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March 2002 Release
345
Virtex-II Pro™ Platform FPGA Documentation
1-800-255-7778
Memory Management
R
Physical Memory
Physical memory
represents the address space of memory installed in a computer system,
including memory-mapped I/O devices. Generally, the amount of physical memory
actually available in a system is smaller than that supported by the processor. When
address translation is supported by the operating system—as it is in virtual-memory
systems—the very-large virtual-address space is translated into the smaller physical-
address space using the memory-management resources supported by the processor.
The PPC405 supports up to four gigabytes of physical memory using a 32-bit physical
address. A hierarchical-memory system involving external (system) memory and the
caches internal to the processor are employed to support that address space. The PPC405
supports separate level-1 (L1) caches for instructions and data. The operation and control
of these caches is described in
Virtual Memory
Virtual memory
is a relocatable address space that is generally larger than the physical-
memory space installed in a computer system. Operating systems relocate (map)
applications and data in virtual memory so it appears that more memory is available than
actually exists. Virtual memory software moves unused instructions and data between
physical memory and external storage devices (such as a hard drive) when insufficient
physical memory is available. The PPC405 supports a 40-bit virtual address that allows
privileged software to manage a one-terabyte virtual-memory space.
Memory Management
Memory management
describes the collection of mechanisms used to translate the addresses
generated by programs into physical-memory addresses. Memory management also
consists of the mechanisms used to characterize memory-region behavior, also referred to
as
storage control
. Memory management is performed by privileged-mode software and is
completely transparent to user-mode programs running in virtual mode.
The PPC405 is a PowerPC embedded-environment implementation. The memory-
management resources defined by the PowerPC embedded-environment architecture (and
its successor, the PowerPC Book-E architecture) differ significantly from the resources
defined by the PowerPC architecture. The resources defined by the PowerPC embedded
environment architecture are well-suited for the special requirements of embedded-system
applications. The resources defined by the PowerPC architecture better meet the
requirements of desktop and commercial-workstation systems.
Generally, the differences between the two memory-management mechanisms are as
follows:
•
The PPC405 supports
software page translation
and provides special instructions for
managing the page tables and the translation look-aside buffer (TLB) internal to the
processor. The page-translation table format, organization, and search algorithms are
software-dependent and transparent to the PPC405 processor. The PowerPC
architecture, on the other hand, defines the page-translation table organization,
format, and search algorithms. It does not define support for the special page table
and TLB instructions but instead assumes the processor hardware is responsible for
searching page tables and updating the TLB.
•
The PPC405 supports
variable-sized pages
. The PowerPC architecture defines fixed-size
pages of 4 KB.
•
The PPC405
does not
support the segment-translation mechanism defined by the
PowerPC architecture.
•
The PPC405
does not
support the block-address-translation (BAT) mechanism defined
by the PowerPC architecture.
Содержание Virtex-II Pro PPC405
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