OPERATING SYSTEM NOTES

3.2. Memory allocation technique

 

---

Swapping Time Example

• Swapping is mechanisms in which a process can be swapped temporarily out of main memory (or move) to secondary storage (disk) and make that memory available to other processes. At some later time, the system swaps back the process from the secondary storage to main memory.
  Though performance is usually affected by swapping process but it helps in running multiple and big processes in parallel and that's the reason Swapping is also known as a technique for memory compaction.

Memory Allocation

Main Memory Layout

• Main memory usually has two partitions:
   Low Memory -- Operating system resides in this memory. 
   High Memory -- User processes are held in high memory.

Type	Description
Single-Partition Allocation	Uses relocation + limit register to protect OS & processes.
Multiple-Partition Allocation	Memory is split into fixed-sized partitions. Each partition holds one process.

---

Fragmentation

Type	Description
External	Enough total memory exists, but not in a contiguous block.
Internal	Memory block > process size, leading to unused space inside the block.

Solutions

• External → Use Compaction (rearrange memory)

• Internal → Allocate best-sized block (just big enough)

---

Contiguous Memory Allocation

Single-Partition

• OS in low memory, user processes in high memory

• Use relocation and limit registers for protection

Multiple-Partition

• Divide memory into fixed partitions

• Each partition runs one process

• OS maintains free/used memory table

 Allocation Strategies

Strategy	Description
First-fit	First block that fits is allocated
Best-fit	Smallest block that fits is allocated
Worst-fit	Largest available block is used

Paging

Paging is a memory management technique used by operating systems to avoid external fragmentation and efficiently manage memory.

Key Points

• Divides memory into fixed-size blocks:

  • Pages (logical)

  • Frames (physical)

• No external fragmentation

• Internal fragmentation may occur

Address Translation

In an operating system that uses paging, every time a program accesses memory (for data or instructions), the address it uses is not the real physical address in RAM. Instead, it uses a logical address (also called a virtual address). The operating system and hardware work together to convert this into a physical address that actually points to a location in RAM.

• Logical Address = Page Number + Offset

• Physical Address = Frame Number + Offset

• Uses Page Table to map pages → frames

Advantages & Disadvantages

Pros	Cons
No external fragmentation	Page Table takes space
Easy swapping (equal size)	May cause internal fragmentation
Efficient implementation	--

---

Segmentation

Key Points

• Divides process into logical segments (e.g., code, stack, data)

• Variable size segments (unlike fixed-size pages)

• Segments loaded into non-contiguous memory, but each segment is contiguous

 Address Format

• Logical Address = Segment Number + Offset

• Segment Table stores:

  • Base (starting address)

  • Length (size)

Paging	Segmentation
Fixed-size pages	Variable-size segments
Based on memory	Based on logical divisions in a program
Less flexible	More flexible, more natural