1. CSI 400/500 Operating Systems Spring 2009
Lecture #8 – Memory Management
February 23rd through 25th, 2009

2. Memory Hierarchy
Primary
Secondary
External

3. Primary Memory Used by processors and running processes Includes:
Registers – holds operation data for processor
Cache – holds immediate storage for process
RAM – processing address space
Cache memory contains the current page or segment of storage being executed by the processor. Designed for quick access, as no translation or disk retrieval is necessary
RAM memory is the entire storage capacity of the computer. It contains the data of all loaded programs, whether running or held. It also contains system storage like interrupt handling routines, system calls, etc.

4. Secondary Memory Memory not allocated to processors or tasks
Includes “virtual memory”, which is extended memory for tasks containing data not currently being used
Used to store data for extended periods of time
Examples: hard drive, disks, flash drives

5. External Memory Storage device external to computer
Located on network, either local or wide
Examples: database server, web page

6. Memory Manager
Component of Operating System that handles allocation, loading, reading, writing, and clearing of primary and secondary memory
Resolves references and/or calls to external data

7. Memory Manager Tasks

8. Loading
Copying process data stored in RAM or secondary memory into cache memory
“Page loading”
Reading external data and loading into cache memory
“Remote accessing”

9. Binding Process of translating virtual address into physical address
Most programs use more storage than cache can hold, so virtual address refers to RAM location without needing extremely long address
Address can still fit into register
Allows for continual extension of “onboard” memory space
We will discuss how this translation works later, as we discuss different ways of sharing total RAM across processes.

10. Saving Restore data to secondary memory Write data to external memory
“Reallocation”
Write data to external memory
“Uploading”
Saving data may involve network communication or file management. We will discuss these concepts in their own lecture topics.

11. Allocation Distributing memory among processes
Process’ available space called “address space”
Three main techniques:
Fixed-Partition
Variable-Partition
Dynamic

12. Fixed Partition Allocation
Called pages
System has predetermined page size
Only one page per process in cache at a time
Common in batch and time-share systems

13. How Paging Works Process allocated a page
Data is filled in from start to end
New data added to end of last data block, allowing for data boundaries
If page fills, new page is allocated
Pages tracked with Page Table
We’ll discuss that in a few sessions

14. Internal Fragmentation
If data freed during process, Memory Manager will attempt to reuse
Desired data must be same size or smaller
Could leave “holes” on page, called “internal fragmentation”
Internal fragmentation cleared by page reallocation
Copies existing data to new page, then clearing old page
Show how these “holes” develop

15. Variable Partition Allocation
Called segments
Memory Manager obtains storage as process produces data or needs external data
All current processes share memory
Each process has its own piece of memory
Common in Client-Server systems

16. How Segmentation Works
System starts with blank page
Processes are created and queued
Initial data for each process allocated a segment
Segments contiguous in storage
New segments added if processes created or processes demand more

17. External Fragmentation
As processes release data or processes complete, data is freed
System tries to reuse data
Request must be at most same size
Could leave “holes” called “external fragmentation”
Freed storage too small to use at current time
Most systems maintain a Free Space table to track these fragments

18. Defragmentation Also called “dynamic relocation”
Moving segments to fill external fragments
Logical addresses reset using dynamic binding
We’ll discuss that with dynamic allocation

19. Dynamic Allocation
Process allowed to specify and change size of its storage
If that much space is not available, process held until space is available
Often requested storage obtained in chunks, using multiple free space fragments
Space chained together via linked list

20. Dynamic Binding
Memory Manager can move segment around as process executes
All process data referenced by symbol name
Variable name if defined, or system-created name
Process maintains Symbol Table to resolve reference
Dynamic Binding means Memory Manager resolves to new physical address when process references data

21. How Dynamic Binding Works
When data placed in segment, entry added to symbol table
Entry is offset in segment
Processor has Relocation Register
Contains start address of segment
Segment moves due to dynamic relocation
Address in relocation register changes
Subsequent reference resolves to new physical location

22. Swapping
Process of saving process’ current address page to secondary storage and loading new address page
Process is blocked during this time, as execution requires new data

23. Swapping Process Process requests non-resident data
Processor blocks process and sends I/O interrupt
Interrupt Handler contacts Memory Manager with data reference
Memory Manager swaps out current page, finds new page, and swaps it in
Tells Interrupt Handler it is done
Interrupt Handler releases process to Ready queue