Updated PDP-11 references

pubs/dec/pdp11/papers/1977_what_have_we_learned/README.md

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+title: What Have We Learned From The PDP-11? (1977)
+permalink: /pubs/dec/pdp11/papers/1977_what_have_we_learned/
+---
+
+What Have We Learned From The PDP-11?
+---
+
+By C. Gordon Bell  
+1977
+
+In the six years that the PDP-11 has been on the market, more than 20,000 units in 10 different models have been
+sold. Although one of the original system design goals was a broad range of models, the actual range of 500 to 1
+(in cost and memory size) has exceeded the design goals.
+
+The PDP-11 was designed to be a small computer, yet its design has been successfully extended to high-performance
+models. This paper recollects the experience of designing the PDP-11, commenting on its success from the point of
+view of its goals, its use of technology, and on the people who designed, built and marketed it.
+
+1. INTRODUCTION
+---
+
+A computer is not solely determined by its architecture; it reflects the technological, economic, and human aspects
+of the environment in which it was designed and built. Most of the non-architectural design factors lie outside the
+control of the designer: the availability and price of the basic electronic technology, the various government and
+industry rules and standards, the current and future market conditions. The finished computer is a product of the
+total design environment.
+
+In this chapter, we reflect on the PDP-11: its goals, its architecture, its various implementations, and the people
+who designed it. We examine the design, beginning with the architectural specifications, and observe how it was
+affected by technology, by the development organization, the sales, application, and manufacturing organizations, and
+the nature of the final users. Figure 1 shows the various factors affecting the design of a computer. The lines
+indicate the primary flow of information for product behavior and specifications. The physical flow of materials is
+along nearly the same lines, but more direct: beginning with the applied technology manufacturers, material moves
+through computer manufacturing and then to service personnel before delivery to the end user.
+
+2. BACKGROUND: THOUGHTS BEHIND THE DESIGN
+---
+
+It is the nature of computer engineering to be goal-oriented, with pressure to produce deliverable products.
+It is therefore difficult to plan for an extensive lifetime. Nevertheless, the PDP-11 evolved rapidly, and over
+a much wider range than we expected. This rapid evolution would have placed unusual stress even on a carefully
+planned system. The PDP-11 was not extremely well planned or controlled; rather it evolved under pressure from
+implementation and marketing groups.
+
+Because of the many pressures on the design, the planning was asynchronous and diffuse; development was distributed
+throughout the company. This sort of decentralized design organization provides a system of checks and balances,
+but often at the expense of perfect hardware compatibility. This compatibility can hopefully be provided in the
+software, and at lower cost to the user.
+
+Despite its evolutionary planning, the PDP-11 has been quite successful in the marketplace: over 20,000 have been
+sold in the six years that it has been on the market (1970-1975). It is not clear how rigorous a test (aside from
+the marketplace) we have given the design, since a large and aggressive marketing organization, armed with software
+to correct architectural inconsistencies and omissions, can save almost any design.
+
+It has been interesting to watch as ideas from the PDP-11 migrate to other computers in newer designs. Although
+some of the features of the PDP-11 are patented, machines have been made with similar bus and ISP structures.
+One company has manufactured a machine said to be "plug compatible" with a PDP-11/40. Many designers have adopted
+the UNIBUS as their fundamental architectural component. Many microprocessor designs incorporate the UNIBUS notion
+of mapping I/O and control registers into the memory address space, eliminating the need for I/O instructions without
+complicating the I/O control logic. When the LSI-11 was being designed, no alternative to the UNIBUS-style architecture
+was even considered.
+
+An earlier paper [Bell et al. 701 described the design goals and constraints for the PDP-11, beginning with a
+discussion of the weaknesses frequently found in minicomputers. The designers of the PDP-11 faced each of these
+known minicomputer weaknesses, and our goals included a solution to each one. In this section we shall review the
+original design goals and constraints, commenting on the success or failure of the PDP-11 at meeting each of them.
+
+The first weakness of minicomputers was their limited addressing capability. The biggest (and most common) mistake
+that can be made in a computer design is that of not providing enough address bits for memory addressing and management.
+The PDP-11 followed this hallowed tradition of skimping on address bits, but it was saved by the principle that a good
+design can evolve through at least one major change.
+
+For the PDP-11, the limited-address problem was solved for the short run, but not with enough finesse to support a
+large family of minicomputers. That was indeed a costly oversight, resulting in both redundant development and lost
+sales. It is extremely embarrassing that the PDP-11 had to be redesigned with memory management only two years after
+writing the paper that outlined the goal of providing increased address space. All predecessor DEC designs have suffered
+the same problem, and only the PDP-10 evolved over a long period (ten years) before a change was needed to increase its
+address space. In retrospect, it is clear that since memory prices decline 26 to 41% yearly, and users tend to buy
+"constant-dollar" systems, then every two or three years another address bit will be required.
