Myth: “Virtual memory” is the mechanism that a kernel uses to make more memory available than is actually physically installed, by setting aside a disk partition for the overflow and copying pages between memory and disk as needed.
I acquired this belief very early in my programming career, but it turns out that swapping pages to disk is merely one of the many things that “virtual memory” makes possible.
Fact: “Virtual memory” is a hardware (CPU) mechanism, which, every single time memory is accessed, references a kernel-specified data structure called a “page table” to arbitrarily frobnicate the high bits of the address, which is called “translating” from a “linear address” to a “physical address”. (The page table gets cached by a translation lookaside buffer, so the lookup is usually quite efficient!)
This fact became very real to me this week as I made a kernel from scratch: I was moderately surprised that I needed to set up a page table, when I had always thought of virtual memory as a somewhat advanced kernel feature. Today, I learned how “relocatable” and “PIC” – terms I’d encountered in the past and never really understood – suddenly make sense in this context.
