We will look at the design philosophies of three historical operating systems in this chapter. Each had achieved success in its respective niche. Each had developed a following of loyal devotees. Each shares some commonality with Unix and Linux, and each bears some characteristics that set it apart, sometimes in striking ways.

For a variety of reasons, not the least of which was limited hardware capacity, these operating systems were written for specific hardware platforms. Users needed strong justification to shell out several thousand to several million dollars for systems that were relatively inflexible. You did not have thousands of applications available at discount retailers and flea markets. Personal computers were not cheap and plentiful. Few people had old computers collecting dust on the tops of piles of unused household goods in their basements.

Newer, faster hardware eventually came along, causing these OSs to fall into disuse. Without the portability of Unix, they could not keep up as hardware designers produced machines with increasing amounts of storage, specialized peripherals,...