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

Memory Management Comparison

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.