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

Memory Hierarchy Primary Secondary External

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.

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

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

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

Memory Manager Tasks

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”

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.

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.

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

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

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

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

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

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

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

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

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

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

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

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

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