PageTables in Windows CE6
Sonu
In the early 80's (and in some cases today), microprocessors usually
accessed physical memory by mapping all physical memory addresses
directly to the external bus. This would mean that the
microprocessor's physical memory addresses were mapped one to one to
the external memory locations on a system. This would create a
situation in which the software running on the microprocessor could
access all of the physical memory on the system at any time. This
would rely heavily on the programmer to ensure that they did not
overwrite code or data that they were not intending to. There was no
form of protection between different pieces of code and the system
memory.
Therefore, the memory management unit was created to solve multiple
problems:
·Memory Protection through the use of Virtual Memory
By adding a layer of abstraction between the microprocessor's "virtual"
memory locations and their actual physical locations, the software
programmer can now define areas of the physical memory map that are
accessible only after it has been preallocated in the virtual memory
map. This means that if the code "accidentally" reads or writes from
memory that is not preallocated, the developer will be warned of a
problem in the execution of this code and can take action.
It also allows the programmer to protect physical memory from certain
pieces of code by changing the mapping between virtual and physical
memory dynamically. This is how most modern operating systems implement
a large portion of their security model.
·Finer Cache Control
By having a MMU, the programmer can define certain regions of virtual
memory that are mapped to either cached or uncached physical memory.
The MMU allows this to occur even at a page level.
· Physical memory fragmentation
Since the MMU creates an abstraction layer between the virtual and
physical addresses, it is possible to create a contiguous region of
memory even when the underlying physical memory is fragmented. This
can give an operating system programmer the flexibility to create a
virtual memory map that is completely independent of the underlying
physical memory structure and usage.
What are page tables?
The mapping of virtual memory to physical memory is accomplished
through the use of what is called "page tables". Let's take the ARMv4
processor architecture in Windows CE6 as a specific example to help
explain this. This architecture relies on two page table levels to map
sections of physical memory to any virtual memory address from 0 to
4GB. These page tables are accessed by the microprocessor's hardware
directly in physical memory to make this translation as inexpensive and
non obtrusive as possible.
First Level Page Table Descriptors
The first level page table contains a number of descriptors that define
a window of memory that represents 1 megabyte of virtual memory. These
1 megabyte windows are called sections. Each of these descriptors take
32bits of physical memory storage space. In some cases, the first
level page table is all that is needed to map physical memory to
virtual memory. The OEMAddressTable is a good example of this. In
other cases the second level page table descriptors are used.
Second Level Page Table Descriptors
Each first level page table descriptor can point to one or more second
level page table descriptors which define a page of virtual memory and
point to a page of physical memory. These page table descriptors can
point to 1KB, 4KB or 64KB regions, but CE6 has chosen to use either 4KB
or 64KB of memory for each of these descriptors depending on the type
and size of the allocation. Each of these descriptors also take 32
bits of physical memory storage space.
Cacheable and Bufferable
Each page table descriptor defines whether or not a given virtual
memory page is cached and/or buffered by the microprocessor when it
accesses the physical memory. You may hear from time to time about the
C and B bits. This refers to the cacheable and bufferable bits
respectively.
Access Permissions
In addition to defining the translation between virtual and physical
memory, the page table descriptors define access permissions to the
physical memory that they describe. This includes protection from
writing, reading and code that is executed in user versus supervisor
mode.
ravi
Dear Friends,
I recently started learning of Win CE. Can you suggest me some books to
study Win CE?
If you have any information (ebooks, soft copies, ppts, pdfs
presentations, papers or any other information), please forward to me.
You can also ive me suggestions to learn Win CE quickly.
Thanks & Regards,
Ravi.
