ARM Cortex-M4 - Updating an MPU Region Using Multi-Word Writes

Cortex-M4 Peripherals
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STR R1, [R0, #0x0] ; Region Number
STR R4, [R0, #0x4] ; Region Base Address
STRH R2, [R0, #0x8] ; Region Size and Enable
STRH R3, [R0, #0xA] ; Region Attribute
Disable a region before writing new region settings to the MPU if you have previously enabled
the region being changed. For example:
; R1 = region number
; R2 = size/enable
; R3 = attributes
; R4 = address
LDR R0,=MPU_RNR ; 0xE000ED98, MPU region number register
STR R1, [R0, #0x0] ; Region Number
BIC R2, R2, #1 ; Disable
STRH R2, [R0, #0x8] ; Region Size and Enable
STR R4, [R0, #0x4] ; Region Base Address
STRH R3, [R0, #0xA] ; Region Attribute
ORR R2, #1 ; Enable
STRH R2, [R0, #0x8] ; Region Size and Enable
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 require 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 .
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