Multics
Multics (Multiplexed Information and Computing Service), one of the foundations for later big guns like Unix, was at one point of time, one of the most influential early time-sharing operating systems. Right from 1964 when it was first setup successfully, it remained an important tool for development for almost two decades. Even at later stages, when it was outdated, old school programmers continued to use it till the dawn of the millenium.
Multics, ancient and obsolete now, employed a humble single level store for data access, almost ignoring the apparent difference between files (called segments in Multics) and process memory. The memory of a process used to consist solely of segments which were mapped into an address space. To control them, the process basically required usual CPU commands, and the operating system then made sure that all the modifications were saved to disk.
In POSIX terminology, it was as if every file was mmap()ed. Conversely, in Multics there was no model of process memory at all, split from the memory used to hold mapped in files, as Unix (a much more advance version) has. The entire memory in the system was part of some segment, which was visible in the file system. Now this included the temporary scratch memory of the process, its kernel stack, etc.
One drawback of this characteristic was that the size of segments was limited to barely 256K 36-bit words (slightly more than 1 megabyte). This was because of the particular hardware architecture of the machines Multics ran on, more of a mechanical default than a software shortcoming, one could say. For files bigger than this, extra code had to be used to work on these multi-segment files. But in the days before large databases and later huge bitmap graphics, this limit was rarely encountered.
The biggest achievement of Multics was the fact that it laid the foundations for the later generations of operating systems. The story of how Unix evolved from this humble Bell Labs project is open sourcing lore. However, the age of Multics is long gone. The last running Multics installation was shut down on October 31, 2000.
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