关于cortex-M3/M4中Bit-banding的笔记

Bit-banding

Bit-banding maps a complete word of memoryonto a single bit in the bit-band region. For example, writing to one of thealias words will set or clear the corresponding bit in the bitband region.

This allows every individual bit in thebit-banding region to be directly accessible from a word-aligned address usinga single LDR instruction. It also allows individual bits to be toggled from Cwithout performing a read-modify-write sequence of instructions.

Address translation

The Cortex-M3 has two 32MB regions that maponto the two 1MB bit-band regions. The two regions are separate, one in theSRAM region and one in the peripheral region.

Each bit in the bit-band region is addressedsequentially in the 32MB alias region. For example, the eighth bit in thebit-band region can be accessed using the eighth word in the 32MB alias region.

Reading and writing to the bit-bandingregion

When writing to the alias regions bit 0 ofthe 32 bit word is used to set the value at the bit-banding region. Readingfrom the alias address will return the value from the bit-band region in bit 0and the other bits will be cleared.

You can also access the base bit-bandregion itself in the same way as normal memory, using word, half word, and byteaccesses.

 


The internal SRAM of the Stellaris® devicesis located at address 0x2000.0000 of the device memory

map. To reduce the number of time consuming read-modify-write (RMW) operations,ARM has

introduced bit-banding technology in the Cortex-M3 processor. With abit-band-enabled processor,

certain regions in the memory map (SRAM and peripheral space) can use addressaliases to access

individual bits in a single, atomic operation. The bit-band base is located ataddress 0x2200.0000.

 

The bit-band alias is calculated by usingthe formula:

bit-band alias = bit-band base + (byte offset * 32) + (bit number * 4)

For example, if bit 3 at address 0x2000.1000 is to be modified, the bit-bandalias is calculated as:

0x2200.0000 + (0x1000 * 32) + (3 * 4) = 0x2202.000C

With the alias address calculated, an instruction performing a read/write toaddress 0x2202.000C

allows direct access to only bit 3 of the byte at address 0x2000.1000.

 

//Hw_types.h   以Bit-banding方式访问双字寄存器x中的第b位

#define HWREGBITW(x, b)         \

       HWREG(((unsigned long)(x) & 0xF0000000) | 0x02000000 | \

              (((unsigned long)(x) &0x000FFFFF) << 5) | ((b) << 2))

 

#define HWREG(x)                                          \

       (*((volatile unsigned long *)(x)))

 

Example:

 

ulValue = 1234;

for(ulIdx = 0; ulIdx < 32; ulIdx++)

{

//set every bitof ulValue(word) to 1

         HWREGBITW(&ulValue,31 - ulIdx) = (0xFFFFFFFF >> (31-ulIdx)) & 1;

         UARTprintf(“\r%08x”,ulValue);

         Delay(1);

}