
6-10
G2 PowerPC Core Reference Manual
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MOTOROLA
MMU Features
bit in MSR is cleared (MSR[IR] for instruction accesses and MSR[DR] for data accesses),
the resulting physical address is identical to the effective address and all other translation
mechanisms are ignored.
6.1.4
Memory Protection Facilities
In addition to the translation of effective addresses to physical addresses, the MMUs
provide access protection of supervisor areas from user access and can designate areas of
memory as read-only, as well as, no-execute or guarded. Table 6-2 shows the eight
protection options supported by the MMUs for pages.
The operating system programs whether instructions can be fetched from an area of
memory by appropriately using the no-execute option provided in the segment descriptor.
Each of the remaining options is enforced, based on a combination of information in the
segment descriptor and the page table entry. Thus, the supervisor-only option allows only
read and write operations generated while the processor is operating in supervisor mode
(corresponding to MSR[PR] = 0) to access the page. User accesses that map into a
supervisor-only page cause an exception to be taken.
Finally, there is a facility in the VEA and OEA that allows pages or blocks to be designated
as guarded, preventing out-of order accesses that may cause undesired side effects. For
example, areas of the memory map that are used to control I/O devices can be marked as
guarded so that accesses (for example, instruction prefetches) do not occur unless they are
explicitly required by the program.
For more information on memory protection, see “Memory Protection Facilities” in
Chapter 7, “Memory Management,” in the the
Programming Environments Manual
.
Table 6-2. Access Protection Options for Pages
Option
User Read
User
Write
Supervisor Read
Supervisor
Write
I-Fetch
Data
I-Fetch
Data
Supervisor-only
—
—
—
√
√
√
Supervisor-only-no-execute
—
—
—
—
√
√
Supervisor-write-only
√
√
—
√
√
√
Supervisor-write-only-no-execute
—
√
—
—
√
√
Both user/supervisor
√
√
√
√
√
√
Both user/supervisor-no-execute
—
√
√
—
√
√
Both read-only
√
√
—
√
√
—
Both read-only-no-execute
—
√
—
—
√
—
Note:
√
access permitted.
— protection violation.
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.