The Memory
The memory system of the first Nord-10s were built up of 8K 16-bit modules housed in a special memory rack. One 19-inch rack could take up to eight 8K modules. It was possible to extend the Nord-10's physical address space beyond 64K up to a maximum of 256K 16-bit words. The paging system translated a 16-bit virtual address into an 18-bit physical address.
The hardware paging system made it possible for one user to write programs up to 64K (virtual memory), and only parts of the program to be present in physical memory at any time (using dynamic memory allocation). The paging system divided memory into 1K pages. The 4 page index tables were found in a 256 word extremely fast memory block. The calculation of a physical address resulted in no appreciable delay in the effective memory cycle time.
The Nord-10 had two independent protection systems. Each individual page could be protected against being read from, written into (type data or type instructions), or against reading of instructions. In addition, there was a system which divided the pages into four different categories, called rings. The rings had a priority from 0 to 3. A program on a lower ring was never allowed to access the pages on a higher ring. Programs which ran on rings 2 and 3 could use the whole Nord-10 instruction set, while programs on rings 0 and 1 only had a limited instruction set available. The different rings were displayed on the operator's panel. For example, ring 0 (User) may have held a user program, while compilers and assemblers ran in ring 1 (Protected User). The bulk of the operating system could run in ring 2 (System), and the kernel in ring 3 (Protected System). If one attempted to execute privileged instructions in ring 0 or 1, or attempts were made to accessed a protected page, a hardware status interrupt would automatically be generated on program level 14 indicating the error.
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