STMicroelectronics STM32F405, Reference Manual

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RM0090 Memory and bus architecture
112
are only available for Cortex
®
-M4 with FPU accesses, and not from other bus masters (e.g.
DMA).
A mapping formula shows how to reference each word in the alias region to a corresponding
bit in the bit-band region. The mapping formula is:
bit_word_addr = bit_band_base + ( byte_offset x 32) + ( bit_number × 4)
where:
–
bit_word_addr is the address of the word in the alias memory region that maps to
the targeted bit
– bit_band_base is the starting address of the alias region
– byte_offset is the number of the byte in the bit-band region that contains the
targeted bit
– bit_number is the bit position (0-7) of the targeted bit
Example
The following example shows how to map bit 2 of the byte located at SRAM address
0x20000300 to the alias region:
0x22006008 = 0x22 (0x300*32) + (2*4)
Writing to address 0x22006008 has the same effect as a read-modify-write operation on bit
2 of the byte at SRAM address 0x20000300.
Reading address 0x22006008 returns the value (0x01 or 0x00) of bit 2 of the byte at SRAM
address 0x20000300 (0x01: bit set; 0x00: bit reset).
For more information on bit-banding, please refer to the Cortex
®
-M4 with FPU programming
manual (see Related documents on page 1 ).
2.4 Boot configuration
Due to its fixed memory map, the code area starts from address 0x0000 0000 (accessed
through the ICode/DCode buses) while the data area (SRAM) starts from address
0x2000 0000 (accessed through the system bus). The Cortex
®
-M4 with FPU CPU always
fetches the reset vector on the ICode bus, which implies to have the boot space available
only in the code area (typically, Flash memory). STM32F4xx microcontrollers implement a
special mechanism to be able to boot from other memories (like the internal SRAM).
In the STM32F4xx, three different boot modes can be selected through the BOOT[1:0] pins
as shown in
Table 2 .
Table 2. Boot modes
Boot mode selection pins
Boot mode Aliasing
BOOT1 BOOT0
x 0 Main Flash memory Main Flash memory is selected as the boot space
0 1 System memory System memory is selected as the boot space
1 1 Embedded SRAM Embedded SRAM is selected as the boot space