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Most of these ideas were suggested by previous work.
A paper by Dijkstra [D2] provided us with a glimpse of the beauty of a
system described as an abstract machine. His paper also suggested the
use of layered design to reduce the complexity of any single layer.
Capability based protection was described by Dennis and Van Horn [D1].
The idea of storing capabilities in special regions of memory, and
providing actions to move and manipulate the capabilities was provided
by the Chicago machine project [F1, F2, F3].
Multics provided the inspiration for a mapped address space,
protection regions within processes (Multics rings), and distributed
system code [B1, G1, S1].
Finally it should be mentioned that our audacity in beginning this
project which had very modest resources compared with the Multics
project, derived from the example of project Genie [L2]. This project
constructed a time sharing system on an SDS 940 with 3 programmers.