Initialized vs uninitialized global variables, in which part of the RAM are the stored? - c++

Question

The problem is that When I define the arrays like this :
float Data_Set_X[15000];
float Data_Set_Y[15000];
float Data_Set_Z[15000];
I get the RAM overflow error which is: .bss will not fit in region RAM Timer-Blink-Test_CM7 C/C++ Problem
When I initilaze at least one of the arrays or three of them , the error will be disappeared.
float Data_Set_X[15000]={0};
float Data_Set_Y[15000];
float Data_Set_Z[15000];
My variables are global.
In linker script file it is written that:
/* Specify the memory areas */
MEMORY
{
RAM_EXEC (rx) : ORIGIN = 0x24000000, LENGTH = 256K
RAM (xrw) : ORIGIN = 0x24040000, LENGTH = 256K
}
/* The startup code goes first into RAM_EXEC */
/* The program code and other data goes into RAM_EXEC */
/* Constant data goes into RAM_EXEC */
/* Initialized data sections goes into RAM, load LMA copy after code */
And there is a separated part of the RAM for /* Uninitialized data section */ according to the linker script.
The RAM size is 1MB and around 800KB is accessible for the user. MCU has dual core and I use the M7 Core. this core can access to a 512KB RAM area as it is mentioned in the Linker Script file. the whole size of these three arrays are 180KB
Here is the linker Script file of my Micro Controller
/*
******************************************************************************
**
** File : LinkerScript.ld
**
**
** Abstract : Linker script for STM32H7 series
** 256Kbytes RAM_EXEC and 256Kbytes RAM
**
** Set heap size, stack size and stack location according
** to application requirements.
**
** Set memory bank area and size if external memory is used.
**
** Target : STMicroelectronics STM32
**
** Distribution: The file is distributed as is, without any warranty
** of any kind.
**
*****************************************************************************
** #attention
**
** Copyright (c) 2019 STMicroelectronics.
** All rights reserved.
**
** This software component is licensed by ST under BSD 3-Clause license,
** the "License"; You may not use this file except in compliance with the
** License. You may obtain a copy of the License at:
** opensource.org/licenses/BSD-3-Clause
**
****************************************************************************
*/
/* Entry Point */
ENTRY(Reset_Handler)
/* Highest address of the user mode stack */
_estack = 0x24080000; /* end of RAM */
/* Generate a link error if heap and stack don't fit into RAM */
_Min_Heap_Size = 0x200 ; /* required amount of heap */
_Min_Stack_Size = 0x400 ; /* required amount of stack */
/* Specify the memory areas */
MEMORY
{
RAM_EXEC (rx) : ORIGIN = 0x24000000, LENGTH = 256K
RAM (xrw) : ORIGIN = 0x24040000, LENGTH = 256K
}
/* Define output sections */
SECTIONS
{
/* The startup code goes first into RAM_EXEC */
.isr_vector :
{
. = ALIGN(4);
KEEP(*(.isr_vector)) /* Startup code */
. = ALIGN(4);
} >RAM_EXEC
/* The program code and other data goes into RAM_EXEC */
.text :
{
. = ALIGN(4);
*(.text) /* .text sections (code) */
*(.text*) /* .text* sections (code) */
*(.glue_7) /* glue arm to thumb code */
*(.glue_7t) /* glue thumb to arm code */
*(.eh_frame)
KEEP (*(.init))
KEEP (*(.fini))
. = ALIGN(4);
_etext = .; /* define a global symbols at end of code */
} >RAM_EXEC
/* Constant data goes into RAM_EXEC */
.rodata :
{
. = ALIGN(4);
*(.rodata) /* .rodata sections (constants, strings, etc.) */
*(.rodata*) /* .rodata* sections (constants, strings, etc.) */
. = ALIGN(4);
} >RAM_EXEC
.ARM.extab : { *(.ARM.extab* .gnu.linkonce.armextab.*) } >RAM_EXEC
.ARM : {
__exidx_start = .;
*(.ARM.exidx*)
__exidx_end = .;
} >RAM_EXEC
.preinit_array :
{
PROVIDE_HIDDEN (__preinit_array_start = .);
KEEP (*(.preinit_array*))
PROVIDE_HIDDEN (__preinit_array_end = .);
} >RAM_EXEC
.init_array :
{
PROVIDE_HIDDEN (__init_array_start = .);
KEEP (*(SORT(.init_array.*)))
KEEP (*(.init_array*))
PROVIDE_HIDDEN (__init_array_end = .);
} >RAM_EXEC
.fini_array :
{
PROVIDE_HIDDEN (__fini_array_start = .);
KEEP (*(SORT(.fini_array.*)))
KEEP (*(.fini_array*))
PROVIDE_HIDDEN (__fini_array_end = .);
} >RAM_EXEC
/* used by the startup to initialize data */
_sidata = LOADADDR(.data);
/* Initialized data sections goes into RAM, load LMA copy after code */
.data :
{
. = ALIGN(4);
_sdata = .; /* create a global symbol at data start */
*(.data) /* .data sections */
*(.data*) /* .data* sections */
. = ALIGN(4);
_edata = .; /* define a global symbol at data end */
} >RAM AT> RAM_EXEC
/* Uninitialized data section */
. = ALIGN(4);
.bss :
{
/* This is used by the startup in order to initialize the .bss secion */
_sbss = .; /* define a global symbol at bss start */
__bss_start__ = _sbss;
*(.bss)
*(.bss*)
*(COMMON)
. = ALIGN(4);
_ebss = .; /* define a global symbol at bss end */
__bss_end__ = _ebss;
} >RAM
/* User_heap_stack section, used to check that there is enough RAM left */
._user_heap_stack :
{
. = ALIGN(8);
PROVIDE ( end = . );
PROVIDE ( _end = . );
. = . + _Min_Heap_Size;
. = . + _Min_Stack_Size;
. = ALIGN(8);
} >RAM
/* Remove information from the standard libraries */
/DISCARD/ :
{
libc.a ( * )
libm.a ( * )
libgcc.a ( * )
}
.ARM.attributes 0 : { *(.ARM.attributes) }
}
Thank you!

Answer 1

Then using this linker script the initialized static storage objects will be stored in the RAM memory section as the .data segment is stored there.
The initial values will be stored in the RAM_EXEC memory section. The startup code will have to copy this data from the RAM_EXEC to RAM.
The problem is that something (probably the debug probe or the bootloader) will have to program the RAM_EXEC memory section first
The not initialized static storage objects will be stored in the RAM memory section as the .bss section is located there. The startup code will have to zero those
The best way is to see what is actually in the memory and where. Generate the .map file to see the exact placement (you can increase the size of the RAM to pass the linking. It will not be the valid executable, but you will be able to generate the .map file)