Updated PDP-11 references

pubs/dec/pdp11/papers/1975_what_have_we_learned/README.md

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+layout: page
+title: What Have We Learned From The PDP-11? (1975)
+permalink: /pubs/dec/pdp11/papers/1975_what_have_we_learned/
+---
+
+Computer Structures: What Have We Learned From The PDP-11?
+---
+
+By Gordon Bell, William D. Strecker  
+November 8, 1975
+
+Over the PDP-11's six year life about 20,000 specimens have been built based on 10 species (models).  Although
+range was a design goal, it was unquantified; the actual range has exceeded expectations (500:1 in memory size and
+system price).  The range has stressed the basic mini(mal) computer architecture along all dimensions.  The main
+PMS structure, i.e. the UNIBUS, has been adopted as a de facto standard of interconnection for many micro and
+minicomputer systems.  The architectural experience gained in the design and use of the PDP-11 will be described
+in terms of its environment (initial goals and constraints, technology, and the organization that designs, builds
+and distributes the machine).
+
+1.0  INTRODUCTION
+---
+
+Although one might think that computer architecture is the sole determinant of a machine, it is merely the focal point
+for a specification.  A computer is a product of its total environment.  Thus to fully understand the PDP-11, it is
+necessary to understand the environment.
+
+Figure Org. shows the various groups (factors) affecting a computer.  The lines indicate the primary flow of
+information for product functional behavior and for product specifications.  The physical flow of goods is nearly
+along the same lines, but more direct: starting with applied technology (e.g., semiconductor manufacturers), going
+through computer manufacturing, and finally to the service personnel before being turned over to the final user.
+ 
+The relevant parts, as they affect the design are:
+
+1. The basic technology--it is important to understand the components that are available to build from, as they
+directly affect the resultant designs.
+2. The development organization--what is the fundamental nature of the organization that makes it behave in a
+particular way?  Where does it get inputs?  How does it formulate and solve problems?
+3. The rest of the DEC organization--this includes applications groups associated with market groups, sales, service
+and manufacturing.
+4. The user, who receives the final output.
+
+Note, that if we assume that a product is done sequentially, and each stage has a gestation time of about two years,
+it takes roughly eight years for an idea from basic research to finally appear at the user's site.  Other organizations
+also affect the design: competitors (they establish a design level and determine the product life); and government(s)
+and standards.
+
+There are an ever increasing number of groups who feel compelled to control all products bringing them all to a common
+norm: the government(s), testing groups such as Underwriters Laboratory, and the voluntary standards groups such as
+ANSI and CBEMA.  Nearly all these groups affect the design in some way or another (e.g. by requiring time).
+
+2.0  BACKGROUND
+---
+
+It is the nature of engineering projects to be goal-oriented--the 11 is no exception, with much pressure on
+deliverable products.  Hence, it is difficult to plan for a long and extensive lifetime.  Nevertheless, the 11
+evolved more rapidly and over a wider range than we expected, placing unusual stress on even a carefully planned
+system.  The 11 family has evolved under market and implementation group pressure to build new machines.  In this way
+the planning has been asynchronous and diffuse, with distributed development.  A decentralized organization provide
+checks and balances since it is not all under a single control point, often at the expense of compatibility.
+Usually, the hardware has been designed, and the software is modified to provide compatibility.
+
+Independent of the planning, the machine has been very successful in the marketplace, and with the systems programs
+written for it.  In the paper (Bell et al, 1970) we are first concerned with market acceptance and use.  Features
+carried to other designs are also a measure of how it contributes to computer structures and are of secondary
+importance.
+
+The PDP-11 has been successful in the marketplace with over 20,0000 computers in use (1970-1975).  It is unclear how
+rigid a test (aside from the marketplace) we have given the design since a large and aggressive marketing and sales
+organization, coupled with software to cover the architectural inconsistencies and omissions, can save almost any
+design.  There was difficulty in teaching the machine to new users; this required a large sales effort.  On the
+other hand, various machine and operating systems capabilities still are to be used.
+
+2.1  GOALS AND CONSTRAINTS - 1970
+---
+
+The paper (Bell et al, 1970) described the design, beginning with weaknesses of minicomputers to remedy other goals
+and constraints.  These will be described briefly in this section, to provide a framework, but most discussion of
+the individual aspects of the machine will be described later.
