In
computer science
Computer science is the study of computation, information, and automation. Computer science spans Theoretical computer science, theoretical disciplines (such as algorithms, theory of computation, and information theory) to Applied science, ...
, an offset within an
array or other
data structure
In computer science, a data structure is a data organization and storage format that is usually chosen for Efficiency, efficient Data access, access to data. More precisely, a data structure is a collection of data values, the relationships amo ...
object is an
integer
An integer is the number zero (0), a positive natural number (1, 2, 3, ...), or the negation of a positive natural number (−1, −2, −3, ...). The negations or additive inverses of the positive natural numbers are referred to as negative in ...
indicating the distance (displacement) between the beginning of the object and a given element or point, presumably within the same object.
The concept of a distance is valid only if all elements of the object are of the same size (typically given in
bytes
The byte is a unit of digital information that most commonly consists of eight bits. Historically, the byte was the number of bits used to encode a single character of text in a computer and for this reason it is the smallest addressable un ...
or
words
A word is a basic element of language that carries meaning, can be used on its own, and is uninterruptible. Despite the fact that language speakers often have an intuitive grasp of what a word is, there is no consensus among linguists on its ...
).
For example, if
A is an
array of characters containing
"abcdef", the fourth element containing the character
'd' has an offset of three from the start of
A.
In assembly language
In
computer engineering
Computer engineering (CE, CoE, or CpE) is a branch of engineering specialized in developing computer hardware and software.
It integrates several fields of electrical engineering, electronics engineering and computer science.
Computer engi ...
and low-level programming (such as
assembly language
In computing, assembly language (alternatively assembler language or symbolic machine code), often referred to simply as assembly and commonly abbreviated as ASM or asm, is any low-level programming language with a very strong correspondence bet ...
), an offset usually denotes the number of address locations added to a
base address
In computing, a base address is an address serving as a reference point ("base") for other addresses. Related addresses can be accessed using an ''addressing scheme''.
Under the ''relative addressing'' scheme, to obtain an absolute address, the ...
in order to get to a specific
absolute address
In computing, a memory address is a reference to a specific memory location in memory used by both software and hardware. These addresses are fixed-length sequences of digits, typically displayed and handled as unsigned integers. This numeric ...
. In this (original) meaning of offset, only the basic address unit, usually the 8-bit
byte
The byte is a unit of digital information that most commonly consists of eight bits. Historically, the byte was the number of bits used to encode a single character of text in a computer and for this reason it is the smallest addressable un ...
, is used to specify the offset's size. In this context an offset is sometimes called a relative address.
In
IBM System/360
The IBM System/360 (S/360) is a family of mainframe computer systems announced by IBM on April 7, 1964, and delivered between 1965 and 1978. System/360 was the first family of computers designed to cover both commercial and scientific applicati ...
instructions, a 12-bit offset embedded within certain instructions provided a range of between 0 and 4096 bytes. For example, within an unconditional branch instruction (X'47F0Fxxx'), the xxx 12-bit hexadecimal offset provided the byte offset from the base register (15) to branch to. An odd offset would cause a
program check
Program (American English; also Commonwealth English in terms of computer programming and related activities) or programme (Commonwealth English in all other meanings), programmer, or programming may refer to:
Business and management
* Program m ...
(unless the base register itself also contained an odd address)—since instructions had to be aligned on half-word boundaries to execute without a program or hardware
interrupt
In digital computers, an interrupt (sometimes referred to as a trap) is a request for the processor to ''interrupt'' currently executing code (when permitted), so that the event can be processed in a timely manner. If the request is accepted ...
.
The previous example describes an indirect way to address to a memory location in the format of segment:offset. For example, assume we want to refer to memory location 0xF867. One way this can be accomplished is by first defining a segment with beginning address 0xF000, and then defining an offset of 0x0867. Further, we are also allowed to shift the hexadecimal segment to reach the final absolute memory address. One thing to note here is that we can reach our final absolute address in many ways.
An offset is not always relative to the base address of the module, for example: If you have a class and you want to retrieve the "color" attribute of this class, the offset may be 0x0100, but this offset has to be added to the offset of the class itself, not the base address. If the class' offset is 0xFF881 and the base address is 0x0A100, then to retrieve the "color" attribute both offsets are added to the base address. 0x0A100 (base) + 0xFF881 (class) + 0x0100 (attribute). Ultimately the attribute's address will be 0x109A81.
See also
*
Array Index
In computer science, an array is a data structure consisting of a collection of ''elements'' ( values or variables), of same memory size, each identified by at least one ''array index'' or ''key'', a collection of which may be a tuple, known ...
References
Computer memory
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