503, the Elliott 803, 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 JUMP 0 into a JUMP 20, 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. Glossary See also Instruction set architecture Address bus Notes References External links Addressing modes in assembly language Addressing modes Computer architecture Machine code Assembly languages