Virtual-Memory Management - Xilinx MicroBlaze Reference Manual

Virtual-Memory Management

Programs running on MicroBlaze use effective addresses to access a flat 4 GB address
space. The processor can interpret this address space in one of two ways, depending on the
translation mode:
• In real mode, effective addresses are used to directly access physical memory
• In virtual mode, effective addresses are translated into physical addresses by the
virtual-memory management hardware in the processor
Virtual mode provides system software with the ability to relocate programs and data
anywhere in the physical address space. System software can move inactive programs and
data out of physical memory when space is required by active programs and data.
Relocation can make it appear to a program that more memory exists than is actually
implemented by the system. This frees the programmer from working within the limits
imposed by the amount of physical memory present in a system. Programmers do not need
to know which physical-memory addresses are assigned to other software processes and
hardware devices. The addresses visible to programs are translated into the appropriate
physical addresses by the processor.
Virtual mode provides greater control over memory protection. Blocks of memory as small
as 1 KB can be individually protected from unauthorized access. Protection and relocation
enable system software to support multitasking. This capability gives the appearance of
simultaneous or near-simultaneous execution of multiple programs.
In MicroBlaze, virtual mode is implemented by the memory-management unit (MMU),
available when
(Performance) or 2 (Frequency). The MMU controls effective-address to physical-address
mapping and supports memory protection. Using these capabilities, system software can
implement demand-paged virtual memory and other memory management schemes.
The MicroBlaze MMU implementation is based upon the PowerPC™ 405 processor.
The MMU features are summarized as follows:
• Translates effective addresses into physical addresses
• Controls page-level access during address translation
• Provides additional virtual-mode protection control through the use of zones
• Provides independent control over instruction-address and data-address translation
and protection
• Supports eight page sizes: 1 kB, 4 kB, 16 kB, 64 kB, 256 kB, 1 MB, 4 MB, and 16 MB. Any
combination of page sizes can be used by system software
• Software controls the page-replacement strategy
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