X86 memory segmentation: Difference between revisions

Revision as of 19:53, 3 May 2025

Revision as of 22:24, 14 May 2025

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While real mode segments are ''technically'' always 64 [[Kilobyte|KB]] long, the practical effect is only that no segment can be ''longer'' than 64 KB, rather than that every segment as actually used in a program ''must'' be treated as 64 KB long – dealing with effectively smaller segments is possible: usable sizes range from 16 through 65,536 bytes, in 16-byte steps. Because there is no protection or privilege limitation in real mode, it is still entirely up to the program to coordinate and keep within the bounds of any segments. This is true both when a segment is programmatically treated as smaller than, or the full 64 KB, but it is also true that any program can always access any memory by just changing segments, since it can arbitrarily set segment selectors to change segment addresses with absolutely no supervision. Therefore, while real mode can be thought of as allowing different segment lengths, and as allowing segments to be overlapping or non-overlapping as desired, none of this is restrictively enforced by the CPU.

The effective 20-bit [[address space]] of PC/XT-generation CPUs limits the [[memory address|addressable memory]] to 2²⁰ bytes, or 1,048,576 bytes (1 [[MegabyteMebibyte|MBMiB]]). This derived directly from the hardware design of the Intel 8086 (and, subsequently, the closely related 8088), which had exactly 20 [[address bus|address pins]]. (Both were packaged in 40-pin DIP packages; even with only 20 address lines, the address and data buses were multiplexed to fit all the address and data lines within the limited pin count.)

{{anchor|Paragraph}}{{resize|105%|{{sidebox|above='''Example calculation:'''|text=A segment value of 0Ch (12) would give a linear address at C0h (192) in the linear address space. The address offset can then be added to this number. 0Ch:0Fh (12:15) would be C0h+0Fh=CFh (192+15=207), CFh (207) being the linear address.}}}}Each segment begins at a multiple of 16 bytes, called a ''paragraph'', from the beginning of the linear (flat) address space. That is, at 16 byte intervals. Since all segments are technically 64 KB long, this explains how overlap can occur between segments and why any ___location in the linear memory address space can be accessed with many segment:offset pairs. The actual ___location of the beginning of a segment in the linear address space can be calculated with ''segment'' × 16. Such address translations are carried out by the segmentation unit of the CPU.