Editing Addressing mode (section)

==Caveats==
{{Unreferenced section|date=May 2012}}
There are no generally accepted names for addressing modes: different authors and computer manufacturers may give different names to the same addressing mode, or the same names to different addressing modes. Furthermore, an addressing mode which, in one given architecture, is treated as a single addressing mode may represent functionality that, in another architecture, is covered by two or more addressing modes. For example, some [[complex instruction set computer]] (CISC) architectures, such as the [[Digital Equipment Corporation|Digital Equipment Corporation (DEC)]] [[VAX]], treat registers and [[value (computer science)|literal or immediate constants]] as just another addressing mode. Others, such as the [[IBM System/360]] and its successors, and most [[reduced instruction set computer]] (RISC) designs, encode this information within the instruction. Thus, the latter machines have three distinct instruction codes for copying one register to another, copying a literal constant into a register, and copying the contents of a memory location into a register, while the VAX has only a single "MOV" instruction.

The term "addressing mode" is itself subject to different interpretations: either "memory address calculation mode" or "operand accessing mode". Under the first interpretation, instructions that do not read from memory or write to memory (such as "add literal to register") are considered not to have an "addressing mode". The second interpretation allows for machines such as VAX which use operand mode bits to allow for a register or for a literal operand. Only the first interpretation applies to instructions such as "load effective address," which loads the address of the operand, not the operand itself.

The addressing modes listed below are divided into code addressing and data addressing. Most computer architectures maintain this distinction, but there are (or have been) some architectures which allow (almost) all addressing modes to be used in any context.

The instructions shown below are purely representative in order to illustrate the addressing modes, and do not necessarily reflect the mnemonics used by any particular computer.

Some computers, e.g., [[IBM 709]], RCA 3301,<ref>{{cite manual
 | title       = System Reference Manual - RCA 3301 REALCOM EDP
 | id          = 94-16-000-1
 | date        = September 1967
 | url         = http://bitsavers.org/pdf/rca/3301/94-16-000_RCA_3301_System_Reference_Manual_Sep67.pdf
 | publisher   = [[RCA]]
 | access-date = December 21, 2023
 }}
</ref> do not have a single address mode field but rather have separate fields for indirect addressing and indexing.