ARM Compiler toolchain Using the Linker - ARM Information Center

4.14 Demand paging
4.14.1 Example
4.14.2 See also
Image structure and generation
In operating systems that support virtual memory an ELF file can be loaded by mapping the ELF
files into the address space of the process loading the file. When a virtual address in a page that
is mapped to the file is accessed the operating system loads that page from disk. ELF files that
are to be used this way must conform to a certain format.
Use the --paged command-line option to enable demand paging mode. This helps produce ELF
files that can be demand paged efficiently.
Note
ELF files produced with the --sysv option are already demand-paged compliant. --arm_linux
also implies --sysv.
The basic constraints on the ELF file are:
• there is no difference between the load and execution address for any Output Section
• all PT_LOAD Program Headers have a minimum alignment, pt_align, of the page size
for the operating system
• all PT_LOAD Program Headers have a file offset, pt_offset, that is congruent to the
virtual address (pt_addr) modulo pt_align.
When --paged is on:
• The linker automatically generates the Program Headers from the execution region base
addresses. The usual situation where one load region generates one Program Header no
longer holds.
• The operating system page size is controlled by the --pagesize command-line option.
• The linker attempts to place the ELF Header and Program Header in the first PT_LOAD
program header, if space is available.
This is an example of a demand paged scatter file:
LR1 GetPageSize() + SizeOfHeaders()
{
ER_RO +0
{
*(+RO)
}
ER_RW +GetPageSize()
{
*(+RW)
}
ER_ZI +0
{
*(+ZI)
}
}
Concepts
• About scatter-loading on page 8-3
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