Memory Management in Operating Systems

Memory Management

• Memory management is a function of the operating system (OS)
• Manages the movement of processes between main and secondary memory
• Tracks memory allocation (allocated or free) for each process
• Facilitates process allocation and updates memory statuses

Types of Addresses

• Symbolic addresses: Used in source code (variable names, constants, instruction labels)
• Relative addresses: Addresses at compilation time; compiler converts symbolic addresses
• Physical addresses: Addresses generated by the loader when a program is loaded into memory

Memory Management Terms

• Frame: Fixed-length block of main memory
• Page: Fixed-length block of data in secondary memory (can be copied to a frame)
• Segment: Variable-length block of data in secondary memory (can be copied, or divided into pages)
• Swapping: Mechanism to move processes temporarily between main and secondary memory for more space
• Fragmentation: Memory spaces become smaller fragments due to continuous loading/removal of processes.

Memory Management Requirements

• Relocation: Programs can be moved in memory during execution (for swapping), avoiding programmer knowledge of exact locations
• Protection: Each process's memory is protected from interference by other processes, preventing unauthorized access
• Sharing: Multiple processes can access the same memory area (for cooperation or shared data)
• Logical organization: Programs organized into modules (some are fixed and some are modifiable), offering an advantage to the OS and hardware
• Physical organization: System responsibility to move and organize information between main and secondary memory.

Memory Types

• Main memory: Volatile memory, fast access, relatively high cost, holds currently used programs and data
• Secondary memory: Non-volatile memory, cheaper than main memory, slower access, stores programs and data for long-term storage

Memory Partitioning Techniques

• Fixed Partitioning: Pre-defined partitions of a fixed size for loading (less efficient use of memory - common internal fragmentation)
• Dynamic Partitioning: Dynamically created partitions to fit process size; more efficient but prone to external fragmentation
• Placement Algorithms (Dynamic Partitioning):
  • Best-fit: Choose the partition closest in size to that required.
  • First-fit: Scan the memory from beginning and select the first available partition.
  • Next-fit: Scan memory starting from last placement for next available partition.

Simple Paging

• Divides memory into equally sized frames
• Processes divided into equally sized pages
• Pages loaded to available frames

Simple Segmentation

• Processes divided into segments
• Segments loaded to dynamic partitions
• No internal fragmentation (better memory utilization)

Virtual Memory (abstraction of main memory)

• Provides private address space for processes and kernel
• Allows multitasking
• Supports oversubscription and mapping between main and secondary memory (on demand)
• Virtual Memory Paging (similar to simple paging), but loads only necessary pages to reduce memory use.
• Virtual Memory Segmentation is similar, loading only required segments.