implemented by us". The constraint that higher layers can only depend on lower layers was imposed by the designers in order to make reasoning about the system (using quasi-formal methods) more tractable, and also to facilitate building and testing the system incrementally. The layers were implemented in order, layer 0 first, with thorough testing of the abstractions provided by each layer in turn. This division of the kernel into layers was similar in some ways to Multics' later ring-segmentation model. Several subsequent operating systems have used layering to some extent, including Windows NT and macOS, although usually with fewer layers. The code of the system was written in assembly language for the Dutch Electrologica X8 computer. This computer had a word size of 27 bits, 32 kilowords of core memory, 512 kilowords of drum memory providing backing store for the LRU cache algorithm, paper tape readers, paper tape punches, plotters, and printers. See also RC 4000 Multiprogramming System Ring (computer security) Timeline of operating systems References Assembly language software Discontinued operating systems Dutch inventions Computer science in the Netherlands Information technology in the Netherlands Edsger W. Dijkstra 1968 software