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families that span a range* but are incompatible. In addition to the range, the initial machine was constrained to fall within the small-computer product line, which means to have about the same performance as a PDP-8. The initial machine outperforms the PDP-5, LINC, and PDP-4 based families. Performance, of course, is both a function of the instruction set and the technology. Here, we are fundamentally only concerned with the instruction set performance because faster hardware will always increase performance for any family. Unlike the earlier DEC families, the PDP-11 had to be designed so that new models with significantly more performance can be added to the family.

A rather obvious goal is maximum performance for a given model. Designs were programmed using benchmarks, and the results were compared with both DEC and potentially competitive machines. Although the selling price was constrained to lie in the $5,000 to $10,000 range, it was realized that the decreasing cost of logic would allow a more complex organization than that of earlier DEC computers. A design that could take advantage of medium- and eventually large-scale integration was an important consideration. First, it could make the computer perform well; second, it would extend the computer family's life. For these reasons, a general register organization was chosen.

Interrupt Response. Since the PDP-l1 will be used for real-time control applications, it is important that devices can communicate with one another quickly (i.e., the response time of a request should be short). A multiple priority level, nested interrupt mechanism was selected; additional priority levels are provided by the physical position of a device on the Unibus. Software polling is unnecessary because each device interrupt corresponds to a unique address.


The total system including software is, of course, the main objective of the design. Two techniques were used to aid programmability. First, benchmarks gave a continuous indication as to how well the machine interpreted programs; second, systems programmers continually evaluated the design. Their evaluation considered: what code the compiler would produce; how would the loader work; ease of program relocatability; the use of a debugging program; how the compiler, assembler, and editor would be coded - in effect, other benchmarks; how real-time monitors would be written to use the various facilities and present a clean interface to the users; finally, the ease of coding a program.


Structural flexibility (sometimes called modularity) for a particular model was desired. A flexible and straightforward method for interconnecting components had to be used because of varying user needs (among user classes and over time). Users should have the ability to configure an optimum system based on cost, performance, and reliability, both by inter connection and, when necessary, constructing new components. Since users build special hardware, a computer should be interfaced easily. As a by-product of modularity, computer components can be produced and stocked, rather than tailor-made on order. The physical structure is almost identical to the PMS structure discussed in the following section; thus,


*PDP-4 ,7,9, 15 family: PDP-5. 8,8/S. 8/I. 8/L family: LINC. PDP-8/LINC. PDP-12 family: and PDP-6. 10 family. The initial PDP-l did not achieve family status.

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