In memory management, what are placement algorithms and replacement algorithms, and what does each type of algorithm decide?

Introduction / Context:
Memory management in operating systems involves two broad kinds of decisions: where to put new processes or data when they are loaded, and which existing items to evict when memory becomes full. These decisions are handled by placement algorithms and replacement algorithms respectively. Understanding the difference between these two classes of algorithms is important for analyzing fragmentation, page fault rates, and overall system performance.

Given Data / Assumptions:

• The memory manager must allocate space for new processes, segments, or pages.
• Physical memory is finite and may fill up.
• When memory is full and a new page or process must be loaded, some existing page or block may need to be removed.
• The question asks what placement and replacement algorithms are and what each decides.

Concept / Approach:
Placement algorithms are used when a process or block is first loaded into memory. In variable partition systems, placement algorithms such as first fit, best fit, and worst fit determine where in the available free memory regions the new allocation should go. Replacement algorithms, such as FIFO, LRU, and optimal replacement, come into play when memory is full and a new page must be loaded; they decide which current page or block to evict to make room. The key distinction is that placement chooses the initial location, while replacement chooses what to remove later under pressure.

Step-by-Step Solution:
Step 1: Identify placement algorithms as those that work with free memory regions to find a suitable location for a new process, segment, or block.Step 2: Note that examples of placement algorithms include first fit, best fit, and worst fit in variable partition memory management.Step 3: Identify replacement algorithms as those that select which existing page or block should be removed when memory space is needed for a new one.Step 4: Note that examples of replacement algorithms include FIFO, LRU, clock, and optimal page replacement in paging systems.Step 5: Conclude that the correct answer must clearly assign initial placement decisions to placement algorithms and eviction decisions to replacement algorithms.

Verification / Alternative check:
Memory management chapters typically first discuss placement strategies for contiguous allocation, presenting diagrams that show how first fit or best fit choose holes in free memory. Later, when paging and segmentation are covered, the focus shifts to page replacement policies that determine how to handle page faults when all frames are occupied. These sections clearly separate placement and replacement concerns, confirming the distinction described in the correct option.

Why Other Options Are Wrong:
Option B is incorrect because CPU scheduling and process termination are handled by different components, not by placement or replacement algorithms in memory management. Option C is wrong because both placement and replacement algorithms operate on main memory or virtual memory, not solely on file systems or CPU scheduling. Option D is incorrect because using the same name for both would ignore the clear conceptual difference between initial placement and later eviction.

Common Pitfalls:
Students sometimes confuse placement algorithms in contiguous allocation systems with page replacement algorithms in paging systems, since both involve choosing locations or victims. Another pitfall is thinking that placement decisions are made only once and never revisited; in reality, compaction or dynamic relocation can change placement, and replacement decisions can indirectly influence fragmentation and locality. Keeping the initial placement versus eviction distinction in mind helps organize these topics.

Final Answer:
Placement algorithms decide where in memory to place a new process, segment, or block when it is first loaded, while replacement algorithms decide which existing page or block to remove when memory is full and new space is needed.