Xenix 286?

On 2/23/2006 at 3:09 PM Liam Proven wrote: We're suffering a bit from nomenclature problems. Generally speaking, paging consists of dividing addressing space up into equally-sized chunks of memory, and swapping these chunks of memory from a secondary store. Optimization (i.e. making sure that related pages are in primary memory at the same time) is left to the operating system (hence, you'll see terms like "demand paging" (basically dumb paging) or "working set paging" (an attempt to predict the behavior of a program based on its history)). Segmentation operates at a higher level. A segment may represent all or part of a DLL or data structure--in other words, contents of a segment are more likely to be related. Segments are not usually of equal size. Since a segment is swapped in as unit, contiguous physical memory must be available for it--hence Windows insistence on using handles for memory allocation (the implication is that things can move). Consider a segment to be a chapter in a book--it's a logical entity. In general, a well-designed program operating in segmented mode has the potential of swapping less than one that is paged. However, paging frees the programmer from worrying about organizing things in segmented fashion (and having to deal with selectors)--what is seen is simply a huge linear address space (Usually CS=DS=ES=FS=GS and nobody changes them). But when we talk about segmentation, don't confuse this with Intel's unfortunate labeling of CS DS ES FS and GS as "segment" registers in real mode. That was a bad name for them--better to have called them "paragraph registers". In fact, when you get into 286 segmented mode, Intel calls them "selectors", not segment registers. Yes, in 286 segmented mode, the size of a segment is limited to 64K, but that's because the segment length field is 16 bits. Segments can be smaller than 64K. Cheers, Chuck