replaced, and data contained
in
that page was modified, system software generally must write the
contents of the modified page to the backing store before replacing its contents. System software
must maintain records to control the environment.
Similarly, when system software manages TLB entries, the software often must know whether a
particular TLB entry was referenced. When the system software must select a TLB entry to cast out,
previously referenced entries are more
likely to be maintained than entries which were never
referenced. System software must also maintain records for this purpose.
The
PPC405GP does not provide hardware reference
or
change bits, but TLB miss interrupts and
data storage interrupts enable system software to maintain reference information for TLB entries and
their associated pages, respectively.
A possible algorithm
follows. First, the TLB entries are built, with each TLB_entry[V, WR] =
O.
System
software retains the index and EPN of each entry.
The first attempt by application code to access a page causes a TLB miss interrupt, because its TLB
entry
is
marked invalid. The TLB miss handler records the reference to the TLB entry (and to the
associated page)
in
a data structure, then sets TLB_entry[V] =
1.
(Note that TLB_entry[V] can be
considered a reference bit for the TLB entry.) Subsequent read accesses to the page associated with
the TLB entry proceed
normally.
In
the example just given for recording TLB entry references, the first write access to the page using
the TLB entry, after the entry is made
valid, causes a data storage interrupt because write access was
turned off. The TLB miss handler records the write to the page
in
a data structure, for use as a
"changed" flag, then sets TLB_entry[WR] = 1 to enable write access. (Note that TLB_entry[WR] can
be considered a change bit for the page.) Subsequent write accesses to the page proceed
normally.
6.7 Access Protection
The PPC405GP provides virtual-mode access protection. The TLB entry enables system software to
control general access for programs
in
the problem state, and control write and execute permissions
for
all pages. The TLB entry can specify zone protection that can override the other access control
mechanisms supported
in
the TLB entries.
TLB entry and zone protection methods also support access controls for cache operation and string
loads/stores.
6.7.1
Access
Protection Mechanisms in
the
TLB
For MMU access protection to be
in
effect, one
or
both of MSR[IR]
or
MSR[OR] must be set to one to
enable address translation.
MSR[IR] enables protection on instruction fetches, which are inherently
read-only. MSR[OR] enables protection on data accesses (loads/stores).
6.7.1.1
General Access Protection
The translation
10
(TLB_entry[TIO]) provides the first level of MMU access protection. This a-bit field,
if non-zero, is compared to the contents of TLB_entry[PIO]. These fields must match
in
a valid TLB
entry
if
any access is to be allowed.
In
typical use, it is assumed that a program
in
the supervisor
state, such as a real-time operating system, sets the
PIO before starting a problem state program that
is subject to access control. .
6·12 PPC405GP User's Manual Preliminary
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