Main Memory Management

Objectives To provide a detailed description of various ways of organizing memory To discuss various memory-management techniques, including paging and segmentation

Background Program must be brought (from disk) into memory and placed within a process for it to be run Main memory, cache and registers are only storage CPU can access directly Cache sits between main memory and CPU registers Protection of memory required to ensure correct operation

Binding of Instructions and Data to Memory Address binding of instructions and data Compile time: If memory location known, absolute code can be generated; must recompile code if starting location changes Load time: Must generate relocatable code if memory location is not known at compile time Execution time: Binding delayed until run time if the process can be moved during its execution from one memory segment to another. Need hardware support for address maps

Multistep Processing of a User Program

Logical vs. Physical Address Space The concept of a logical address space that is bound to a separate physical address space is central to proper memory management Logical address – generated by the CPU; also referred to as virtual address Physical address – address seen by the memory unit Compile time & load time binding – same logical and physical addresses Execution time binding – different logical and physical addresses

Memory-Management Unit (MMU) Hardware device that maps virtual to physical address In MMU scheme, the value in the relocation register is added to every address generated by a user process at the time it is sent to memory The user program deals with logical addresses; it never sees the real physical addresses

Dynamic Loading Routine is not loaded until it is called Better memory-space utilization; unused routine is never loaded Useful when large amounts of code are needed to handle infrequently occurring cases No special support from the operating system is required implemented through program design

Dynamic Linking Linking postponed until execution time Small piece of code, stub, used to locate the appropriate memory-resident library routine Stub replaces itself with the address of the routine, and executes the routine Operating system needed to check if routine is in processes’ memory address Dynamic linking is particularly useful for libraries

Swapping A process can be swapped temporarily out of memory to a backing store, and then brought back into memory for continued execution Backing store – fast disk large enough to accommodate copies of all memory images for all users; must provide direct access to these memory images Roll out, roll in – swapping variant used for priority-based scheduling algorithms; lower-priority process is swapped out so higher-priority process can be loaded and executed Major part of swap time is transfer time; total transfer time is directly proportional to the amount of memory swapped Modified versions of swapping are found on many systems (i.e., UNIX, Linux, and Windows) System maintains a ready queue of ready-to-run processes which have memory images on disk

Schematic View of Swapping

Basic Memory Management Concepts Mono programming vs multiprogramming Fixed vs variable size partitions Contiguous vs. non-contiguous blocks Cache vs. RAM (main memory) vs External storage

Mono vs Multiprogramming Mono Programming Single process shares memory with OS Security – only to protect OS Very simple implementation Multiprogramming N processes share memory with OS Security – protect all processes from each other NO FRIENDLY processes Boundary registers can be used to check processes

Mono-programming Multiprogramming

Multiprogramming Hole – block of available memory; holes of various size are scattered throughout memory When a process arrives, it is allocated memory from a hole large enough to accommodate it Operating system maintains information about: a) allocated partitions b) free partitions (hole) OS OS OS OS process 5 process 5 process 5 process 5 process 9 process 9 process 8 process 10 process 2 process 2 process 2 process 2

Fixed vs. Variable partitions Fixed Size partitions Memory divided into fixed (possibly unequal) sizes Queuing options for processes One queue for RAM Large jobs waiting longer for short jobs using large partitions Load- time binding can be used One queue for each partition Best use of memory Some partitions empty while processes are waiting Compile time binding can be used Fragmentation – internal (inside partitions)

Fixed Partitions

Fixed vs. Variable partitions Variable Size partitions Process given as much memory as needed Initially better utilization – no gaps Large processes can use all of memory No room for growth unless it is added into partition Fragmentation – external (holes created as processes complete)

Variable Size Partitions

Variable partitions 5K free P2 terminates: 2k free Total: 22K free P6(20k) > 22K free Cannot load P6(20k) - coalesce Could load p7(4k)

Variable partitions Coalescing - combining of adjacent free space don’t have to move any executing jobs but doesn’t make use of all free space Compaction - “burping” RAM (garbage collection) combine all free space at one end of RAM Requires that processes be moved Uses CPU time

Contiguous vs non-contiguous Contiguous Blocks Each process occupies a single contiguous block Relocation registers used to protect user processes from each other, and from changing operating-system code and data Base register contains value of smallest physical address Limit register contains range of logical addresses – each logical address must be less than the limit register MMU maps logical address dynamically

Base and Limit Registers A pair of base and limit registers define the logical address space

Base and Limit Registers

Base and Limit Registers

Non-contiguous Non-contiguous Blocks Each process is divided into segments that may not reside next to each other in memory. Harder to implement (how to divide) Better utilization of RAM

Memory Management Strategies Fetch Strategies When to put things into RAM When to fetch the next piece of data or program to insert into RAM   demand fetching - Next item fetched when referenced and not resident anticipatory fetching - Retrieve larger quantities than referenced to minimize number of fetches.

Memory Management Strategies Placement Strategies Where to place items in RAM First-fit: Allocate the first hole that is big enough (fast) Next fit - Similar, keeps track of where in List left off. Quick fit - maintains separate lists for common sizes. Best-fit: Allocate the smallest hole that is big enough must search entire list, unless ordered by size Produces the smallest leftover hole Must search all of RAM Most extreme external fragmentation (smallest left over hole) But - leaves larger holes for larger items.

Memory Management Strategies Placement Strategies Worst-fit: Allocate the largest hole must also search entire list ( or maintain pointer to largest) Divides large chunks that are useful for large items Produces the largest leftover hole (most useful fragment)

Memory Management Strategies Replacement Strategies Determine which item to replace with the incoming item Examples: FIFO LRU (Least Recently Used) Variable size partitions – MORE complicated!

Memory Usage Strategies Bit maps Memory is divided into allocation units (blocks) Bit map contains a bit for each allocation unit 0 - represents it is free, 1 - represents it is in use.   Linked Lists Linked list contains nodes sorted by address Node contains: type indicator (hole or process), starting address, length, and pointer to previous and next node.

End of part 1