diff -r 43e37759235e -r 51a74ef9ed63 Symbian3/SDK/Source/GUID-7287F1D4-023E-5C90-91E1-6D8DF78D8241.dita --- /dev/null Thu Jan 01 00:00:00 1970 +0000 +++ b/Symbian3/SDK/Source/GUID-7287F1D4-023E-5C90-91E1-6D8DF78D8241.dita Wed Mar 31 11:11:55 2010 +0100 @@ -0,0 +1,67 @@ + + + + + +Virtual +machine modelThe Kernel provides a 'virtual machine' environment to user processes +where each process accesses its data in the same virtual address range. +

The Kernel provides a 'virtual machine' environment to user processes. +Each process accesses its data in the same virtual address range, called the +data section, which ranges from:

+0x00400000 to 0x3FFFFFFF +

but note that the static data always appears at:

+0x00400000 +

The code chunk for RAM loaded processes always appears at:

+0x20000000 +

This allows multiple processes to run, each executing the same code (from +the same code chunk) for each of the processes. For example, multiple word +documents open at the same time, each in a separate instance of the word application. +This reduces RAM usage.

+

In effect, each user process has the same kind of view. Code instructions +address data using the virtual address; the Memory Management Unit (MMU) is +responsible for the translation of the virtual address to the physical RAM +address.

+

Only one chunk can occupy a given virtual address range at a time, so a +context switch between different processes involves re-mapping the chunks. +The process chunks of the old process are re-mapped to their home addresses. +These are in the home section, which is the virtual address range from:

+0x80000000 to 0xFFFFFFFF +

ROM code is normally mapped into the address range:

+0x50000000 to 0x5FFFFFFF +

The process chunks of the new process are mapped from their home addresses +back to the data section. Chunks which are not accessible by the current user +process reside in the home section, and they have supervisor-mode-only access +permissions, so that only the kernel can access them.

+

The Kernel's data and stack/heap chunks also reside in the home section. +These are never visible to user processes. Code chunks for RAM-loaded libraries +reside at the top end of the home section and have user read-only access, +so that all user processes can execute code from a loaded library.

+

A context switch between processes therefore involves:

+ +

This is best seen graphically. In the first diagram shown below, user-process +1 is running and can 'see' all chunks in the clear boxes. Those boxes with +dark background represent chunks which are not visible to user-process 1.

+

When user-process 2 is running, the context switch re-maps the user-process +2 data to the data section and user process 1 data is re-mapped to the home +section as the second diagram shows.

+ +User process 1's view. + + + +User process 2's view. + + + \ No newline at end of file