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452
March 2002 Release
1-800-255-7778
Virtex-II Pro™ Platform FPGA Documentation
Chapter 5:
Memory-System Management
R
See
Software Management of Cache Coherency
, for more information on
memory coherency.
Guarded (G)
The guarded storage attribute controls speculative accesses into a memory region.
When the G attribute is cleared to 0, speculative accesses from the memory region can
occur.
When the G attribute is set to 1, speculative memory accesses (instruction prefetches and
data loads) are not permitted. The G storage attribute is typically used to protect memory-
mapped I/O from improper access. An instruction fetch from a guarded region does not
occur until all previous instructions have completed execution, guaranteeing that the
access is not speculative. Prefetching is disabled for a guarded region. Performance is
degraded significantly when executing out of guarded regions, and software should avoid
unnecessarily marking instruction regions as guarded.
See
Preventing Inappropriate Speculative Accesses
for more information on
guarded storage.
User Defined (U0)
The user-defined storage attribute controls implementation-dependent (processor and/or
system) behavior of an access into a memory region. For example, some embedded-system
implementations use the U0 attribute to identify memory regions containing compressed
instructions. In those implementations, memory regions with U0
=
1 contain compressed
instructions, and memory regions with U0
=
0 contain uncompressed instructions.
If desired, system software can cause an exception to occur when a data store is performed
to U0 memory locations. This exception condition can be enabled using the U0-exception
enable bit (U0XE) in the CCR0 register (see
). When
CCR0[U0XE]
=
1, a store to memory locations with U0
=
1 cause a data-storage interrupt to
occur. When CCR0[U0XE]
=
0, stores to U0 memory locations do not cause an exception. See
Data-Storage Interrupt (0x0300)
for information on identifying U0 exceptions.
If no U0 behavior is implemented by the embedded system, setting and clearing the U0
attribute has no effect on instruction fetches or data loads. However, the U0-exception
enable can be used to trigger data-storage interrupts as described above whether the
system defines U0 behavior.
Endian (E)
The endian attribute controls the byte ordering of accesses into a memory region.
When the E attribute is cleared to 0, memory accesses use big-endian byte ordering. When
the E attribute is set to 1, memory accesses use little-endian byte ordering. See
for more information on big-endian and little-endian memory
accesses.
Storage-Attribute Control Registers
The storage-attribute control registers specify the real-mode storage attributes. In virtual
mode, these registers are ignored and storage attributes are taken from the page translation
entries (TLB entries). See
for information on virtual-
mode storage attributes.
The storage-attribute control-registers are 32-bit registers. Each bit is associated with a 128
MB memory region: bit 0 controls the lowest 128 MB region, bit 1 controls the next-lowest
128 MB region, and so on. Together, the 32 register bits provide storage control across the
entire 4 GB physical-address space. The five most-significant effective-address bits (EA
0:4
)
are used to select a specific bit within the register.
shows the address ranges
associated with each register bit.
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