of a second) to read the record in from a fairly fast modern disk. The disk is 100,000 times slower than memory.

So you have $450 to spend to make a system faster. You could buy a faster CPU, and get 1.5 times as much speed when the data is in memory. Or you could get more memory, and get 100,000 times as much speed when the data would previously pushed out to disk. It doesn't take a very sharp pencil to decide that extra memory is the safer choice.

Although there are specific operations, such as compression and encryption, whose performance is entirely driven by CPU power, across the broad range of modern application a 75 MHz Pentium with 16 megabytes of memory will outperform a 133 MHz Pentium with 8 megabytes. It seems unlikely that anyone would ever try and put an eight cylinder engine in a Saturn, but most computer vendors overload their systems with CPU power.

Even when memory is not fully used by applications, modern operating systems allow the user to allocate a "disk cache" which keeps copies of the most frequently used data from the disk. This cache can significantly improve performance when starting programs or loading files. So extra memory always has some use.

Low-Tech is Important

Vendors fill their adds with high-tech terms. A user may go shopping for a DX2 with a PCI bus and CD- ROM. However, normal business applications do not challenge even an average system. When the computer is used all the time, the most important issues may be much less complicated.

For the full-time computer user, the most important part of the machine is often the keyboard. This is not an advanced, modern device. The design of today's keyboard is not much different from the electric typewriter of thirty years ago. Good design is still important. A moderately fast typist may press 200 keys each minute, all day long. It is not very difficult to make a keyboard that can record the keystroke. It is much more complicated to make a keyboard that feels good and is comfortable to use.

How far does the key move when it is pressed? What resistance does it provide. How does it feel when it hits bottom? The least expensive keyboards feel soft and squishy. A good keyboard has a substantial feel. In the modern era, where employees complain of hand injuries due to poorly designed keyboards, an extra $100 may be much less expensive than an insurance claim.

The quality of the screen is also important. A larger screen is easier to read. A faster refresh rate reduces flicker. A few disreputable vendors try to sneak by an "interlaced" monitor to trim the cost of their least expensive systems by another $100.

For a secretary, using the system for word processing and casual queries, the keyboard and screen are much more important than anything in the system unit. Performance may be less significant than comfort.

The Q Bridge

In downtown New Haven, CT where I-91 meets I-95, the "Q Bridge" crosses the harbor area. It must be one of the hottest attractions in southern New England, because every morning and afternoon cars line up for miles to cross it. It defines the rush hour commute, and nothing that you do to the other roads or exits in West Haven or East Haven will materially speed things up.

Inside your computer there is an electronic version of the Q Bridge. Depending on the application, some component will become the choke point, and all the data bytes will line up waiting to get through. But while the real Q Bridge never changes, the PC choke point moves as you change use.

A Porche and a Yugo get caught in the backup at the same point in West Haven. Twenty minutes later they cross the bridge at the same time. It doesn't do any good to spend a lot of money on a fast car and a big engine if the limiting factor is traffic moving five miles an hour. Yet customers often select a server with a fast CPU, without first considering what the bottleneck will be.

Database performance will probably be limited by disk speed and cache memory. Windows applications depend on the speed of the display adapter. The CPU tends to get busy only when data is encrypted or compressed. The CPU chip is upgradable, but changing it won't help much if your Q Bridge is located at a different component.

Topics to discuss include:

  1. The CPU and Memory
  2. The I/O Bus
  3. Video Adapters
  4. IDE or SCSI Disk


Copyright 1995 PCLT -- Introduction to PC Hardware -- H. Gilbert

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