Datasheet

59
11011B–ATARM–21-Feb-12
SAM3N
• Memory map switching. If the system contains a memory map switching mechanism, use a
DSB instruction after switching the memory map in the program. This ensures subsequent
instruction execution uses the updated memory map.
• Dynamic exception priority change. When an exception priority has to change when the
exception is pending or active, use DSB instructions after the change. This ensures the
change takes effect on completion of the DSB instruction.
• Using a semaphore in multi-master system. If the system contains more than one bus
master, for example, if another processor is present in the system, each processor must use
a DMB instruction after any semaphore instructions, to ensure other bus masters see the
memory transactions in the order in which they were executed.
Memory accesses to Strongly-ordered memory, such as the system control block, do not require
the use of DMB instructions.
10.5.5 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 1MB of the SRAM and peripheral memory regions.
The memory map has two 32MB alias regions that map to two 1MB bit-band regions:
• accesses to the 32MB SRAM alias region map to the 1MB SRAM bit-band region, as shown
in Table 10-6
• accesses to the 32MB peripheral alias region map to the 1MB peripheral bit-band region, as
shown in Table 10-7.
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.
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)
Table 10-6. 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 10-7. Peripheral memory bit-banding regions
Address
range
Memory
region Instruction and data accesses
0x40000000-
0x400FFFFF
Peripheral bit-band
alias
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
region
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.