previous | contents | next

C. G. Bell / J. C. Mudge

In the original 1970 PDP-11 paper (Chap. 38), a set of design goals and constraints were given, 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 their goals included a solution to each one. This section reviews the original goals, commenting on the success or failure of the PDP-11 in meeting each of them.

The weaknesses of prior designs that were noted were limited addressability, a small number of registers, absence of hardware stack facilities, elementary I/O processing, absence of growth-path family members, and high programming costs.

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 embarassing that the PDP-11 had to be redesigned with memory management² only two years after writing the paper that outlined the goal of providing increased address space. All earlier DEC designs suffered from the same problem, and only the PDP-10 evolved over a long period (15 years) before a change occurred to increase its address space. In retrospect, it is clear that another address bit is required every two or three years, since memory prices decline about 30 percent yearly, and users tend to buy constant price successor systems.

A second weakness of minicomputers was their tendency to skimp on registers. This was corrected for the PDP-11 by providing eight 16-bit registers. Later, six 64-bit registers were added as the accumulators for floating-point arithmetic. This number seems to be adequate: there are enough registers to allocate two or three registers (beyond those already dedicated to program counter and stack pointer) for program global purposes and still have registers for local statement computation.³ More registers would increase the context switch time and worsen the register allocation problem for 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, has proved to be exceptionally useful, and has been copied by other designers. The stack limit check, however, has not been widely used by DEC operating systems.

A fourth weakness, limited interrupt capability and slow context switching, was essentially solved by the Unibus interrupt vector design. 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. Implementations could go further and save the general registers, for example, in memory or in special registers. This was not specified in the architecture and has not been done in any of the implementations to date. VAX-11 provides explicit load and save process context instructions.

A fifth weakness of earlier minicomputers, inadequate character handling capability, was met in the PDP-11 by providing direct byte addressing capability. String instructions were not provided in the hardware, but the common string operations (move, compare, concatenate) could be programmed with very short loops. Early benchmarks showed that this mechanism was adequate. However, as COBOL compilers have improved and as more understanding of operating systems string handling has been obtained, a need for a string instruction set was felt, and in 1977 such a set was added.

A sixth weakness, the inability to use read-only memories as primary memory, was avoided in the PDP-11. Most code written for the PDP-11 tends to be reentrant without special effort by the programmer, allowing a read-only memory (ROM) to be used directly. Read-only memories are used extensively for bootstrap loaders, program debuggers, and for simple functions. Because large read-only memories 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 completely general solution of I/O computers has not yet been implemented. The I/O processor concept is used extensively in display processors, in communication processors, and in signal processing.