RM0440
90/2126 RM0440 Rev 4
The BOOT0 pin or user option bit (depending on the nSWBOOT0 bit value in the
FLASH_OPTR register), and nBOOT1 bit are also re-sampled when exiting from Standby
mode. Consequently, they must be kept in the required Boot mode configuration in Standby
mode. After this startup delay has elapsed, the CPU fetches the top-of-stack value from
address 0x0000 0000, then starts code execution from the boot memory at 0x0000 0004.
Depending on the selected boot mode, main Flash memory, system memory or SRAM1 is
accessible as follows:
• Boot from main Flash memory: the main Flash memory is aliased in the boot memory
space (0x0000 0000), but still accessible from its original memory space
(0x0800 0000). In other words, the Flash memory contents can be accessed starting
from address 0x0000 0000 or 0x0800 0000.
• Boot from system memory: the system memory is aliased in the boot memory space
(0x0000 0000), but still accessible from its original memory space (0x1FFF 0000).
• Boot from the embedded SRAM1: the SRAM1 is aliased in the boot memory space
(0x0000 0000), but it is still accessible from its original memory space (0x2000 0000).
PB8/BOOT0 GPIO is configured in:
• Input mode during the complete reset phase if the option bit nSWBOOT0 is set into the
FLASH_OPTR register and then switches automatically in analog mode after reset is
released (BOOT0 pin).
• Input mode from the reset phase to the completion of the option byte loading if the bit
nSWBOOT0 is cleared into the FLASH_OPTR register (BOOT0 value coming from the
option bit). It switches then automatically to the analog mode even if the reset phase is
not complete.
Note: When the device boots from SRAM, in the application initialization code, you have to
relocate the vector table in SRAM using the NVIC exception table and the offset register.
When booting from the main Flash memory, the application software can either boot from
bank 1 or from bank 2 (only for category 3 devices). By default, boot from bank 1 is selected.
To select boot from Flash memory bank 2, set the BFB2 bit in the user option bytes. When
this bit is set and the boot pins are in the boot from main Flash memory configuration, the
device boots from system memory, and the boot loader jumps to execute the user
application programmed in Flash memory bank 2. For further details, please refer to
AN2606.
See Table 13: Access status versus protection level and execution modes for bootloader
function for different RDP levels
Forcing boot from user Flash memory
Regardless the boot configuration it is possible to force booting from a unique entry point in
main Flash memory. See Section Embedded Flash memory (FLASH).
Physical remap
Once the boot pins mode is selected, the application software can modify the memory
accessible in the code area (in this way the code can be executed through the ICode bus in
place of the System bus). This modification is performed by programming the
Section 10.2.1: SYSCFG memory remap register (SYSCFG_MEMRMP) in the SYSCFG
controller.
The following memories can thus be remapped:
• Main Flash memory
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