Datasheet

SAM3X / SAM3A [DATASHEET]
Atmel-11057C-ATARM-SAM3X-SAM3A-Datasheet_23-Mar-15
208
Software must use memory barrier instructions:
 before MPU setup if there might be outstanding memory transfers, such as buffered writes, that might be
affected by the change in MPU settings
 after MPU setup if it includes memory transfers that must use the new MPU settings.
However, memory barrier instructions are not required if the MPU setup process starts by entering an exception
handler, or is followed by an exception return, because the exception entry and exception return mechanism cause
memory barrier behavior.
Software does not need any memory barrier instructions during MPU setup, because it accesses the MPU through
the PPB, which is a Strongly-Ordered memory region.
For example, if you want all of the memory access behavior to take effect immediately after the programming
sequence, use a DSB instruction and an ISB instruction. A DSB is required after changing MPU settings, such as
at the end of context switch. An ISB is required if the code that programs the MPU region or regions is entered
using a branch or call. If the programming sequence is entered using a return from exception, or by taking an
exception, then you do not require an ISB.
10.23.8.2 Updating an MPU region using multi-word writes
You can program directly using multi-word writes, depending on how the information is divided. Consider the
following reprogramming:
; R1 = region number
; R2 = address
; R3 = size, attributes in one
LDR R0, =MPU_RNR ; 0xE000ED98, MPU region number register
STR R1, [R0, #0x0] ; Region Number
STR R2, [R0, #0x4] ; Region Base Address
STR R3, [R0, #0x8] ; Region Attribute, Size and Enable
Use an STM instruction to optimize this:
; R1 = region number
; R2 = address
; R3 = size, attributes in one
LDR R0, =MPU_RNR ; 0xE000ED98, MPU region number register
STM R0, {R1-R3} ; Region Number, address, attribute, size and enable
You can do this in two words for pre-packed information. This means that the RBAR contains the required region
number and had the VALID bit set to 1, see “MPU Region Base Address Register” on page 203. Use this when the
data is statically packed, for example in a boot loader:
; R1 = address and region number in one
; R2 = size and attributes in one
LDR R0, =MPU_RBAR ; 0xE000ED9C, MPU Region Base register
STR R1, [R0, #0x0] ; Region base address and
; region number combined with VALID (bit 4) set to 1
STR R2, [R0, #0x4] ; Region Attribute, Size and Enable
Use an STM instruction to optimize this:
; R1 = address and region number in one
; R2 = size and attributes in one
LDR R0,=MPU_RBAR ; 0xE000ED9C, MPU Region Base register
STM R0, {R1-R2} ; Region base address, region number and VALID bit,
; and Region Attribute, Size and Enable
10.23.8.3 Subregions
Regions of 256 bytes or more are divided into eight equal-sized subregions. Set the corresponding bit in the SRD
field of the RASR to disable a subregion, see “MPU Region Attribute and Size Register” on page 204. The least
significant bit of SRD controls the first subregion, and the most significant bit controls the last subregion. Disabling