PM0056 datasheet(28/156 Pages) STMICROELECTRONICS

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The Cortex-M3 processor

PM0056

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DocID15491 Rev 5

that if the exception is taken immediately after being enabled the processor uses the

new exception vector.

•

Self-modifying code. If a program contains self-modifying code, use an ISB

instruction immediately after the code modification in the program. This ensures

subsequent instruction execution uses the updated program.

•

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.

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 13

•

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

region, as shown in Table 14.

Table 13. 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 14. 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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