Process Execution and Memory Management

Key Concepts

• Process Execution: For a process to execute, it must be in the main memory (physical memory).
• Main Memory (Physical Memory): Finite in size, sometimes smaller than the process size.
• Secondary Memory: Where processes reside before being loaded into main memory.

Handling Large Processes

• Loading only parts of a process into main memory can be more efficient.
• Processes can be divided into pages (equal-sized portions).
  • Example: If CPU needs page 3, only that page is loaded into main memory.
• Paging: Process of breaking a process into pages.
• Demand Paging: Pages loaded on demand (only when needed).

Virtual Memory

• By loading only required pages, it seems main memory is larger than it is.
• Virtual Memory: Concept where a portion of the hard disk is used as an extension of main memory.

Page Faults

• Occur when the required page is not in main memory.
• Handling Page Faults:
  • OS contacts the hard disk to locate the required page.
  • If main memory is full, a page must be swapped out to load the new one.
  • Swapping: Swapping a page into main memory and another out into secondary memory.

Page Replacement Algorithms

• First-In-First-Out (FIFO): Replaces the oldest page in memory.
• Page Table: Keeps track of which pages are in main memory and which are in secondary.
  • Valid/Invalid Bits: Indicate if a page is in main memory (valid) or not (invalid).

Example Scenario

• Frames: Allocated slots in main memory.
• Page References: Order in which CPU requests pages.
• FIFO Algorithm Application:
  • Load pages into frames sequentially.
  • When frames are full, oldest page is swapped out.

Page Faults and Hit Ratio

• Page Fault: Page not found in main memory, leading to a swap/miss.
• Hit Ratio: Number of hits (page found in memory) divided by total requests.
  • Example Calculation:
    • Total requests: 19
    • Hits: 6
    • Hit Ratio: 6/19

Practical Example

• When fast-forwarding a movie in VLC player, a short delay occurs due to page faults.

Further Topics

• Discussion on LRU (Least Recently Used) and Optimal Page Replacement techniques in upcoming lectures.