Editing Memory address (section)

==Contents of each memory location==
{{see also|binary data}}
Each memory location in a [[stored-program computer]] holds a [[binary number]] or [[decimal number]] ''of some sort''.  Its interpretation, as data of some [[data type]] or as an instruction, and use are determined by the [[instruction (computer science)|instructions]] which retrieve and manipulate it.

Some early programmers combined instructions and data in words as a way to save memory, when it was expensive: The [[Manchester Mark 1]] had space in its 40-bit words to store little bits of data – its processor ignored a small section in the middle of a word – and that was often exploited as extra data storage.{{citation needed|date=October 2011}} [[Self-replicating]] programs such as [[Computer virus|viruses]] treat themselves sometimes as data and sometimes as instructions. [[Self-modifying code]] is generally [[deprecated]] nowadays, as it makes testing and maintenance disproportionally difficult to the saving of a few bytes, and can also give incorrect results because of the compiler or processor's assumptions about the machine's [[State (computer science)|state]], but is still sometimes used deliberately, with great care.

===Address space in application programming===
In modern [[computer multitasking|multitasking]] environment, an [[application program|application]] [[process (computing)|process]] usually has in its address space (or spaces) chunks of memory of following types:
* '''[[Machine code]]''', including:
** program's own code (historically known as ''[[code segment]]'' or ''text segment'');
** [[shared libraries]].
* '''[[Data (computing)|Data]]''', including:
** initialized data ([[data segment]]);
** [[.bss|uninitialized (but allocated)]] variables;
** [[run-time stack]];
** [[heap (programming)|heap]];
** [[Shared memory (interprocess communication)|shared memory]] and [[memory mapped file]]s.
Some parts of address space may be not mapped at all.

Some systems have a "split" [[memory architecture]] where machine code, constants, and data are in different locations, and may have different address sizes.
For example, [[PIC18]] microcontrollers have a 21-bit program counter to address machine code and constants in Flash memory, and 12-bit address registers to address data in SRAM.