Memory segment
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On the Intel x86 architecture, a memory segment is the portion of memory which may be addressed by a single index register without changing a 16-bit segment selector. In real mode or protected mode on the 80286 processor (or V86 mode on the 80386 and later processors), a segment is 64 kilobytes in size (using 16-bit index registers). In 32-bit protected mode, available in 80386 and subsequent processors, a segment is 4 gigabytes (due to 32-bit index registers).
In 16-bit mode, enabling applications to make use of multiple memory segments (in order to access more memory than available in any one 64K-segment) was quite complex, but was viewed as a necessary evil for all but the smallest tools (which could do with less memory). The root of the problem was that no appropriate address-arithmetic instructions suitable for flat addressing of the entire memory range were available. Flat addressing is possible by applying multiple instructions, which however leads to slower programs.
The introduction of 32-bit operating systems and the more comfortable 32-bit flat memory model has resulted in the almost elimination in use of segmented addressing towards the end of the 1990s. However, using the flat memory model has resulted in the 4 gigabyte limit not being far from everyday use. Segmentation allows operating systems to make the limit a per-process virtual address space issue, utilizing up to a maximum of 64 gigabytes of system memory, but the reluctance to eventually return to segmentation is often cited as motivation to move towards 64-bit processors.