Online Computer Terms Dictionary - A

address

1. <networking> e-mail address.

2. <networking> Internet address.

3. <networking> MAC address.

4. <storage, programming> An unsigned integer used to select one fundamental element of storage, usually known as a word from a computer's main memory or other storage device. The CPU outputs addresses on its address bus which may be connected to an address decoder, cache controller, memory management unit, and other devices.

While from a hardware point of view an address is indeed an integer most strongly typed programming languages disallow mixing integers and addresses, and indeed addresses of different data types. This is a fine example for syntactic salt: the compiler could work without it but makes writing bad programs more difficult.

(1997-07-01)

address bus

<processor> The connections between the CPU and memory which carry the address from/to which the CPU wishes to read or write. The number of bits of address bus determines the maximum size of memory which the processor can access.

See also data bus.

(1995-03-22)

addressed call mode

<communications> (ACM) A mode that permits control signals and commands to establish and terminate calls in V.25bis.

(1997-05-07)

addressee

<communications> One to whom something is addressed. E.g. "The To, CC, and BCC headers list the addressees of the e-mail message". Normally an addressee will eventually be a recipient, unless there is a failure at some point (an e-mail "bounces") or the message is redirected to a different addressee.

(2000-03-22)

addressing mode

1. <processor, programming> One of a set of methods for specifying the operand(s) for a machine code instruction. Different processors vary greatly in the number of addressing modes they provide. The more complex modes described below can usually be replaced with a short sequence of instructions using only simpler modes.

The most common modes are "register" - the operand is stored in a specified register; "absolute" - the operand is stored at a specified memory address; and "immediate" - the operand is contained within the instruction.

Most processors also have indirect addressing modes, e.g. "register indirect", "memory indirect" where the specified register or memory location does not contain the operand but contains its address, known as the "effective address". For an absolute addressing mode, the effective address is contained within the instruction.

Indirect addressing modes often have options for pre- or post- increment or decrement, meaning that the register or memory location containing the effective address is incremented or decremented by some amount (either fixed or also specified in the instruction), either before or after the instruction is executed. These are very useful for stacks and for accessing blocks of data. Other variations form the effective address by adding together one or more registers and one or more constants which may themselves be direct or indirect. Such complex addressing modes are designed to support access to multidimensional arrays and arrays of data structures.

The addressing mode may be "implicit" - the location of the operand is obvious from the particular instruction. This would be the case for an instruction that modified a particular control register in the CPU or, in a stack based processor where operands are always on the top of the stack.

2. In IBM System 370/XA the addressing mode bit controls the size of the effective address generated. When this bit is zero, the CPU is in the 24-bit addressing mode, and 24 bit instruction and operand effective addresses are generated. When this bit is one, the CPU is in the 31-bit addressing mode, and 31-bit instruction and operand effective addresses are generated.

["IBM System/370 Extended Architecture Principles of Operation", Chapter 5., 'Address Generation', BiModal Addressing].

(1995-03-30)

address mask

<networking> (Or "subnet mask") A bit mask used to identify which bits in an IP address correspond to the network address and subnet portions of the address. This mask is often referred to as the subnet mask because the network portion of the address can be determined by the class inherent in an IP address. The address mask has ones in positions corresponding to the network and subnet numbers and zeros in the host number positions.

(1996-03-21)

address resolution

<networking> Conversion of an Internet address into the corresponding physical address (Ethernet address). This is usually done using Address Resolution Protocol.

The resolver is a library routine and a set of processes which converts hostnames into Internet addresses, though this process in not usually referred to as resolution. See DNS.

(1996-04-09)

Address Resolution Protocol

<networking, protocol> (ARP) A method for finding a host's Ethernet address from its Internet address. The sender broadcasts an ARP packet containing the Internet address of another host and waits for it (or some other host) to send back its Ethernet address. Each host maintains a cache of address translations to reduce delay and loading. ARP allows the Internet address to be independent of the Ethernet address but it only works if all hosts support it.

ARP is defined in RFC 826.

The alternative for hosts that do not do ARP is constant mapping.

See also proxy ARP, reverse ARP.

(1995-03-20)

address space

<operating system, architecture> The range of addresses which a processor or process can access, or at which a device can be accessed. The term may refer to either physical address or virtual address.

The size of a processor's address space depends on the width of the processor's address bus and address registers.

Each device, such as a memory integrated circuit, will have its own local address space which starts at zero. This will be mapped to a range of addresses which starts at some base address in the processor's address space.

Similarly, each process will have its own address space, which may be all or a part of the processor's address space. In a multitasking system this may depend on where in memory the process happens to have been loaded. For a process to be able to run at any address it must consist of position-independent code. Alternatively, each process may see the same local address space, with the memory management unit mapping this to the process's own part of the processor's address space.

(1999-11-01)

Address Strobe

<storage> (AS) One of the input signals of a memory device, especially semiconductor memory, which is asserted to tell the memory device that the address inputs are valid. Upon receiving this signal the selected memory device starts the memory access (read/write) indicated by its other inputs.

It may be driven directly by the processor or by a memory controller.

(1996-10-02)