Memory and bus architecture RM0033
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instance, the Ethernet MAC can read/write from/to the 16 KB SRAM while the CPU is
reading/writing from/to the 112 KB SRAM.
The CPU can access the system SRAM through the System Bus or through the I-Code/D-
Code buses when boot from SRAM is selected or when physical remap is selected
(SYSCFG memory remap register (SYSCFG_MEMRMP) in the SYSCFG controller). To get
the max performance on SRAM execution, physical remap should be selected (boot or
software selection).
2.3.2 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 STM32F20x and STM32F21x both the peripheral registers and the SRAM are
mapped to a bit-band region, so that single bit-band write and read operations are allowed.
The operations are only available for Cortex
®
-M3 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 = 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 programming manual
(see Related documents on page 1).
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