In classic Unix memory management terminology, which statement best describes the swap space map data structure referred to as a map?

Introduction / Context:
Traditional Unix kernels used simple but effective data structures to manage free space in memory and swap areas. One such structure was called a map, which maintained information about free segments on a device or in a region of memory. Understanding the idea behind the swap space map helps clarify how the system tracks which parts of swap are available for use when paging or swapping processes.

Given Data / Assumptions:

• The system uses a swap device to store process images or pages that do not fit in main memory.
• The kernel needs to know where free space exists on this swap device.
• A map is described as an array that contains addresses and counts of free units.

Concept / Approach:
In this context, a map is a simple array of entries, each describing a contiguous area of free space on the swap device. Each entry typically contains a starting block address and a size measured in blocks or other resource units. When the operating system needs to allocate space on the swap device, it scans the map to find an entry that is large enough and then splits or updates that entry to reflect the allocation. When space is freed, the map is updated and adjacent free regions may be coalesced. This scheme allows efficient management of free resources without complex data structures.

Step-by-Step Solution:
Step 1: Interpret the original statement that a map is an array containing addresses of free space and the number of allocatable resource units there.Step 2: Recognize that this description matches a free space map, where each entry describes a region of available space.Step 3: Understand that the map is used by the kernel to satisfy requests for swap space when processes are swapped or pages are written out.Step 4: Exclude alternatives that describe unrelated structures such as device driver lists or hardware registers.Step 5: Choose the option that clearly defines the map as an array like structure recording starting addresses and sizes of free regions on the swap device.

Verification / Alternative check:
Classic Unix implementation descriptions refer to map data structures for both core memory and swap space. They often show code that walks an array of map entries, searching for an entry with a large enough size field. When an allocation is made, the starting address is returned and the entry is adjusted. When space is freed, new entries are added and adjacent free segments are merged. These operations align directly with the explanation that the map tracks free segments as address and size pairs.

Why Other Options Are Wrong:
Option B: Suggests that the map stores the complete contents of all running processes, which would require far more information than a simple free space map and is not how swap management is implemented.Option C: Describes a list of device driver names, which has nothing to do with free space on the swap device.Option D: Refers to a hardware register pointing to physical memory; such a register cannot describe multiple free segments and is not called a map.

Common Pitfalls:
Learners sometimes confuse maps of free space with page tables or other data structures that track allocated memory. A free space map focuses only on available regions, not on every allocated block. Another confusion is thinking that modern systems use exactly the same structures as early Unix. While implementations have evolved, the underlying idea of tracking free segments with address and size pairs remains important.

Final Answer:
The correct answer is It is an array like data structure that records the starting addresses and sizes of free space segments in the swap device so that allocatable swap resources can be found efficiently.