Data segment: Difference between revisions

Revision as of 04:53, 28 March 2020

Revision as of 22:03, 22 April 2020

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. Uninitialized 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.

Line 8:

==Program memory==

A computer program memory can be largely categorized into two sections: read-only and read-/write. This distinction grew from early systems holding their main program in [[read-only memory]] such as [[Mask ROM]], [[Programmable read-only memory|PROM]] or [[EEPROM]]. As systems became more complex and programs were loaded from other media into RAM instead of executing from ROM the idea that some portions of the program's memory should not be modified was retained. These became the ''.text'' and ''.rodata'' segments of the program, and the remainder which could be written to divided into a number of other segments for specific tasks.

===Text===