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the experience with the extended PDP-11 designs, it was decided to drop the constraint of the PDP-l1 instruction format in designing the extended virtual address space, or Native mode, of the VAX-1l architecture. However, in order to run existing PDP-11 programs, VAX-11 includes PDP-l1 Compatibility mode. This mode provides the basic PDP-11 instruction set without privileged instructions (as defined by the RSX-11M operating system) and floating-point instructions. Nor is the former memory management architecture (KT- 11) preserved in this mode.

Preserving the existing PDP-11 instruction formats with VAX-11 would have required too high a price in dynamic bit efficiency. Whereas the PDP-11 has a high level of efficiency in this area, adding the new operation codes for the anticipated data-types, access modes, and different length addresses would have lowered the instruction stream bit efficiency. An operation code extension field would have been required. It was also felt that data stream bit efficiency could be improved. For example, measurements showed that 98 percent of all literals were 6 bits or less in length.

Besides the desire to add the data-types for string, 32- and 64-bit integers, and decimal arithmetic, there were many other extensions proposed. These included a common procedure CALL instruction, demand paging, true indexing, context-sensitive indexing, and more I/O addressing.

Along the way, some major perturbations to the PDP-11 style were considered and rejected, often because they violated the notion of compatibility with PDP-l1. Typed data and descriptor addressing were rejected on the grounds of dynamic bit efficiency. Although system software costs may be lower with such architectures, it was not possible to quantify the gain convincingly. Also, such an architecture destroyed any compatibility, cultural or other wise, with PDP-11.

The experience with PDP-l1 (floating point, in particular) led the VAX designers to reject a soft-machine architecture, i.e., one with an instruction set (and highly microprogrammed implementations) for general purpose emulation. Their PDP-11 experience showed that embedding a data-type (once it is understood) in the architecture gives a higher performance gain than embedding the higher level language control constructs. There was also a general objection to soft machines: the problem of controlling a proliferation of instruction sets in vented by many small software groups was felt to be unmanageable. Moreover, higher level instruction sets jeopardize the ability to communicate between programs that are written in different languages. This compatibility is a major goal of VAX.

A capabilities-based architecture was rejected because it was not fully understood and because there was no performance or reliability data available from the few experimental machines which had been built.


We gratefully acknowledge the suggestions of Roger Cady, Dick Clayton, and Bruce Delagi who were eminently qualified and intimately involved in the PDP-l l's evolution.

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