+
+A second weakness of minicomputers was their tendency not to have enough registers. This was corrected for the PDP-11
+by providing eight 16-bit registers. Later, six 32-bit registers were added for floating-point arithmetic. This number
+seems to be adequate: there are enough registers to allocate two or three (beyond those already dedicated to program
+counter and stack pointer) for program global purposes and still have registers for local statement computation. More
+registers would increase the multiprogramming context switch time and confuse the user.
+
+A third weakness of minicomputers was their lack of hardware stack capability. In the PDP-11, this was solved with the
+autoincrement/autodecrement addressing mechanism. This solution is unique to the PDP-11 and has proven to be
+exceptionally useful. (In fact, it has been copied by other designers.)
+
+A fourth weakness, limited interrupt capability and slow context switching, was essentially solved with the device of
+UNIBUS interrupt vectors, which direct device interrupts. Implementations could go further by providing automatic
+context saving in memory or in special registers. This detail was not specified in the architecture, nor has it evolved
+from any of the implementations to date. The basic mechanism is very fast, requiring only four memory cycles from the
+time an interrupt request is issued until the first instruction of the interrupt routine begins execution.
+
+A fifth weakness of prior minicomputers, inadequate character-handling capability, was met in the PDP-11 by providing
+direct byte addressing capability. Although string instructions are not yet provided in the hardware, the common string
+operations (move, compare, concatenate) can be programmed with very short loops. Benchmarks have shown that systems
+which depend on this string-handling mechanism do not suffer for it.
+
+A sixth weakness, the inability to use read-only memories, was avoided in the PDP-11. Most code written for the PDP-11
+tends to be pure and reentrant without special effort by the programmer, allowing a read-only memory (ROM) to be used
+directly. ROMs are used extensively for bootstrap loaders, program debuggers, and for normal simple functions. Because
+large ROMs were not available at the time of the original design, there are no architectural components designed
+specifically with large ROMs in mind.
+
+A seventh weakness, one common to many minicomputers, was primitive I/O capabilities. The PDP-11 answers this to a
+certain extent with its improved interrupt structure, but the more general solution of I/O processors has not yet been
+implemented. The I/O-processor concept is used extensively in the GT4X display series, and for signal processing. Having
+a single machine instruction that would transmit a block of data at the interrupt level would decrease the CPU overhead
+per character by a factor of three, and perhaps should have been added to the PDP-11 instruction set.
+
+Another common minicomputer weakness was the lack of system range. If a user had a system running on a minicomputer and
+wanted to expand it or produce a cheaper turnkey version, he frequently had no recourse, since there were often no
+larger and smaller models with the same architecture. The problem of range and how it is handled in the PDP-11 is
+discussed extensively in a later section.
+
+A ninth weakness of minicomputers was the high cost of programming them. Many users program in assembly language,
+without the comfortable environment of editors, file systems, and debuggers available on bigger systems. The PDP-11
+does not seem to have overcome this weakness, although it appears that more complex systems are being built successfully
+with the PDP-11 than with its predecessors, the PDP-8 and PDP-15. Some systems programming is done using higher-level
+languages; the optimizing compiler for BLISS-11, however, runs only on the PDP-10.
+
+One design constraint that turned out to be expensive, but probably worth it in the long run, was that the word length
+had to be a multiple of eight bits. Previous DEC designs were oriented toward 6-bit characters, and DEC has a large
+investment in 12-, 18-, and 36-bit systems. The notion of word length is somewhat meaningless in machines like the
+PDP-11 and the IBM System/360, because data types are of varying length, and instructions tend to be multiples of 16
+bits.
+
+Microprogrammability was not an explicit design goal, partially since the large ROMs which make it feasible were not
+available at the time of the original Model 20 implementation. All subsequent machines have been microprogrammed, but
+with some difficulty and expense.
+
+Understandability as a design goal seems to have been minimized. The PDP-11 was initially a hard machine to understand,
+and was marketable only to those who really understood computers. Most of the first machines were sold to knowledgeable
+users in universities and research laboratories. The first programmers' handbook was not very helpful, and the second,
+arriving in 1972, helped only to a limited extent. It is still not clear whether a user with no previous computer
+experience can figure out how to use the machine from the information in the handbooks. Fortunately, several computer
+science textbooks [Gear 74, Eckhouse 75, and Stone and Siewiorek 75] have been written based on the PDP-11; their
+existence should assist the learning process.
+
+We do not have a very good understanding of the style of programming our users have adopted. Since the machine can be
+used so many ways, there have been many programming styles. Former PDP-8 users adopt a one-accumulator convention;
+novices use the two-address form; some compilers use it as a stack machine; probably most of the time it is used as
+a memory-to-register machine with a stack for procedure calling.
+
+Structural flexibility (modularity) was an important goal. This succeeded beyond expectations, and is discussed
+extensively in the UNIBUS section.
+
+...