Personal Distributed Computing: The Alto and Ethernet Software

1.2.Schemes

Systems resemble the organizations that produce them.
—Conway

The Alto system was affected not only by the ideas its builders had about what kind of system to build, but also by their ideas about how to do computer systems research. In particular, we thought that it is important to predict the evolution of hardware technology, and start working with a new kind of system five to ten years before it becomes feasible as a commercial product. If the Alto had been an arti­cle of commerce in 1974, it would have cost $40,000 and would have had few buyers. In 1984 a personal computer with four times the mem­ory and disk storage, and a display half the size, could be bought for about $3500. But there is still very little software for that computer that truly exploits the potential of personal distributed computing, and much of what does exist is derived from the Alto system. So we built the most capable system we could afford within our generous research budget, confident that it could be sold at an affordable price before we could figure out what to do with it. This policy was continued in the late 1970s, when we built the Dorado, a machine with about ten times the computing power of an Alto. Affordable Dorados are not yet available, but they surely will be by the late 1980s. Our insistence on work­ing with tomorrow’s hardware accounts for many of the differences between the Alto system and the early personal computers that were coming into existence at the same time.

Another important idea was that effective computer system design is much more likely if the designers use their own systems. This rule imposes some constraints on what can be built. For instance, we had little success with database research, because it was hard to find signifi­cant applications for databases in our laboratory. The System R project [17] shows that excellent work can be done by ignoring this rule, but we followed it fairly religiously.

A third principle was that there should not be a grand plan for the system. For the most part, the various parts of the Alto system were developed independently, albeit by people who worked together every day. There is no uniform design of user interfaces or of system-wide data abstractions such as structured documents or images. Most pro­grams take over the whole machine, leaving no room for doing any­thing else at the same time, or indeed for communicating with any other program except through the file system. Several different lan­guage environments are available, and programs written in one lan­guage cannot call programs in another. This lack of integration is the result of two considerations. First, many of the Alto’s capabilities were new, and we felt that unfettered experimentation was the best way to explore their uses. Second, the Alto has strictly limited memory and address space. Many applications are hard enough to squeeze into the whole machine and could not have been written if they had to share the hardware resources. Nor is swapping a solution to the shortage of memory: The Alto’s disk is slow, and even fast swapping is inconsist­ent with the goal of fast and highly predictable interaction between the user and the machine.

There was some effort to plan the development of the Alto system sufficiently to ensure that the basic facilities needed for daily work would be available. Also, there was a library of packages for perform­ing basic functions like command line parsing. Documentation for the various programs and subroutine packages was maintained in a uni­form way and collected into a manual of several hundred pages. But the main force for consistency in the Alto system was the informal in­teraction of its builders.

The outstanding exception to these observations is the Smalltalk system, which was built by a tightly knit group that spent a lot of effort developing a consistent style, both for programming and for the user interface. Smalltalk also has a software-implemented virtual memory scheme that considerably relaxes the storage limitations of the Alto. The result is a far more coherent and well-integrated world than can be found in the rest of the Alto system, to the point that several of the Alto’s successors modeled their user interfaces on Smalltalk. The price paid for this success was that many Smalltalk applications are too slow for daily use. Most Smalltalk users wrote their papers and read their mail using other Alto software.

There was much greater consistency and integration of compo­nents in later systems that evolved from the Alto, such as Star and Cedar. This was possible because their designers had a lot of experi­ence to draw on and 4 to 16 times as much main storage in which to keep programs and data.

The Alto system was built by two groups at PARC: the Computer Science Laboratory (CSL), run by Robert Taylor and Jerome Elkind, and the Learning Research Group (LRG), run by Alan Kay. LRG built Smalltalk, and CSL built the hardware and the rest of the system, often in collaboration with individuals from the other computer-related labo­ratory at PARC, the Systems Science Laboratory. George Pake, as manager first of PARC and then of all Xerox research, left the researchers free to pursue their ideas without interference from the rest of Xerox.

Table 1: Systems and chronology (approximate)

The Star system, a Xerox product based on the Alto [43], was built by the System Development Division (SDD), run by David Liddle. The Dorado, a second-generation personal computer [27] and Cedar, a second-generation research system based on the Alto [53], were built in CSL. Table 1 is a chronology of these systems.