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Table A.2 summarizes the determining factors.Table A.2 Factors Affecting the Run-time Impact of Increased Code SizeDevice ConfigurationNon-XIPPotential Impactsof IncreasedXIPCode SizeMoving MemoryModelMultiple MemoryModelCode is DemandPagedDisk spaceParticularlyimpairedImpairedImpairedImpairedRAM usageNo significantimpactIncreasesIncreasesindependentlyof how manyprocesses usethe codeReducesimpact of thememorymodelTime to load the No significantcodeimpactImpairedImpairedVariable(see below)2 Characteristics, such as maintainability, testability, etc., can also be influenced byhaving more code but they’re not significantly influenced by how the device is configured.RUN-TIME IMPACT OF INCREASED CODE SIZE401As you can see the most important factor is whether the code is loadedfrom XIP or non-XIP flash. If it’s on XIP flash then the Loader has to do verylittle to enable the code to be executed.
If the code is on non-XIP flashthen, by definition, the code cannot be executed in place and the Loaderhas to allocate RAM in which to hold the code that will be executed. Inaddition, it’s not a simply case of copying the code from flash to RAM;the code may need to be decompressed in addition to having all of itsimplicit linkages resolved.
Note that because code can be compressedwhilst stored on disk this means XIP code has a relatively bigger impacton disk space than non-XIP code.For code stored on non-XIP flash, the different memory models impactthe RAM used to hold the code to be executed in different ways. Themultiple memory model copies each DLL into RAM once and then eachprocess references a single copy of the code. However, this isn’t truefor the moving memory model, in which multiple copies may be loadedalthough only if absolutely necessary.3 Hence the multiple memorymodel will always use the same or less RAM than the moving memorymodel when representing executable code at run time.Another factor affecting code stored on non-XIP flash is demandpaging4 which can make a big difference by reducing the impact ofincreased code size on RAM usage and the time to load code. Thisis because, prior to Symbian OS v9.3, entire DLLs are copied intoRAM when they are needed; when demand paging is enabled only therequired ’page’ within the DLL is loaded into RAM when a reference ismade to it (this is known as ’paging on demand’).
This can significantlyreduce the code loaded at any one time and hence the correspondingRAM usage. The effect on the time to load the code is less easy toquantify. Certainly it is common for processes to load faster becauseless code is loaded all at once. However, when demand paging isn’tenabled code is loaded once and then no further time is spent loadingcode. With demand paging enabled, code can be paged in at any pointand hence impact performance at any point. Note that on Symbian OSv9.3, only code built into the device can be demand paged but onSymbian OS v9.5 code that is installed onto the device can also beaffected.3 For example, when a DLL uses writeable static data or is loaded into multiple processesin such a way that the data cannot be at the same address in all those processes, such as fora fixed process.4 www.symbian.com/symbianos/demandpagingand www.iqmagazineonline.com/article.php?issue=23&article id=706.ReferencesAlexandrescu, A.
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