Memory Allocation & Paging

Memory management is the OS function that handles allocation and deallocation of memory to processes. It tracks which parts of memory are in use, allocates memory to new processes, and frees memory when processes terminate.

Contiguous vs Non-Contiguous Allocation

Scheme	Description	Fragmentation
Contiguous	Each process occupies a single contiguous block of memory	External fragmentation
Paging	Process divided into fixed-size pages, loaded into arbitrary frames	Internal fragmentation (page boundary waste)
Segmentation	Process divided into variable-size segments (code, data, stack)	External fragmentation

Fragmentation

• Internal fragmentation: allocated memory block is larger than requested — the rest is wasted inside the block. Common with fixed-size partitions and paging (last page is rarely full).
• External fragmentation: total free memory is sufficient, but it’s scattered in small holes — no contiguous block large enough for a request. Solved by paging.

Paging

Physical memory is divided into fixed-size frames. Logical memory is divided into pages of the same size. A page table maps page → frame.

Logical address: [page number | offset]
                        ↓
                  Page table lookup
                        ↓
Physical address: [frame number | offset]

TLB (Translation Lookaside Buffer): A fast hardware cache that stores recent page→frame translations. Without a TLB, every memory access requires two physical accesses (one for the page table, one for the actual data). The TLB reduces this to one in the common case.

Segmentation

Segments are variable-sized logical units: code segment, data segment, stack segment. Each segment has a base address and a limit. Segmentation matches the programmer’s view of memory — different segments for different purposes.