Bit-banding is a feature in some Arm Cortex processors that allows single bit modifications to be made to memory mapped peripherals without the overhead of a read-modify-write sequence. This provides an efficient mechanism to set or clear individual bits in registers. Bit-band regions map a complete word in memory to a single bit in the bit-band alias memory region. Writes to bit-band alias memory modify only the targeted bit in the corresponding word in the bit-band region.
What are Bit-band Regions?
Bit-band regions are sections of physical memory that are aliased into bit-band alias memory. Each word in a bit-band region is mapped to a bit in the bit-band alias memory. This allows atomic read-modify-write access to a single bit within a word of memory. Without bit-banding, setting or clearing a single bit requires a read of the entire word, modifying the bit, then writing the word back. Bit-banding eliminates this overhead by providing single-bit access through the alias memory. Writes to the alias memory affect only the mapped bit in the bit-band region.
Bit-band Region Address Mapping
The bit-band regions are defined in the Arm system architecture. For Cortex-M processors, two 1MB bit-band regions are defined – one for SRAM and one for peripherals. The bit-band alias regions are offset from the bit-band regions by a constant value. This allows conversion between bit-band addresses and alias addresses using simple formulas.
For example, to convert a SRAM address to a bit-band alias address:
- SRAM bit-band region base: 0x20000000
- SRAM bit-band alias offset: 0x02000000
- Bit-band alias address = Bit-band region base + (byte_offset x 32) + (bit_number x 4)
So to access bit 5 of byte 0x100 in SRAM:
- SRAM address: 0x20000100
- Byte offset: 0x100
- Bit number: 5
- Alias address = 0x20000000 + (0x100 x 32) + (5 x 4) = 0x22000114
A write to 0x22000114 will set or clear only bit 5 in 0x20000100.
Advantages of Bit-band Regions
The main advantages of bit-band regions are:
- Atomic read-modify-write for single bit access
- Faster than read-modify-write sequence for peripheral registers
- Simplify setting/clearing bits in control registers
- Avoid software locks when multiple threads access shared bits
Bit manipulation using bit-band alias registers helps optimize performance and code size compared to traditional read-modify-write sequences.
Bit-banding in Cortex-M
The Arm Cortex-M series of processors include bit-band regions in their memory maps. Two 1MB regions are defined – one for SRAM and one for peripherals. The SRAM bit-band region maps to internal SRAM while the peripheral region maps to memory-mapped registers.
Cortex-M Bit-band Memory Map
In Cortex-M, the bit-band regions are defined as:
- SRAM bit-band base: 0x20000000
- SRAM bit-band size: 1MB
- Peripheral bit-band base: 0x40000000
- Peripheral bit-band size: 1MB
The corresponding alias regions are:
- SRAM alias base: 0x22000000
- Peripheral alias base: 0x42000000
So the SRAM and peripheral bit-band regions occupy the lower 1MB of each region, with the alias regions following immediately after.
Accessing Bit-band Aliases
To access a bit-band alias location, convert the target address to a bit-band alias address using the formulas:
- SRAM: Alias = 0x22000000 + (Byte_offset x 32) + (Bit_num x 4)
- Peripheral: Alias = 0x42000000 + (Byte_offset x 32) + (Bit_num x 4)
Then simply read from or write to this alias address to access the targeted bit. The processor handles the address translation automatically.
Bit-banding Usage Examples
Here are some examples of bit-banding in Cortex-M:
- Set bit 5 in GPIO port A:
- GPIOA base: 0x40020000
- Byte offset: 0x0000
- Bit number: 5
- Alias address = 0x42000000 + (0x0000 x 32) + (5 x 4) = 0x42000114
- Toggle bit 0 in NVIC ISER0 register:
- ISER0 address: 0xE000E100
- Byte offset: 0xE100
- Bit number: 0
- Alias address = 0x42000000 + (0xE100 x 32) + (0 x 4) = 0x4200E100
- Clear pending bit 3 in EXTI_PR:
- EXTI_PR address: 0x40013C14
- Byte offset: 0x3C14
- Bit number: 3
- Alias address = 0x42000000 + (0x3C14 x 32) + (3 x 4) = 0x42003C34
This demonstrates how bit-banding can simplify peripheral register access and bit manipulation for Cortex-M cores.
Limitations of Bit-banding
While bit-banding is useful, there are some limitations to consider:
- Only 1MB of SRAM and 1MB of peripherals are bit-addressable
- Not all ARM Cortex CPUs include bit-band regions
- Bit-band accesses are slower than direct SRAM access
- Not all peripheral registers are mapped into the bit-band region
- Aliased bits can only be accessed as full 32-bit words
So bit-banding does not replace normal memory access methods. Rather, it supplements them by providing atomic single-bit manipulation for selected memory regions.
Bit-band Region Size
The relatively small 1MB size of the bit-band regions means only a portion of memory is aliased. So larger memories and peripherals are not fully contained in the regions. Aliased access is only possible where addresses overlap the bit-band area.
Performance Overhead
The address translation and bus arbitration for bit-band accesses introduce a small performance penalty. Accessing SRAM directly will be faster than going via the bit-band alias.
32-bit Access Only
The bit-band alias regions can only be accessed using full 32-bit reads and writes. 8-bit and 16-bit accesses are not supported. So some overhead is introduced when only accessing a single bit.
Not All Peripherals Included
Vendor implementations differ in which peripheral registers are mapped into the alias memory region. So bit-banding is not universally available across every register in the memory map.
Conclusion
Bit-banding provides an efficient way to manipulate single bits in registers without the overhead of read-modify-write sequences. By mapping memory words to individual bits in an alias region, Cortex-M processors enable atomic access to set, clear or toggle bits in peripheral and SRAM locations. This can simplify peripheral configuration and control, state machines, interrupt handling and other single-bit operations.
Understanding the bit-band address mapping formulas allows targeting specific bits with single 32-bit writes. However, the technique has limitations including size restrictions, performance overhead and lack of support for 8/16-bit access. Bit-banding complements, rather than replaces, conventional memory access methods in Cortex-M based systems.
