***************************
Chapter 8: Main Memory
***************************

Main Concepts
=============

- **Main memory** provides the CPU with fast access to instructions and data.
- The **operating system (OS)** manages memory allocation between processes.
- Memory management ensures:
  - Efficient utilization of available memory
  - Process isolation and protection
  - Fast address translation

---------------------------------------------------
Background: Logical vs. Physical Address Space
---------------------------------------------------

**Logical Address (Virtual Address)**
  - Generated by the CPU during program execution.
  - Used by a process as if it has its own continuous memory space.

**Physical Address**
  - The actual location in main memory (RAM).
  - Determined after translation from the logical address.

**Address Binding**
  - The process of mapping logical to physical addresses.
  - Occurs at one of three stages:
    - **Compile-time binding:** If memory location known at compile time.
    - **Load-time binding:** Performed when program is loaded.
    - **Execution-time binding:** Requires hardware support (MMU).

**Memory-Management Unit (MMU)**
  - Hardware that translates logical addresses to physical addresses at runtime.
  - Allows process relocation and protection.

------------------------------
Swapping
------------------------------

- **Swapping** moves processes between main memory and a **backing store** (e.g., disk).
- Used to free memory for other processes.

**Key Points**
  - Entire processes are swapped out/in.
  - **Swap time** is proportional to process size and disk transfer speed.
  - Can cause high I/O overhead.

**Swap Space**
  - A dedicated area on disk used for temporarily holding swapped-out processes.

------------------------------
Contiguous Memory Allocation
------------------------------

- Each process is allocated one continuous block of physical memory.

**Memory Partitioning Methods**
  - **Fixed Partitioning:**
    - Memory divided into fixed-sized blocks.
    - Can cause **internal fragmentation** (unused space inside allocated region).
  - **Dynamic Partitioning:**
    - Partitions created as needed.
    - Can cause **external fragmentation** (free space scattered between blocks).

**Allocation Strategies**
  - **First Fit:** Allocate first block large enough.
  - **Best Fit:** Allocate smallest suitable block.
  - **Worst Fit:** Allocate largest available block.

**Compaction**
  - A technique to reduce external fragmentation by moving processes together.

------------------------------
Paging
------------------------------

- Solves the problem of external fragmentation.
- Divides physical memory into fixed-sized **frames**.
- Divides logical memory into **pages** of the same size.

**Address Translation**
  - Logical address = (page number, offset)
  - Page number used to index the **page table**.
  - Page table maps logical pages to physical frames.

**Advantages**
  - No external fragmentation.
  - Easy process relocation.
  - Efficient memory use.

**Disadvantages**
  - Requires additional memory for the page table.
  - Can cause **internal fragmentation** (unused space in frames).

**Hardware Support**
  - **Translation Lookaside Buffer (TLB):** Cache for fast page-table lookups.

------------------------------
Segmentation
------------------------------

- Divides memory into **variable-sized segments** based on program structure (e.g., code, data, stack).

**Logical Address Structure**
  - (segment number, offset)

**Segment Table**
  - Each entry has:
    - **Base address:** Start of the segment in physical memory.
    - **Limit:** Length of the segment.

**Advantages**
  - Reflects program’s logical organization.
  - Supports protection and sharing easily.

**Disadvantages**
  - Can cause external fragmentation.
  - More complex memory management than paging.

------------------------------
Summary
------------------------------

.. list-table:: Memory Management Comparison
   :widths: 25 35
   :header-rows: 1

   * - **Technique**
     - **Fragmentation Type / Notes**
   * - Contiguous Allocation
     - Internal and External Fragmentation
   * - Paging
     - Internal Fragmentation Only
   * - Segmentation
     - External Fragmentation Only
   * - Swapping
     - Performance Overhead (Disk I/O)

**Key Takeaways**
  - Logical and physical addresses are distinct.
  - Paging and segmentation provide flexibility.
  - Swapping enables multitasking at the cost of performance.
  - The OS balances efficiency, speed, and protection in memory management.