diff -r 51a74ef9ed63 -r ae94777fff8f Symbian3/SDK/Source/GUID-7287F1D4-023E-5C90-91E1-6D8DF78D8241.dita --- a/Symbian3/SDK/Source/GUID-7287F1D4-023E-5C90-91E1-6D8DF78D8241.dita Wed Mar 31 11:11:55 2010 +0100 +++ b/Symbian3/SDK/Source/GUID-7287F1D4-023E-5C90-91E1-6D8DF78D8241.dita Fri Jun 11 12:39:03 2010 +0100 @@ -1,67 +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. - - + + + + + +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:

+
    +

  • moving the old process +chunks to the home section, and changing their access permissions to supervisor-only

    • +

  • moving the new process +chunks to the data section and changing their access permissions back to user-accessible.

    • +
+

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