Editing Addressing mode (section)

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The [[Elliott 503]],<ref name="brooks">Dave Brooks. [https://web.archive.org/web/20141101211113/http://members.iinet.com.au/~daveb/history.html#puzzle "Some Old Computers"].</ref> the [[Elliott 803]],<ref name="brooks"/><ref>Bill Purvis. [https://web.archive.org/web/20080616173228/http://bil.members.beeb.net/inst803.html "Some details of the Elliott 803B hardware"]</ref> and the [[Apollo Guidance Computer]] only used absolute addressing, and did not have any index registers.
Thus, indirect jumps, or jumps through registers, were not supported in the instruction set. Instead, it could be instructed to ''add the contents of the current memory word to the next instruction''. Adding a small value to the next instruction to be executed could, for example, change a <code>JUMP 0</code> into a <code>JUMP 20</code>, thus creating the effect of an indexed jump. Note that the instruction is modified on-the-fly and remains unchanged in memory, i.e. it is not [[self-modifying code]]. If the value being added to the next instruction was large enough, it could modify the opcode of that instruction as well as or instead of the address.