In CPU addressing modes, which mode decrements the specified register first and then uses its new contents as the effective address (pre-decrement)?

Introduction / Context:
Addressing modes define how instructions locate operands. Pre and post modification addressing modes are common on many architectures because they combine address calculation with implicit updates to address registers, improving code density and performance for stack and array operations.

Given Data / Assumptions:

• The instruction references a register, modifies it, and uses the modified value for memory access.
• The modification described is decrement first, then use as an address.
• We use standard naming: auto decrement for pre-decrement and auto increment for post-increment or pre-increment variants by ISA.

Concept / Approach:

Auto decrement addressing means the CPU subtracts an amount (often the operand size) from the register before forming the effective address. This is typical for pushing values onto a descending stack. Auto increment does the opposite, often used after reading to step through arrays.

Step-by-Step Solution:

Verification / Alternative check:

Many ISAs (for example, 68k, ARM with pre indexing) document pre decrement and pre increment forms explicitly and name the decrement form as auto decrement or pre-indexed decrement.

Why Other Options Are Wrong:

• index addressing: uses base plus index; no automatic decrement of the register.
• indirect addressing: uses a register as a pointer but does not modify it.
• auto increment: increments, not decrements.

Common Pitfalls:

• Confusing pre and post modification semantics.
• Assuming all ISAs use identical names; the underlying behavior is the key.

Final Answer:

auto decrement.