13.5.1.2 Privilege violation determinationWhile the access evaluation macro is determining region hit, the logic is also evaluatingif the current access is allowed by the permissions defined in the region descriptor. Usingthe master and supervisor/user mode signals, a set of effective permissions is generatedfrom the appropriate fields in the region descriptor. The protection violation logic thenevaluates the access against the effective permissions as shown in the following table.Table 13-4. Protection violation definitionDescription MxUM Protection violation?r w xInstruction fetch read — — 0 Yes, no execute permission— — 1 No, access is allowedData read 0 — — Yes, no read permission1 — — No, access is allowedData write — 0 — Yes, no write permission— 1 — No, access is allowed13.5.2 Putting it all together and error terminationsFor each slave port monitored, the MPU performs a reduction-AND of all the individualterms from each access evaluation macro. This expression then terminates the bus cyclewith an error and reports a protection error for three conditions:• If the access does not hit in any region descriptor, a protection error is reported.• If the access hits in a single region descriptor and that region signals a protectionviolation, a protection error is reported.• If the access hits in multiple (overlapping) regions and all regions signal protectionviolations, a protection error is reported.As shown in the third condition, granting permission is a higher priority than denyingaccess for overlapping regions. This approach is more flexible to system software inregion descriptor assignments. For an example of the use of overlapping regiondescriptors, see Application information.Chapter 13 Memory Protection Unit (MPU)MWCT101xS Series Reference Manual, Rev. 3, 07/2019NXP Semiconductors 233
