When does my microcontroler uses my Flash or my RAM? - c

Question

I am currently developing an embedded application on the Atmel SAML21J microcontroller, and I have 256KB Flash memory, and a 40KB SRAM memory. When I program my app on the MCU, I have the following message :
Program Memory Usage 66428 bytes 24,6 % Full
Data Memory Usage 29112 bytes 71,1 % Full
It seems to mean that even before I start to run my code, I already have a 71% full RAM.
I would like to know the following things:
what is defined in the RAM, and what is defined in the Flash ?
can I do something to use more of my Flash (that is only 24% full) to save space on the SRAM, and how ?
I saw a ".ld" file that specifies the size of my stack : will it leave me more space in the RAM if I make it higher ?
In this .ld file, is the memory (Flash + SRAM) considered as one unique memory entity ? (meaning that the addresses of the SRAM starts and the end of the flash, for example ?)
Even if I read a lot of things on this subject, this is still shady to me, and I would really appreciate if you guys enligthened me on that.
Thanks.

Answer 1

Where and what placed (defined):
Stack (local variables placed in stack), all global variables, functions that specificied with special keyword (for ex. __ramfuc for IAR) as runned from RAM - are placed in RAM.
All functions (no differents where it's will run), all constants, variables initialization values are placed in Flash. Worth mentioning for AVR you need to use keyword PROGMEM to place any constant to Flash (functions don't need that), while for ARM keyword const will be enough.
For save RAM space you can (in order of effectiveness):
place big tables and text constants (debug messages too) in Flash
merge global buffers (with unions) and use it for differents task in different time
reduce size of stack, there could be problems with stack overflow - so you must reduce functions nesting
use bitmasks for global flags instead of bytes
If you insrease stack size: since stack placed in RAM so you increase RAM usage.
Flash and RAM memories have different address ranges, so from .ld file
you can know where each variable or function aligned by linker:
/* Memories definition in *.ld file */
MEMORY
{
RAM (xrw) : ORIGIN = 0x20000000, LENGTH = 128K
ROM (rx) : ORIGIN = 0x8000000, LENGTH = 1024K
}
/* Sections */
SECTIONS
{
/* The program code and other data into ROM memory */
.text :
{
...
} >ROM
}
There we have:
128Kb of RAM address range [0x20000000, 0x2001FFFF]
1Mb of Flash address range [0x08000000, 0x080FFFFF]
And example how section text placed to Flash memory.
And theh after success compile project you can open file ./[Release|Debug]/output.map for see where each functions and variables are placed:
.text.main 0x08000500 0xa4 src/main.o
0x08000500 main
...
.data 0x20000024 0x124 src/main.o
0x20000024 io_buffer
Function main is placed in Flash memory, global variable io_buffer is placed in RAM memory.