AVR uses Hardward architecture with separate address space for program
memory (flash) and data memory (RAM). Normal program flow can only
access data memory which means that all variables - including const
variables - have to be copied into RAM to be accessible. (This happens
automatically during startup.)
It is possible to work around this limitation in software but that
can have severe impact on performance and/or API complexity. It is hardly
feasible to change NuttX interfaces in a way that would allow to make use
of this workaround.
On newer AVR families, there is an alternative option enabled by this patch.
These chips map part of their program memory (a 32kB window) into data
memory address space. This patch leverages this feature and adds support
for placing const variables into the mapped window. No copy to RAM is done
for them.
Const variables are therefore loaded directly from flash (not consuming
RAM) while still being available to be used by any NuttX interface.
Linker script of breadxavr board is changed to make use of these changes.
Tested by verifying string addresses - parameters in printf call
in a custom application (and also by running the application and verifying
its output.)
Documentation tested by build.
Signed-off-by: Kerogit <kr.git@kerogit.eu>
This patch adds a document that describes why const variables need to be
copied into the RAM in AVR architecture for normal program flow in NuttX.
It then describes options of accessing them directly from the flash without
need to do any copying.
Patch was tested by building the documentation.
Signed-off-by: Kerogit <kr.git@kerogit.eu>
The document context-switch-notes.rst was leaking into "Supported Platforms"
table of contents. To fix this, it was moved to separate common directory
and referenced from AVR index.rst
Similar to that, the gpio_intr_mux.rst document was leaking into list
of supported AVR families. This too was fixed by moving it to separate
docs directory.
The change was tested by building the Documentation using make html
Signed-off-by: Kerogit <kr.git@kerogit.eu>
