Smartphone Operating System (779883), страница 35
Текст из файла (страница 35)
Domainsare recorded in the page directory and the MMU enforces accesspermissions for each domain. While segmentation is not explicitlyused, there is an organization to the layout of memory: there is a datasection for user-allocated data and a kernel section for kernel-allocateddata.• The multiple model was developed for versions 6 and later of theARM architecture. The MMU in these versions differs from that usedin earlier versions. For example, the page directory requires differenthandling, since it can be sectioned into two pieces, each referencingtwo different sets of page tables: user-page tables and kernel-pagetables.
The new version of the ARM architecture revised and enhancedthe access bits on each page frame and deprecated the domainconcept.• The direct model assumes that there is no MMU at all. This model israrely used and is not allowed on real smartphones. The lack of anMMU would cause severe performance issues.
This model is usefulfor development environments where the MMU must be disabled forsome reason.• The emulator model was developed to support the Symbian OSemulator on Microsoft Windows. As one might expect, the emulator162MEMORY MANAGEMENThas a few restrictions in comparison to a real target CPU. The emulatorruns as a single Microsoft Windows process, therefore the addressspace is restricted to 2 GB, not 4 GB. All memory provided to theemulator is accessible to any Symbian OS process and therefore nomemory protection is available. Symbian OS libraries are provided asMicrosoft Windows DLLs and, therefore, Microsoft Windows handlesthe allocation and management of memory.7.6 Memory Use in LinuxLinux is designed to run on the Intel architecture and, therefore, it supportsthe memory models designed into the Intel 80x86 line of processors.Linux is a 32-bit operating system and therefore can access up to 4 GBof memory.
The processor MMU supports segmentation and memoryframes; Linux supports frame sizes of 4 KB.Linux implements a free list of pages and supports placement of application pages anywhere in memory. It uses a ‘buddy system’ algorithm toallocate page frames. The goal of this system is to allocate contiguous pageframes as often as possible and it does this by maintaining as many adjacent free page frames as possible. The buddy system keeps multiple lists offree blocks of various sizes, joining blocks, and thus moving them betweenlists, whenever blocks are freed up next to other free blocks. The intent isthat blocks are allocated faster because the right blocks at the right sizeare found by checking the correct list (instead of searching the entire list).Like Symbian OS, Linux uses a page-directory–page-table structure fortranslating logical addresses into physical ones.
As we have discussed,this two-level approach to address translation saves memory but doublesthe time for address calculation. Hardware helps with this situation; theIntel architecture provides address computation in its MMU as well as aTLB to find pages quickly.Linux implements the idea of reserved-page frames. These framesrepresent an area of memory reserved for the Linux kernel and itsdata structures. This area can never be relinquished or allocated to auser process. Linux tries as much as possible to keep its page framescontiguous; it even chooses an area of memory that is unlikely to be usedto load itself into at boot time. The kernel typically starts at the 2 MB markand reserves as many frames as it needs.
Because it cannot predict howmany dynamically assigned pages it needs, these are used where theycan be found.SUMMARY163Linux implements a swapping strategy for memory pages and mostoften uses disk space for virtual memory. Swap space is implementedas either a partition on a disk drive or as a large file. Within this space,Linux keeps pages for each process as close to each other as possible.This minimizes disk I/O time. Pages are moved to swap space when thenumber of free memory pages drops below a predefined threshold (Linuxdoes not wait for the number to become zero).The selection of pages to remove from memory is interesting. Thegeneral rule for Linux is to choose one of the pages held by the processwith the most pages in memory.
Within this collection of pages, LRUis used. A counter is added to the page table that stores the amount oftime that has elapsed since the last access to the page. The page with thelargest amount of elapsed time is chosen.Linux uses several types of segmented memory. It supports codeand data segments for the kernel, code and data segments for userprocesses, and global and local segment tables.
Segments are shared byall processes. The segment tables contain entries that direct the operatingsystem to segments for individual processes. Any hardware propertiesthat support segmentation are exploited.Protections are designated inside page or segment tables. Each pageor segment is coded with a specific access value and the type of accessis always compared to the access value of the page or segment on whichthe operating is done.Linux has been adapted to run on 64-bit machines, which affectsseveral parts of memory management. The address space expands from4 GB to 16 EB,2 making much more memory possible.
Larger memorymeans larger swap space allocation. More memory means larger pagingand segment tables. Rather than take up huge amounts of reserved operating system memory for larger page tables, 64-bit Linux implementationstypically use a three-level paging scheme, which extends the two-levelscheme used in 32-bit systems.7.7 SummaryThis chapter has discussed how operating systems manage memory.Like a computer’s CPU, memory must be shared between the processes2That is, 16 exabytes. An exabyte is slightly more than one billion gigabytes.
The newaddress space is 4 billion times the size of the former 4 GB address space.164MEMORY MANAGEMENTthat execute on a computer. We began by introducing the terminologyand background of memory management, including how a programprogresses from source code to an in-memory executing image, thedifference between logical and physical addressing, and the mechanismsused for dynamic loading and linking. We then discussed paging and howpaging is used to manage memory. Issues surrounding paging includethe use of virtual memory, how to keep track of pages and how memoryand page allocation affect fragmentation. We then discussed how thelack of support for virtual memory on smaller computer platforms affectsoperating system design.
We introduced segmentation and saw how theconcept affected design. We concluded the chapter with discussions ofmemory management in Symbian OS and Linux.Exercises1.Can a logical address also be a physical address? If not, why not? Ifso, what type of address binding would be used for this?2.When physical memory is allocated as frames, can external fragmentation exist? Explain.3.Let’s say that a program makes many requests for dynamic memory,but they are all small (10 bytes). Which memory allocation schemeis best for this pattern of memory allocation?4.How are relocatable binding and execution-time binding different?5.Consider a configuration with a logical address space that has4096 words, mapped onto a physical memory with 64 pages.a. How many bits should the logical address have?b.
How many bits should the physical address have?6.Consider a new page-replacement algorithm, Not Recently Used(NRU), that favors keeping pages which have been recently used.This algorithm works on the following principle: when a page isreferenced, a bit is set for that page, marking it as referenced.Similarly, when a page is modified (written to), a modified bit is set.By using a mix of these bits, the system can guess about recent usage(e.g.
referenced but not modified or modified but not referenced).How is this strategy different from Least Recently Used?EXERCISES1657.Give at least three situations in which memory space needs to beprotected from programs.8.Smartphones are being designed with hard disks (e.g., the NokiaN91). What would the effect be if these devices started using virtualmemory?9.Why is loading all application pages and dependent modules rightaway important for a smartphone device?10.How many page tables are possible in Symbian OS?11.How many memory frames are possible in Symbian OS?12.What is the alternative to the reserved page frames that Linux uses?What effect does the alternative have on an operating system?8File Systems and StorageWe introduced operating systems in Chapter 1 saying that using one waslike viewing hardware through ‘rose-tinted glasses’.
Operating systemspresent a view of the system resources to users and enable applicationsto use those resources in a constrained and organized way.File systems are perhaps the greatest example of this abstracted viewof system resources. A file system takes a collection of bits and bytes froma storage medium and creates an organized hierarchy of files and directories, enabling complete use of this abstract concept. All storage mediahas the idea of files and directories and it is an incredibly useful concept.This chapter explores how this abstraction is built and used byoperating systems.
![](https://s.studizba.com/z.php?f=/templates/si/i/noavatar.png&w=100&h=100&t=1"){kind=avatar}
