Addressing modes describe how an instruction identifies the data it uses or determines the location of an operand. Understanding them helps explain how machine instructions, compilers, and processors work together.
An instruction needs to identify its operands. The operand may be a value embedded in the instruction, a register, or data stored in memory. An addressing mode specifies how the processor locates or obtains that operand, often by combining values from registers with constants encoded in the instruction.
All flashcards
What is an addressing mode?
A rule describing how an instruction identifies or obtains an operand, including how it may calculate the operand's effective address.
What is the effective address?
The memory location calculated for an operand access. It identifies where the operand is stored, rather than necessarily being the operand's value.
How do code and data addressing differ?
Code addressing selects an instruction location or control-flow target. Data addressing selects a location from which an instruction reads or to which it writes data.
Why do addressing-mode names vary?
Different authors and manufacturers use different terminology, and architectures may divide or combine similar operand-access behavior in different ways.
What is the purpose of LEA?
Load effective address computes an address and puts it in a register without accessing the memory at that address. It is useful for pointers and address-related arithmetic.
Quiz questions
A PC-relative branch uses the next instruction address and a signed displacement. How is its target calculated?Answer: Add the displacement to the next instruction address.
In a register-indirect jump, what supplies the control-flow destination?Answer: The address stored in a register.
For an array with 8-byte elements, how does scaled indexing convert an element index into a byte displacement?Answer: Multiply the index by 8 before adding it to the base.
In register autoincrement indirect addressing, when is the pointer register increased?Answer: After the current address has been used for the access.
What additional operation is required in memory-indirect addressing before accessing the final operand?Answer: Read a pointer from memory to obtain the final effective address.
Based on the Wikipedia article Addressing mode, licensed CC BY-SA 4.0. Adapted into a summary, flashcards and quizzes.