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The data segment is read-write, since the values of variables can be altered at run time. This is in contrast to the ''read-only data segment'' (''{{visible anchor|rodata}} segment'' or ''.rodata''), which contains static constants rather than variables; it also contrasts to the [[code segment]], also known as the text segment, which is read-only on many architectures. Zero-initialized data, both variables and constants, is instead in the [[BSS segment]].
Historically, to be able to support memory address spaces larger than the native size of the internal address register would allow, early CPUs implemented a system of segmentation whereby they would store a small set of indexes to use as offsets to certain areas. The [[Intel 8086]] family of CPUs provided four segments: the code segment, the data segment, the stack segment and the extra segment. Each segment was placed at a specific ___location in memory by the software being executed and all instructions that operated on the data within those segments were performed relative to the start of that segment. This allowed a 16-bit address register, which would normally be able to access 64 KB of memory space, to access
This segmenting of the memory space into discrete blocks with specific tasks carried over into the programming languages of the day and the concept is still widely in use within modern programming languages.
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