In computer science, an offset within an

array An array is a systematic arrangement of similar objects, usually in rows and columns. Things called an array include: {{TOC right Music * In twelve-tone and serial composition, the presentation of simultaneous twelve-tone sets such that the ...

or other

data structure In computer science, a data structure is a data organization, management, and storage format that is usually chosen for efficient access to data. More precisely, a data structure is a collection of data values, the relationships among them, a ...

object is an

integer An integer is the number zero (), a positive natural number (, , , etc.) or a negative integer with a minus sign (−1, −2, −3, etc.). The negative numbers are the additive inverses of the corresponding positive numbers. In the language o ...

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 uni ...

words A word is a basic element of language that carries an objective or practical 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 consen ...

). For example, if A is an

of characters containing "abcdef", the fourth element containing the character 'd' has an offset of three from the start of A.

In assembly language

computer engineering Computer engineering (CoE or CpE) is a branch of electrical engineering and computer science that integrates several fields of computer science and electronic engineering required to develop computer hardware and software. Computer engineers ...

and low-level programming (such as

assembly language In computer programming, assembly language (or 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 b ...

), 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 used at various levels by software and hardware. Memory addresses are fixed-length sequences of digits conventionally displayed and manipulated as unsigned integers. ...

. 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 uni ...

, 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 that was announced by IBM on April 7, 1964, and delivered between 1965 and 1978. It was the first family of computers designed to cover both commercial and scientific applica ...

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 12bit hexadecimal offset provided the byte offset from the base register (15) to branch to. An odd offset would cause a

program check Program, programme, programmer, or programming may refer to: Business and management * Program management, the process of managing several related projects * Time management * Program, a part of planning Arts and entertainment Audio * Programm ...

(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 (GUI) 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's 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.

In assembly language

See also