Addressing Modes

Assembler Addressing Modes — Quick Reference

Table explains common addressing modes, assembler syntax, effective-address (EA) formula and short usage notes.

Name	Assembler Syntax	Addressing Function (EA = Effective Address)	Explanation and Use
Index	X(Ri)	EA = [Ri] + X	EA is generated by adding a constant offset X to the contents of the index register Ri. The register contents are not modified. Common when accessing arrays: Ri holds base address, X selects element/field. (p. 38)
Base with Index	(Ri, Rj)	EA = [Ri] + [Rj]	The effective address is the sum of two registers' contents — often one acts as base, the other as index. Provides flexibility since both components can change; useful for segmented addressing or multi-level referencing. (p. 39)
Base with Index and Offset	X(Ri, Rj)	EA = [Ri] + [Rj] + X	EA is sum of two registers plus a constant offset. Can model multi-dimensional addressing (e.g., 3-D arrays) or complex frame offsets. (p. 40)
Relative	X(PC)	EA = [PC] + X	This variant uses the Program Counter (PC) instead of a general register. It commonly specifies branch/jump targets as an offset from the current PC. (p. 40) Enables position-independent code and PC-relative addressing for branch targets or data in the same code segment.

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