PM0214 datasheet(31/245 Pages) STMICROELECTRONICS

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PM0214

The Cortex-M4 processor

244

2.2.5

Bit-banding

A bit-band region maps each word in a bit-band alias region to a single bit in the bit-band

region. The bit-band regions occupy the lowest 1 MB of the SRAM and peripheral memory

regions.

The memory map has two 32 MB alias regions that map to two 1 MB bit-band regions:

•

Accesses to the 32 MB SRAM alias region map to the 1 MB SRAM bit-band region, as

shown in Table 14

•

Accesses to the 32 MB peripheral alias region map to the 1 MB peripheral bit-band

region, as shown in Table 15.

Note:

A word access to the SRAM or peripheral bit-band alias regions map to a single bit in the

SRAM or peripheral bit-band region.

Bit band accesses can use byte, halfword, or word transfers. The bit band transfer size

matches the transfer size of the instruction making the bit band access.

The following formula shows how the alias region maps onto the bit-band region:

bit_word_offset = (byte_offset x 32) + (bit_number x 4)

bit_word_addr = bit_band_base + bit_word_offset

Table 14. SRAM memory bit-banding regions

Address

range

Memory

region

Instruction and data accesses

0x20000000-

0x200FFFFF

SRAM bit-band region

Direct accesses to this memory range behave as SRAM memory

accesses, but this region is also bit addressable through bit-band alias.

0x22000000-

0x23FFFFFF

SRAM bit-band alias

Data accesses to this region are remapped to bit band region. A write

operation is performed as read-modify-write. Instruction accesses are not

remapped.

Table 15. Peripheral memory bit-banding regions

Address

range

Memory

region

Instruction and data accesses

0x40000000-

0x400FFFFF

Peripheral

bit-band region

Direct accesses to this memory range behave as peripheral memory

accesses, but this region is also bit addressable through bit-band

alias.

0x42000000-

0x43FFFFFF

Peripheral

bit-band alias

Data accesses to this region are remapped to bit-band region. A write

operation is performed as read-modify-write. Instruction accesses are

not permitted.

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