ARM Cortex R4F Instrukcja Użytkownika Pobierz pdf (Strona 27)

Bit Banding

• Memory mapped I/O, 4GB memory address space organized in bytes.

• 4GB is very large for small embedded applications.

• Bit-banding happens by taking advantage of this large memory space.

• Uses two different regions of the address space to refer the same physical

data in the memory.

• In primary bit-band region each address corresponds to single data byte.

• In the bit-band alias each address corresponds to 1-bit of the same data.

• It allows the access of a bit of data (read or write) by a single instruction.

• LDR can load a single bit and STR can write a single bit of data.

• Two bit band alias regions can be used to access individual status and

control bit of I/O devices or to implement a set of 1-bit Boolean flags

that can be used to implement a set of mutex objects.

• Bit-band hardware does not allow interruption of read-modify write.

Bit_band alias address = Bit_band base +128 x word_offset + 4 x bit #

If bit-3 at address 20001000

is to be modified the bit-band alias is

22000000

+ 128

x 1000

+ 4 x 3 = 2208000C

1 2 ... 22 23 24 25 26 27 28 29 30 31 32 ... 63 64

Brak uwag

ARM Cortex R4F Instrukcja Użytkownika Strona 27