Demand Paging on Symbian OS

Overview

Symbian delivers Demand Paging support in Symbian OS versions 9.3, 9.4 and 9.5. Demand Paging enables more efficient usage of RAM by loading read-only code and data only when required (“on demand”).

What is Demand Paging?

• Data must be copied from NAND Flash memory into RAM in order to execute
• Previously, Symbian OS copied entire DLLs into RAM when they were needed
• Demand Paging means that only the required “page” within the DLL is loaded into RAM
• Demand Paging only loads a page into RAM when a reference is made to it (paging on demand)
• Loading by page means less RAM is used at any one time
  
  

Demand Paging is a more efficient way of loading data into RAM;
Code that does need to be executed is not loaded into RAM

NAND Flash and Paging

• ROMFS contains whole image
• Core image is a wired subset
• Pages from DLLs are copied into RAM
• Idle code pages are automatically unloaded
• RAM only used by executing code

Benefits of Demand Paging

Demand Paging performance may vary

Performance depends on:

• size of the ROM
• size of the core ROM image compared to the primary ROFS image
• amount of code in the ROM marked as 'unpaged'
• size of the paging cache
• whether Code paging or XIP ROM paging is being used (or both)
• use-case currently being run

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