IBM 610 workstation computer 3445

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In most (but not all) systems less than 10% of the memory contains writeable global data. The application software knows the name of these areas and gains access to them via the operating system. I am handling those global areas by clbuttifying all access as I-O. The operating system would tell the chip's memory management unit by setting a bit in the memory map table.

On a virtual memory fault i.e. the page is not in ram, the memory management hardware makes an interrupt into the operating system. The operating system suspends the task and instructs the disk to read back the page. The loader then copies the page to the appropriate area of ram and updates the pointers, memory management unit and scheduler. To make room for the required page other pages may have to be written to disk and deleted from ram. I am using the same mechanism with on chip ram.

On current hardware designs to permit the CPU to access a single byte entire pages have to be moved to and from the disk. This high level design decision was taken by computer architects and electronic engineers.

BAH we can hold philosophical debates about whether the loader is or is not part of the operating system but it is definitely not an applications program. In a multi-CPU system there may be a master operating system with a master application program running somewhere that the local operating system talks to but that is off topic.

Andrew Swallow