Hello @Jakub Standarski,
the reason for using that much memory is because of the workaround needed to avoid an issue related to the firewall IP itself.
This issue is described in the errata sheet ES0393 chapter 2.4.2 Spurious Firewall reset.
You can also find in Projects\B-L4S5I-IOT01A\Applications\2_Images_KMS\Linker_Common\STM32CubeIDE\mapping_sbsfu.ld the explanation and some details about this workaround.
/* RAM section */
/* SE RAM1 region protected by firewall */
/* SE stack is placed 1st in RAM, stack overflow does not write on other RAM area */
__ICFEDIT_SE_region_RAM_start__ = 0x20000000;
__ICFEDIT_SE_region_RAM_stack_top__ = 0x20002000;
__ICFEDIT_SE_region_RAM_end__ = 0x2001FFBF;
/* See errata sheet: ES0393 - Rev 6 - October 2019 */
/* Only 128 Kbytes - 1 byte and with the 6 lowest bits set to 0 of SRAM1 can be protected by the firewall instead of the full memory space (192 Kbytes of SRAM1). */
/* Use SRAM2 or SRAM3 as system RAM and avoid accessing SRAM1 before any access to an address outside the protected SRAM1 area, */
/* of which the 18 LSBs correspond to an address within the protected SRAM1 area. */
/* So we declare all SRAM1 to avoid spurious Firewall reset but only 0x1FFC0 bytes can be actually used (under Firewall). */
__ICFEDIT_SE_region_SRAM1_do_not_use_start__ = 0x2001FFC0;
__ICFEDIT_SE_region_SRAM1_do_not_use_end__ = 0x2002FFFF; /* 0x10040 bytes we cannot use to avoid spurious firewall reset */
/* SBSFU RAM dummy memory access: See errata sheet: ES0393 - Rev 6 - October 2019 : */
/* - if multiple RAM banks are available: select another RAM bank than the RAM bank used by FWALL */
/* AND */
/* - select a memory range with 18 LSB outside the 18 LSB range protected by FWALL */
__ICFEDIT_SB_region_RAM_dummy_access_start__ = 0x20030000;
__ICFEDIT_SB_region_RAM_dummy_access_end__ = __ICFEDIT_SB_region_RAM_dummy_access_start__ + 3;
Best regards
Jocelyn
