Data segment: Difference between revisions

Revision as of 14:07, 17 July 2019

Revision as of 02:11, 11 October 2019

Line 1:

In [[computing]], a '''data segment''' (often denoted '''.data''') is a portion of an [[object file]] or the corresponding [[virtual address space]] of a program that contains initialized [[static variable]]s, that is, [[global variable]]s and [[static local variable]]s. The size of this segment is determined by the size of the values in the program's source code, and does not change at [[Run time (program lifecycle phase)|run time]].

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-initializedUninitialized 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 1 MB of memory space.