X86 memory segmentation: Difference between revisions

Revision as of 18:52, 15 April 2025

Revision as of 18:53, 15 April 2025

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=== End-of-address-space quirkiness ===

{{main article|A20 line}}

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 8088 CPU's 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 (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.