Virtual Memory; Paging; Segmentation

Virtual Memory

What is Virtual Memory?

Virtual Memory allows executing processes that are not completely in main memory.

• Processes see a virtual address space larger than physical RAM
• OS swaps parts of processes between RAM and disk

Benefits:

• Run programs larger than physical RAM
• More processes can run simultaneously
• Memory protection between processes

Swapping

Swapping moves entire processes between main memory and disk (swap space/page file).

RAM: [Active Process A] [Active Process B] [Free]
Disk: [Swapped Process C] [Swap Space]

When process A needs to swap out:
A → written to disk swap area
C → loaded from disk into RAM

Performance impact: Disk I/O is much slower than RAM → excessive swapping = "thrashing"

Virtual Address Space

Virtual Address Space (per process):
+------------------+
|      Stack       |   0xFFFF...
|        ↓         |
|                  |
|        ↑         |
|       Heap       |
+------------------+
|    Data/BSS      |
+------------------+
|      Text        |   0x0000...
+------------------+

Physical RAM:
+------------------+
|   OS Kernel      |
+------------------+
|   Process Pages  |   (only needed pages loaded)
+------------------+

Demand Paging: Load pages into RAM only when accessed ("lazy loading").

Paging

What is Paging?

Paging divides:

• Physical memory into fixed-size blocks called frames

Virtual address space into fixed-size blocks called pages

Same size: page size = frame size (typically 4KB)

How Paging Works

Virtual Address: [Page Number | Offset]
                     |            |
                     ↓            |
               Page Table        |
              +---------+        |
              | Page 0→Frame 5|  |
              | Page 1→Frame 3|  |
              | Page 2→Frame 7|  |
              +---------+        |
                     |            |
                     ↓            ↓
Physical Address: [Frame Number | Offset]

Address Translation:

Virtual address: page_number=2, offset=100
Page Table: page 2 → frame 7
Physical address: frame_number=7, offset=100
Physical addr = 7 × 4096 + 100

Page Table Storage

Large page tables stored in memory; TLB (Translation Lookaside Buffer) caches recent translations.

TLB Hit: Address found in TLB → fast access TLB Miss: Access page table in memory → slower; TLB updated

Page Replacement Algorithms

When RAM is full and new page needed, which page to evict?

Thrashing

Thrashing occurs when process spends more time swapping pages than executing:

• Too many processes → each has too few frames
• Constant page faults → constant disk I/O
• CPU utilization drops drastically

Solution: Reduce degree of multiprogramming; use working set model.

Segmentation

What is Segmentation?

Segmentation divides virtual memory into variable-size segments based on logical divisions of a program:

Program Segments:
+------------------+
|  Code Segment    |   ← program instructions
+------------------+
|  Data Segment    |   ← global variables
+------------------+
|  Stack Segment   |   ← function calls
+------------------+
|  Heap Segment    |   ← dynamic allocation
+------------------+

Segmentation vs Paging

Paged Segmentation

Modern systems combine both:

• Segmented with Paged Segments — each segment is paged

Used by x86-64 architecture