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 need any memory barrier instructions during MPU setup, because it accesses
the MPU through the Private Peripheral Bus (PPB), which is a Strongly Ordered memory region.
For example, if all of the memory access behavior is intended to take effect immediately after the
programming sequence, then a
DSB
instruction and an
ISB
instruction should be used. 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
an
ISB
is not required.
Updating an MPU Region Using Multi-Word Writes
The MPU can be programmed directly using multi-word writes, depending how the information is
divided. Consider the following reprogramming:
; R1 = region number
; R2 = address
; R3 = size, attributes in one
LDR R0, =MPUNUMBER ; 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
An
STM
instruction can be used to optimize this:
; R1 = region number
; R2 = address
; R3 = size, attributes in one
LDR R0, =MPUNUMBER ; 0xE000ED98, MPU region number register
STM R0, {R1-R3} ; Region number, address, attribute, size and enable
This operation can be done in two words for prepacked information, meaning that the
MPU Region
Base Address (MPUBASE)
register (see page 197) contains the required region number and has
the
VALID
bit set. This method can be used when the data is statically packed, for example in a
boot loader:
139
June 18, 2014
Texas Instruments-Production Data
Tiva
™
TM4C1294NCPDT Microcontroller