Bit banding
The Cortex™-M3 memory map includes two bit-band regions. These regions map each
word in an alias region of memory to a bit in a bit-band region of memory. Writing to a word
in the alias region has the same effect as a read-modify-write operation on the targeted bit in
the bit-band region.
In the STM32F10xxx both peripheral registers and SRAM are mapped in a bit-band region.
This allows single bit-band write and read operations to be performed. The operations are
only available for Cortex-M3 accesses, 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 in the alias region:
0x22006008 = 0x22000000 + (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™-M3 Technical Reference
Manual.
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