+
+Weakness 1, that of limited address capability, was solved for its immediate future, but not with the finesse it might
+have been.  Indeed, this has been a costly oversight in redundant development and sales.
+
+There is only one mistake that can be made in a computer design that is difficult to recover from--not providing enough
+address bits for memory addressing and memory management.  The PDP-11 followed the unbroken tradition of nearly every
+known computer.  Of course, there is a fundamental rule of computer (and perhaps other) designs which helps to alleviate
+this problem: any well-designed machine can be evolved through at least one major change.  It is extremely embarrassing
+that the 11 had to be evolved with memory management only two years after the paper was written outlining the goal of
+providing increased address space.  All predecessor DEC designs have suffered the same problem, and only the PDP-10
+evolved over a ten year period before a change was made to increase its address space.  In retrospect, it is clear
+that since memory prices decline at 26% to 41% per year, and many users tend to buy constant dollar systems, then every
+two or three years another bit is required for the physical address space.
+
+Weakness 2 of not enough registers was solved by providing eight 16-bit registers; subsequently six more 32-bit
+registers were added for floating point arithmetic.  The number of registers has proven adequate.  More registers
+would just increase the context switching time, and also perhaps the programming time by posing the allocation dilemma
+for a compiler or a programmer.
+
+Lack of stacks (weakness 3) has been solved, uniquely, with the auto-increment/auto-decrement addressing mechanism.
+Stacks are used extensively in some languages, and generally by most programs.
+
+Weakness 4, limited interrupts and slow context switching has been generally solved by the 11 UNIBUS vectors which
+direct interrupts when a request occurs from a given I/O device.
+
+Byte handling (weakness 5) was provided by direct byte addressing.
+
+Read-only memory (weakness 6) can be used directly without special programming since all procedures tend to be pure
+(and reentrant) and can be programmed to be recursive (or multiply reentrant).  Read-only memories are used extensively
+for bootstrap loaders, debugging programs, and now provide normal console functions (in program) using a standard
+terminal.
+
+Very elementary I/O processing (weakness 7) is partially provided by a better interrupt structure, but so far, I/O
+processors per se have not been implemented.
+
+Weakness 8 suggested that we must have a family.  Users would like to move about over a range of models.
+
+High programming costs (weakness 9) should be addressed because users program in machine language.  Here we have no
+data to suggest improvement.  A reasonable comparison would be programming costs on an 11 versus other machines.  We
+built more complex systems (e.g., operating systems, computers) with the 11 than with simpler structures (e.g. PDP-8
+or 15).  Also, some systems programming is done using higher level languages.
+
+Another constraint was the word length had to be in multiples of eight bits.  While this has been expensive within DEC
+because of our investment in 12, 18 and 36 bit systems, the net effect has probably been worthwhile.  The notion of
+word length is quite meaningless in machines like the 11 and the IBM 360 because data-types are of varying lengths,
+and instructions tend to be in multiples of 16 bits.  However, the addressing of memory for floating point is
+inconsistent.
+
+Structural flexibility (modularity) was an important goal.  This succeeded beyond expectations, and is discussed
+extensively in the part on PMS, in particular the UNIBUS section.
+
+There was not an explicit goal of microprogrammed implementation.  Since large read-only memories were not available
+at the time of the Model 20 implementation, microprogramming was not used.  Unfortunately, all subsequent machines
+have been microprogrammed but with some additional difficulty and expense because the initial design had poorly
+allocated opcodes, but more important the condition codes behavior was over specified.
+
+Understandability was also stated to be a goal, that seems to have been missed.  The initial handbook was terse and as
+such the machine was only saleable to those really understood computers.  It is not clear what the distribution of
+first users was, but probably all had previous computing experience.  A large number of machines were sold to
+extremely knowledgeable users in the universities and laboratories.  The second handbook came out in 1972 and helped
+the learning problem somewhat, but it is still not clear whether a user with no previous computer experience can
+determine how to use a machine from the information in the handbooks.  Fortunately, two computer science textbooks
+(Eckhouse, 75; and Stone and Siewiorek, 75) have been written based on the 11 to assist in the learning problem.
+
+...