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In a system using segmentation, computer memory addresses consist of a segment id and an offset within the segment. [3] A hardware memory management unit (MMU) is responsible for translating the segment and offset into a physical address, and for performing checks to make sure the translation can be done and that the reference to that segment and offset is permitted.
Overlays are not a method of paging RAM to disk but merely of minimizing the program's RAM use. Subsequent architectures used memory segmentation, and individual program segments became the units exchanged between disk and RAM. A segment was the program's entire code segment or data segment, or sometimes other large data structures.
Unlike virtual storage—paging or segmentation, rollout/rollin does not require any special memory management hardware; however, unless the system has relocation hardware such as a memory map or base and bounds registers, the program must be rolled back in to its original memory locations. Rollout/rollin has been largely superseded by virtual ...
If the paging unit is enabled, addresses in a segment are now virtual addresses, rather than physical addresses as they were on the 80286. That is, the segment starting address, the offset, and the final 32-bit address the segmentation unit derived by adding the two are all virtual (or logical) addresses when the paging unit is enabled.
Flat memory model or linear memory model refers to a memory addressing paradigm in which "memory appears to the program as a single contiguous address space." [1] The CPU can directly (and linearly) address all of the available memory locations without having to resort to any sort of bank switching, memory segmentation or paging schemes.
While this article concentrates on modern MMUs, commonly based on demand paging, early systems used base and bounds addressing that further developed into segmentation, or used a fixed set of blocks instead of loading them on demand. The difference between these two approaches is the size of the contiguous block of memory; paged systems break ...
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So segmentation at the hardware level - which, in most processors, provides protection - can be used to implement segmentation at the OS level. It can even be done so on systems that provide both segmentation and paging. Segmentation at the OS level can also be done on a system with only paging support in the MMU.