CSE330 Chapter 9

What are the goals of memory management?

Keep CPU busy; Protect process memory; Allow relocation; Manage allocation efficiently

What does the Memory Management Unit (MMU) do?

Translates logical to physical addresses; Enforces protection; Supports relocation

What is a logical address?

Address generated by CPU; Also called virtual address

What is a physical address?

Actual location in main memory; Used by hardware

When can binding of addresses occur?

At compile time; At load time; At execution time

What are the advantages of dynamic loading and linking?

Load routines when needed; Reduce memory use; Share libraries across programs

What describes contiguous allocation?

Each process in one block; May cause external fragmentation

What causes external fragmentation?

Free memory split into holes; Enough total space but not contiguous

What causes internal fragmentation?

Allocated block larger than needed; Wasted space inside blocks

When is compaction possible?

When relocation is dynamic; To combine free space

What are the three main allocation strategies?

First-fit; Best-fit; Worst-fit

What does paging divide memory into?

Physical frames and logical pages

Advantages of paging?

No external fragmentation; Noncontiguous allocation; Easy sharing

What parts make up a logical address in paging?

Page number; Page offset

What does the page table store?

Frame numbers; Valid/invalid bits; Protection bits

What hardware assists address translation?

MMU; TLB

Disadvantages of paging?

Page-table overhead; Internal fragmentation in last page

Page size trade-offs?

Small pages = less waste but larger tables; Large pages = fewer tables but more waste

Why are large page tables a problem?

Consume memory; Slow lookups

Solutions for large page tables?

Hierarchical paging; Hashed page tables; Inverted page tables

What characterizes an inverted page table?

One entry per frame; Smaller memory use; Needs hashing to search

Purpose of swapping?

Move inactive processes to disk; Increase multiprogramming

Drawbacks of swapping?

High I/O overhead; Slower response time

Requirements of reentrant (shared) code?

Non-self-modifying; Read-only; Separate per-process data

Benefits of shared pages?

Reduced memory usage; Shared libraries; Common code mapped to same frame

How is memory protection achieved?

Valid/invalid bits; Base and limit registers

Fragmentation summary

Contiguous allocation → external; Paging → internal; Segmentation → external

Paging vs segmentation

Paging = fixed-size units; Segmentation = variable-size logical units

Hardware needed for paging

MMU; TLB; Page-table base register

OS responsibilities in memory management

Track used/free space; Allocate and deallocate memory; Manage paging and swapping