ITPEC FE Morning October 2018 Question 11
Which of the following is the addressing mode that references data in main memory as shown in the figure below?
Explanation
Indirect addressing — the address part points to a memory word that holds the address of the data, not the data itself.
Follow the arrows in the figure:
- •The address part of the instruction holds 20
- •Main memory at address 20 holds 25, which is the effective address
- •Main memory at address 25 holds the data
Reaching the operand takes two memory accesses, first for the address and then for the data.
Answer (c)
Why not others:
- •(a) Direct addressing — the address part is itself the effective address, so the operand would be the value 25 stored at address 20
- •(b) Indexed addressing — the effective address is the address part plus the contents of an index register; no register appears in the figure
- •(d) Relative addressing — the effective address is the address part plus the program counter, used for position-independent jumps
Key rule: Count the memory lookups: direct uses the address part as the address, indirect reads an address from memory first, and indexed or relative add a register (index or program counter) to it.
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