ARM architecture concept of Bit-Banding to solve problem of Race-condition in case of register value update

Two 1MB 'bit-band' regions, one in the peripheral memory area and one in the SRAM memory areas are each mapped to a 32MB virtual 'alias' region. Each bit in the bit-band region is mapped to a 32bit word in the alias region.





The first bit in the 'bit-band' peripheral memory is mapped to the first word in the alias region, the second bit to the second word etc.

Code example of using Bit-banding:

// Define base address of bit-band
#define BITBAND_SRAM_BASE 0x20000000
// Define base address of alias band
#define ALIAS_SRAM_BASE 0x22000000
// Convert SRAM address to alias region
#define BITBAND_SRAM(a,b) ((ALIAS_SRAM_BASE + (a-BITBAND_SRAM_BASE)*32 \    + (b*4)))
 
// Define base address of peripheral bit-band
#define BITBAND_PERI_BASE 0x40000000
// Define base address of peripheral alias band
#define ALIAS_PERI_BASE 0x42000000
// Convert PERI address to alias region
#define BITBAND_PERI(a,b) ((ALIAS_PERI_BASE + (a-BITBAND_PERI_BASE)*32 \    + (b*4)))
 
//Define some memory address
#define MAILBOX 0x20004000
//Define a hardware register
#define TIMER 0x40004000
 
// Mailbox bit 0
#define MBX_B0 *((volatile unsigned int *)(BITBAND_SRAM(MAILBOX,0))) 
// Mailbox bit 7
#define MBX_B7 *((volatile unsigned int *)(BITBAND_SRAM(MAILBOX,7))) 
// Timer bit 0
#define TIMER_B0 *((volatile unsigned char *)(BITBAND_PERI(TIMER,0))) 
// Timer bit 7
#define TIMER_B7 *((volatile unsigned char *)(BITBAND_PERI(TIMER,7)))
 
 
int main(void){    
    unsigned int temp = 0;
    MBX_B0 = 1; // Word write    
    temp = MBX_B7; // Word read    
    TIMER_B0 = temp; // Byte write    
    return TIMER_B7; // Byte read
}

This is not the only solution to this problem.All common architectures have implemented mechanisms for atomically setting and clearing bits. ARM’s approach is elegant in that it can be exercised with ANSI C, while most others implementations require special C extensions or the use of assembly language.