The Memory Management Unit (MMU) is hardware that translates logical addresses to physical addresses at runtime.
Feature
Logical Address
Physical Address
Also called
Virtual address
Real address
Generated by
CPU during program execution
Memory unit (RAM)
Visible to
User/program
Hardware only
Translation
Translated to physical by MMU
Actual location in RAM
Address Binding
The process of mapping logical addresses to physical addresses can happen at different times:
Compile Time: If the memory location is known at compile time, absolute code is generated. Must recompile if location changes.
Load Time: If the memory location is not known at compile time, relocatable code is generated. Binding happens when the program is loaded into memory.
Execution Time: Binding is delayed until runtime. Allows the process to be moved during execution. Requires hardware support (MMU). Used by most modern OS.
Contiguous Memory Allocation
Each process occupies a single contiguous block of memory.
Allocation Strategies:
Fixed Partitioning: Memory is divided into fixed-size partitions. Simple but causes internal fragmentation (wasted space within a partition).
Variable Partitioning: Partitions are created dynamically based on process size. Causes external fragmentation (free memory scattered in small pieces).
First Fit: Allocate the first hole that is big enough. Fast.
Best Fit: Allocate the smallest hole that is big enough. Minimizes wasted space but slow.
Worst Fit: Allocate the largest hole. Leaves large remaining holes.
Fragmentation
Internal Fragmentation: Wasted space inside an allocated partition. Occurs when a process is smaller than the partition it's allocated to.
External Fragmentation: Total free memory is enough but not contiguous. Occurs in variable partitioning.
Compaction: Shuffle memory contents to consolidate free space. Expensive - requires moving all processes.
Paging
Paging eliminates external fragmentation by dividing both physical memory and logical memory into fixed-size blocks:
Frame: Fixed-size block of physical memory
Page: Fixed-size block of logical memory (same size as frame)
The OS maintains a page table for each process that maps page numbers to frame numbers
Eliminates external fragmentation but causes internal fragmentation (last page may not be full)
Segmentation
Segmentation divides a program into logical units (segments) of variable size - code segment, data segment, stack segment, heap segment.
Each segment has a name and length
Logical address = (segment number, offset)
OS maintains a segment table with base address and limit for each segment
Supports user's view of memory (logical units)
Causes external fragmentation
Memory OS state trace
Memory OS state trace
Event 1: a process requests a resource related to Memory
Event 2: the kernel checks permissions, availability, and current state
Event 3: the scheduler or manager decides whether to run, wait, block, or fail
Event 4: the process observes the result and continues or handles the error
Memory small simulation
Memory small simulation
Input: P1, P2, P3
Resource/state: limited
Rule: apply the Memory policy step by step
Output: show which process runs, waits, completes, or is denied
Always write the before-state and after-state for each step.
Before you move on
Memory Management in OS Paging, Segmentation Mastery Check
4 checks
Translate a virtual address through page number, offset, page table, and physical frame.
Explain how the TLB, page faults, replacement, and working set affect observed latency.
Compare paging, segmentation, and combined schemes by fragmentation, protection, and sharing.
Calculate frame use and recognize when replacement pressure becomes thrashing.
