vxworks programmer's guide.pdf - DAQ Plone Site

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Virtual Memory Interface
7.5 Using the MMU Programmatically
This section describes the facilities provided for manipulating the MMU
programmatically using low-level routines in vmLib. You can make data private
to a task or code segment, make portions of memory noncacheable, or writeprotect
portions of memory. The fundamental structure used to implement virtual
memory is the virtual memory context (VMC).
For a summary of the VxVMI routines, see the reference entry for vmLib.
7.5.1 Virtual Memory Contexts
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A virtual memory context (VM_CONTEXT, defined in vmLib) is made up of a
translation table and other information used for mapping a virtual address to a
physical address. Multiple virtual memory contexts can be created and swapped
in and out as desired.
Global Virtual Memory
Some system objects, such as text segments and semaphores, must be accessible to
all tasks in the system regardless of which virtual memory context is made current.
These objects are made accessible by means of global virtual memory. Global virtual
memory is created by mapping all the physical memory in the system (the
mapping is defined in sysPhysMemDesc) to the identical address in the virtual
memory space. In the default system configuration, this initially gives a one-to-one
relationship between physical memory and global virtual memory; for example,
virtual address 0x5000 maps to physical address 0x5000. On some architectures, it
is possible to use sysPhysMemDesc to set up virtual memory so that the mapping
of virtual-to-physical addresses is not one-to-one; see 7.3 Virtual Memory
Configuration, p.290 for additional information.
Global virtual memory is accessible from all virtual memory contexts.
Modifications made to the global mapping in one virtual memory context appear
in all virtual memory contexts. Before virtual memory contexts are created, add all
global memory with vmGlobalMap( ). Global memory that is added after virtual
memory contexts are created may not be available to existing contexts.
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