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→End-of-address-space quirkiness: --Missed one. |
→End-of-address-space quirkiness: Clarified: It is actually not complex to use separate segments for program code, data, and the stack, or to store separate hetereogeneous data in multiple 64 KB segments. What becomes quite complex is handling a single data object larger than 64 KB. |
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The last segment, FFFFh (65535), begins at linear address FFFF0h (1048560), 16 bytes before the end of the 20-bit address space, and thus can access, with an offset of up to 65,536 bytes, up to 65,520 (65536−16) bytes past the end of the 20-bit address space of the 8086 or 8088 CPU. A further 4,094 next-highest 64K-segments also still cross that 1MB-threshold, but by less and less. On the 8086 and 8088 CPUs, these address accesses were wrapped around to the beginning of the address space such that 65535:16 would access address 0, and e.g. 65533:1000 would access address 952 of the linear address space. The fact that some programs written for the 8088 and 8086 relied on this quirky wrap-around as a feature led to the [[Gate A20]] compatibility issues in later CPU generations, with the [[Intel 286]] and above, where the linear address space was expanded past 20 bits.
In 16-bit real mode, enabling applications to make use of multiple memory segments for a single data structure (in order to access more memory than available in any one 64K-segment) is 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 is that no appropriate address-arithmetic instructions suitable for flat addressing of the entire memory range are available.{{Citation needed|date=July 2011}} Flat addressing is possible by applying multiple instructions, which however leads to slower programs.
The ''[[x86 memory models|memory model]]'' concept derives from the setup of the segment registers. For example, in the ''tiny model'' CS=DS=SS, that is the program's code, data, and stack are all contained within a single 64 KB segment. In the ''small'' memory model DS=SS, so both data and stack reside in the same segment; CS points to a different code segment of up to 64 KB.
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