> On Feb 16, 2018, at 5:01 PM, Rich Alderson <[email protected]>
> wrote:
>
>> From: Timothe Litt <[email protected]>
>> Date: Fri, 16 Feb 2018 16:15:28 -0500
>
>> Overlays were used to compensate for limited virtual address space.
>
> ITYM "Overlays were used to compensate for limited address space."
>
> I first encountered the notion of overlays in IBM 1401 and System/360
> programs.
> Neither computer had any notion of virtual memory, virtual addressing, memory
> mapping, or any such hoohah. You had the memory you were given, and that was
> most decidedly that.
There was virtual addressing in early systems without hardware help. The
famous THE operating system (mid 1960s) uses virtual addressing, with 27 bit
virtual addresses which are mapped by a page table to 16 bit physical
addresses. Addressing of virtually addressed object is done by way of
subroutines, which works because the compiler makes it so. And as in many
other VM systems, virtual memory is used for buffering I/O, for shared
libraries, for user code and data, and so on. There's a bunch of nice analysis
in the design to deal with deadlock concerns and resource starvation problems;
the papers are in the "EWD archives", the collected archives of Dijkstra.
paul
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