NXP Semiconductors MPC5605BK, Reference Manual

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Chapter 21 Memory Protection Unit (MPU)
MPC5606BK Microcontroller Reference Manual, Rev. 2
386 Freescale Semiconductor
21.5.2.4.4 MPU Region Descriptor n , Word 3 (MPU_RGD n .Word3)
The fourth word of the MPU region descriptor contains the optional process identifier and mask, plus the
region descriptor’s valid bit.
Since the region descriptor is a 128-bit entity, there are potential coherency issues as this structure is being
updated since multiple writes are required to update the entire descriptor. Accordingly, the MPU hardware
assists in the operation of the descriptor valid bit to prevent incoherent region descriptors from generating
spurious access errors. In particular, it is expected that a complete update of a region descriptor is typically
done with sequential writes to MPU_RGD
n .Word0, then MPU_RGD n .Word1,... and finally
MPU_RGD n .Word3. The MPU hardware automatically clears the valid bit on any writes to words {0,1,2}
of the descriptor. Writes to this word set/clear the valid bit in a normal manner.
Since it is also expected that system software may adjust only the access controls within a region descriptor
(MPU_RGD
n .Word2) as different tasks execute, an alternate programming view of this 32-bit entity is
provided. If only the access controls are being updated, this operation should be performed by writing to
MPU_RGDAAC
n (Alternate Access Control n) as stores to these locations do not affect the descriptor’s
valid bit.
M0PE Bus master 0 process identifier enable
If set, this flag specifies that the process identifier and mask (defined in MPU_RGDn.Word3) are to be
included in the region hit evaluation. If cleared, then the region hit evaluation does not include the
process identifier.
M0SM Bus master 0 supervisor mode access control
This field defines the access controls for bus master 0 when operating in supervisor mode. The M0SM
field is defined as:
00 r, w, x = read, write and execute allowed
01 r, –, x = read and execute allowed, but no write
10 r, w, – = read and write allowed, but no execute
11 Same access controls as that defined by M0UM for user mode
M0UM Bus master 0 user mode access control
This field defines the access controls for bus master 0 when operating in user mode. The M0UM field
consists of three independent bits, enabling read, write and execute permissions: {r,w,x}. If set, the bit
allows the given access type to occur; if cleared, an attempted access of that mode may be terminated
with an access error (if not allowed by any other descriptor) and the access not performed.
Table 21-8. MPU_RGD n .Word2 field descriptions (continued)
Field Description