Digi NS9215 - Manual (Page 121)

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W O R K I N G W I T H T H E C P U
Domain access control
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Compatibility
issues
To enable code to be ported easily to future architectures, it is recommended
that no reliance is made on external abort behavior.
The Instruction Fault Status register is intended for debugging purposes only.
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D o m a i n a c c e s s c o n t r o l
MMU accesses are controlled primarily through the use of domains. There are 16
domains, and each has a two-bit field to define access to it. Client users and
Manager users are supported.
The domains are defined in the R3: Domain Access Control register; the register
format in “R3:Domain Access Control register” on page 91 shows how the 32 bits of
the register are allocated to define the 16 two-bit domains.
Specifying access
permissions
This table shows how the bits within each domain are defined to specify access
permissions.
Interpreting
access permission
bits
This table shows how to interpret the access permission (AP) bits, and how the
interpretation depends on the R and S bits in the R1: Control register (see "R1:
Control register," beginning on page 88).
Domain MVA of first aborted address in transfer
Permission MVA of first aborted address in transfer
External about for noncached reads,
or nonbuffered writes MVA of last address before 1KB boundary, if any word of
the transfer before 1 KB boundary is externally aborted.
MVA of last address in transfer if the first externally
aborted word is after the 1 KB boundary.
Domain Fault Address register
Value Meaning Description
0 0 No access Any access generates a domain fault.
0 1 Client Accesses are checked against the access permission bits in the section or
page descriptor.
1 0 Reserved Reserved. Currently behaves like no access mode.
1 1 Manager Accesses are not checked against the access permission bits, so a
permission fault cannot be generated.