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Bit Banding

Bit Banding

• Bit banding is the capability to address a single bit of a memory address.

• This feature is optional. i.e., MCU manufacturer may or may not support it. Consult the reference manual!

• Bit banding provides atomic operations (i.e. will not be interrupted) to bit data.

• To access a word (4 byte), use ldr, str.

  To access a half-word (2 byte), use ldrh, strh.

  To access a byte, use ldrb, strb.

  To access a bit, use bit banding.

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  @ To set the bit-0 of the byte r1 points to (without bit banding):
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  ldrb r0, [r1]       @ load a byte
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  orr  r0, r0, #0x01  @ set bit-0
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  strb r0, [r1]       @ store the result back 
  

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  @ To set the bit-0 of the byte r1 points to using bit banding:
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  ldrb r0, [<alias_address>]
  

• These bit-addressable region is called the "Bit band region", and each bit in the bit band region can be addressed by using the "Bit band alias" address. See the following diagram. Here, the bit band feature is supported only within the Peripheral region and SRAM region.

  (Think about the relationship between 1MB and 32 MB.)

• Example of "Bit" to "Bit band alias" mapping:

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  Bit-band Region                         Alias address
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  ------------------                      -----------------
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  0x20000000 bit[0] <-------------------  0x22000000 bit[0]
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  0x20000000 bit[1] <-------------------  0x22000004 bit[0]
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  0x20000000 bit[2] <-------------------  0x22000008 bit[0]
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  ...                                     ...
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  0x20000000 bit[31] <------------------  0x2200007C bit[0]
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  0x20000004 bit[0] <-------------------  0x22000080 bit[0]
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  0x20000004 bit[1] <-------------------  0x22000084 bit[0]
  

Calculation of Bit Band Alias Address

• Generic formula:

  Alias address = alias_base + (32 * (bit_band_memory_addr - bit_band_base)) + bit * 4

• What is the alias address of the 7th bit position of the memory location 0x20000200?

  $\to$$\to$ Alias address = 0x22000000 + (32 * (0x20000200 - 0x20000000)) + 7 * 4

Bit Banding - Demonstration

• Program to modify the 7th bit position of the memory location 0x20000200 with/without using bit banding.

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  #include <stdint.h>
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  #if !defined(__SOFT_FP__) && defined(__ARM_FP)
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    #warning "FPU is not initialized, but the project is compiling for an FPU. Please initialize the FPU before use."
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  #endif
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  #define ALIAS_BASE 0x22000000U
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  #define BITBAND_BASE 0x20000000U
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  int main(void)
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  {
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      uint8_t *ptr = (uint8_t *)0x20000200;
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      *ptr = 0xff;
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      // 1. without using bit banding
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      *ptr &= ~(1 << 7);  // clearing 7th bit position of a byte in 0x20000200
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      // reset the value
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      *ptr = 0xff;
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      // 2. using bit banding
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      uint8_t *alias_addr = (uint8_t *)(ALIAS_BASE + (32 * (0x20000200 - BITBAND_BASE)) + 7 * 4);
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      *alias_addr = 0;    // clearing 7th bit position of a byte in 0x20000200
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      /* Loop forever */
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      for(;;);
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  }
  

  Without using bit banding generates this many assembly instructions:

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  17                      *ptr &= ~(1 << 7);  // clearing 7th bit position of a byte in 0x20000200
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  080001f8: 7b 68         ldr     r3, [r7, #4]
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  080001fa: 1b 78         ldrb    r3, [r3, #0]
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  080001fc: 03 f0 7f 03   and.w   r3, r3, #127    ; 0x7f
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  08000200: da b2         uxtb    r2, r3
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  08000202: 7b 68         ldr     r3, [r7, #4]
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  08000204: 1a 70         strb    r2, [r3, #0]
  

  Using bit banding generates much less instructions:

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  24                      *alias_addr = 0;    // clearing 7th bit position of a byte in 0x20000200
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  08000210: 3b 68         ldr     r3, [r7, #0]
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  08000212: 00 22         movs    r2, #0
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  08000214: 1a 70         strb    r2, [r3, #0]
  

References

Nayak, K. (2022). Embedded Systems Programming on ARM Cortex-M3/M4 Processor [Video file]. Retrieved from https://www.udemy.com/course/embedded-system-programming-on-arm-cortex-m3m4/