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Chapter 8: Memory-Management Strategies Memory-Management Strategies 8.1 Background 8.2 Swapping Objectives To provide a detailed description of various ways of organizing memory hardware To discuss various memory-management 8.3 Contiguous Memory Allocation 8.4 Paging 8.5 Structure of the Page Table 8.6 Segmentation techniques, including paging and segmentation 8.7 Example: The Intel Pentium To provide a detailed description of the Intel Pentium, which supports both pure segmentation and segmentation with paging Silberschatz, Galvin and Gagne ©2005 8.1 Operating System Principles 8.2 Operating System Principles 8.1 Background Silberschatz, Galvin and Gagne ©2005 Base and Limit Registers Program must be brought (from disk) into memory and placed within a process for it to be run A pair of base and limit registers define the logical address space Main memory and registers are the only storage CPU can access directly Register access in one CPU clock (or less) Main memory can take many cycles Cache sits between main memory and CPU registers size Protection of memory required to ensure correct operation Please refresh the Instruction Execution Cycle Operating System Principles 8.3 Silberschatz, Galvin and Gagne ©2005 Operating System Principles 8.4 Silberschatz, Galvin and Gagne ©2005 Binding of Instructions and Data to Memory HW address protection with base and limit registers Address binding of instructions and data to memory addresses can happen at three different stages z Compile time: If memory location known beforehand, absolute code can be generated; must recompile code if starting location changes z Load time: Must generate relocatable code if memory location is not known at compile time z 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 (e.g., base and limit registers) physical mostly used Conceptually (概念上) Operating System Principles 8.5 Silberschatz, Galvin and Gagne ©2005 Multistep Processing of a User Program Operating System Principles 8.6 Silberschatz, Galvin and Gagne ©2005 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 z Logical address – generated by the CPU; also referred to as virtual address z Physical address – address seen by the memory unit Logical and physical addresses are the same in compile-time and load-time address-binding schemes; logical (virtual) and physical addresses differ in execution-time address-binding scheme Operating System Principles 8.7 Silberschatz, Galvin and Gagne ©2005 Operating System Principles 8.8 Silberschatz, Galvin and Gagne ©2005 Memory-Management Unit (MMU) Example: Assembly Program (supplemented) 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 Operating System Principles 8.9 Silberschatz, Galvin and Gagne ©2005 Example: Machine Code Operating System Principles 8.10 Silberschatz, Galvin and Gagne ©2005 Dynamic relocation using a relocation register MS-DOS used four relocation registers in loading and running processes. Why four, instead of one ??? Operating System Principles 8.11 Silberschatz, Galvin and Gagne ©2005 Operating System Principles 8.12 Silberschatz, Galvin and Gagne ©2005 Dynamic Loading Dynamic Linking Linking postponed until execution time 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 Small piece of code, called 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 No special support from the operating system is required. It is implemented through program design processes’ memory address Dynamic linking is particularly useful for libraries with several versions Operating System Principles 8.13 Silberschatz, Galvin and Gagne ©2005 z Only programs compiled with the new library version are affected by the incompatible changes incorporated in it. Other programs will continue using the older library. z This system is also known as shared libraries Operating System Principles 8.2 Swapping 8.14 Silberschatz, Galvin and Gagne ©2005 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 Context-switch time in such a swapping system is fairly high, see example in p. 323 Operating System Principles 8.15 Silberschatz, Galvin and Gagne ©2005 Operating System Principles 8.16 Silberschatz, Galvin and Gagne ©2005 Schematic View of Swapping 8.3 Contiguous Allocation Main memory usually into two partitions: • Modification of swapping: only enabled when many processes are using a threshold amount of memory z Resident operating system, usually held in low memory with interrupt vector z User processes then held in high memory Relocation registers used to protect user processes from each other, and from changing operating-system code and data • I/O problem: if I/O is asynchronously accessing the user memory for I/O buffers, then the process cannot be swapped. > Latch job in memory while it is involved in I/O > Do I/O only into OS buffers Silberschatz, Galvin and Gagne ©2005 8.17 Operating System Principles HW support for relocation and limit registers limit register z Base register contains value of smallest physical address z Limit register contains range of logical addresses – each logical address must be less than the limit register z MMU maps logical address dynamically Contiguous Allocation Fixed-sized partition z relocation register Silberschatz, Galvin and Gagne ©2005 8.18 Operating System Principles Multiple-partition allocation Hole – block of available memory; holes of various size are scattered throughout memory logical + < physical address z When a process arrives, it is allocated memory from a hole large enough to accommodate it z 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 In reality process 2 Operating System Principles 8.19 Silberschatz, Galvin and Gagne ©2005 Operating System Principles process 2 process 2 8.20 process 2 Silberschatz, Galvin and Gagne ©2005 Fragmentation Dynamic Storage-Allocation Problem External Fragmentation – total memory space exists to How to satisfy a request of size n from a list of free holes z z First-fit: Allocate the first hole that is big enough z Best-fit: Allocate the smallest hole that is big enough; must search entire list, unless ordered by size Produces z the smallest leftover hole Statistical analysis of first fit reveals a 50-percent rule property Internal Fragmentation – allocated memory may be slightly larger than requested memory; this size difference is memory internal to a partition, but not being used Reduce external fragmentation by compaction Worst-fit: Allocate the largest hole; must also search entire list Produces satisfy a request, but it is not contiguous the largest leftover hole • First-fit and best-fit better than worst-fit in terms of speed and storage utilization. First-fit is normally faster. z Shuffle memory contents to place all free memory together in one large block z Compaction is possible only if relocation is dynamic, and is done at execution time z I/O problem Latch Do 8.21 Operating System Principles Silberschatz, Galvin and Gagne ©2005 job in memory while it is involved in I/O I/O only into OS buffers 8.4 Paging Silberschatz, Galvin and Gagne ©2005 8.22 Operating System Principles Address Translation Scheme Logical address space of a process can be noncontiguous; process is allocated physical memory whenever the latter is available Address generated by CPU is divided into: z Page number (p) – used as an index into a page table which contains base address of each page in physical memory z Page offset (d) – combined with base address to define the physical memory address that is sent to the memory unit Divide physical memory into fixed-sized blocks called frames (size is power of 2, between 512 bytes and 8,192 bytes) Divide logical memory into blocks of same size called pages page number Keep track of all free frames To run a program of size n pages, need to find n free frames and load program Set up a page table to translate logical to physical addresses z page offset p d m-n n For given logical address space 2m and page size 2n Internal fragmentation Operating System Principles 8.23 Silberschatz, Galvin and Gagne ©2005 Operating System Principles 8.24 Silberschatz, Galvin and Gagne ©2005 Paging Hardware Operating System Principles 8.25 Paging Model of Logical and Physical Memory Silberschatz, Galvin and Gagne ©2005 Paging Example Free Frames 32-byte memory and 4-byte pages Operating System Principles 8.27 Silberschatz, Galvin and Gagne ©2005 8.26 Operating System Principles after allocation before allocation Silberschatz, Galvin and Gagne ©2005 Operating System Principles 8.28 Silberschatz, Galvin and Gagne ©2005