Code +Paging Overview

Purpose

Demand +paging is a technique where memory appears to application programs to be present +in RAM, but may in fact be stored on some external media and transparently +loaded into RAM when needed. Demand paging trades off increased available +RAM against decreased performance, increased media wear and increased power +usage. More RAM is made available by loading pages only on demand, but a cost +is incurred every time a page is loaded.

Demand paging is used to +reduce the amount of RAM that needs to be shipped with a device and so reduce +its cost.

For the code paging type of demand paging, the executable +is stored in a ROM. Since the memory locations that will be pointed to cannot +be determined ahead of time, the pointers in the executable have to be modified +after the page-in process. This process is known as 'relocation' and 'fix-up'. +It is usually done by loader.

Description

The +executable is in a ROM and so has to be loaded into RAM before it can be executed. +When the required part of the executable is not present in RAM, then a paging +fault is generated which starts the paging in process of specifying which +part of the ROM is to be paged-in along with which RAM page is to be used.

The +above process (in very simple terms) describes how ROM paging works. With +code paging, there is the added complication that the executable is not execute +in place (it is probably stored via the use of an operating system e.g. ROFS) +and so when it is paged in any pointers in the page will not point to any +valid location. Hence a new step has to be carried out that modifies the pointers +in the new page so that will point to meaningful locations once they are RAM. +This process is known as 'relocation' and 'fix-up'. It is usually done by +loader.

Components

These +are the main components of code demand paging:

Rom image - The executable +is stored in a ROM.
RAM - Where the executable +will be executed.
Paging Fault Handler +- To detect that a page-in process is required and to carry it out.
Loader - This usually +does the 'relocation' and 'fix-up' process after the 'page-in' process.

Using ROM Paging

Which +type of paging is used and for which area of memory is first specified the oby and mmp files +and finally build by using specific parameters in the buildrom utility.