Addressing Modes – GATE CSE Computer Organization ArchitecturePractice Questions & PYQs

With only immediate and direct addressing, all memory references must be fixed at compile time, precluding any runtime address calculation. Pointers require indirect addressing to dereference a stored address, which is unsupported. Arrays and records require indexed or displacement addressing (e.g., $Base + Offset$) to access elements or fields dynamically based on variables, which is impossible using only static direct addresses. Recursive procedures necessitate a stack where local variables are accessed relative to a dynamic stack pointer, requiring indirect or register-relative addressing. Since all these features rely on dynamic memory access, none can be implemented.

In PC relative addressing, the effective address ($EA$) of an instruction or operand is calculated as $EA = PC + \text{offset}$, where the $PC$ is the program counter and the offset is a fixed displacement. Because this calculation relies on the relative distance between the current instruction and the target memory location, the relationship remains invariant regardless of where the program resides in physical memory. If the entire program block is moved, both the $PC$ and the target location shift by the same amount, keeping the difference (the offset) constant. Consequently, the instruction references remain accurate without requiring any modifications to the underlying machine code, enabling position-independent execution.

Relative addressing calculates the effective address using the formula $Effective Address = PC + Offset$. Because this offset is defined relative to the current Program Counter ($PC$) rather than an absolute memory address, the code functions correctly regardless of where it is loaded in memory. This specific capability allows a block of code to be executed from any memory location without modification, which is the defining characteristic of position-independent code ($PIC$). Consequently, relative addressing is the essential mechanism used to generate $PIC$, as it ensures jumps and data references maintain consistent relative distances regardless of the absolute base address.