pointing to a memory map location in C

knobby67 · 07-29-2007, 02:17 PM

Hi all,
I hope someone can help me out on this or point me the right direction. I need to get the checksum of a memory range. I understand how to get this if I’m pointing to a buffer or array. However I need to get this for a direct memory address, so it might be memory map location 0x00ff and the next 50 bytes after this. So I need to be able to point via an address rather than from a buffer/array name. I’m really unsure how to do this, can anyone point (no pun intended) me in the right direction. Thanks

BTW I’m using C on an embedded processor. Thanks

paulsm4 · 07-29-2007, 02:38 PM

Hi -

If you're doing this from user space, then the virtual address *IS* the address. Trying to compute a checksum of an arbitrary memory range with respect to using "physical memory" ranges is nonsense:

a) It's a lot of extra work (you can already easily get a user space pointer)

b) It isn't even necessarily valid (the "real" addresses can change - many times - under your feet while you're computing the checksum. Your final "sum" would be pure fantasy!)

There are (as always) exceptions ... and you might indeed have a legitimate requirement in here.

But based on the information you've given ... don't do it! Just compute the user-space checksum from your user-space virtual address range ... and you should be Happy.

IMHO .. PSM

PS:
Otherwise, if I've misunderstood the question, you might be interested in looking at:

a) mmap
... and/or ...
b) the C/C++ "volatile" keyword
... and/or ...
c) Writing your own kernel module to access physical hardware (memory and registers)

Here's a link that might be of interest:
http://www.simtec.co.uk/appnotes/AN0014/

theNbomr · 08-01-2007, 07:25 PM

If this is a typical 8-bit microcontoller or the like, where you are not running under a protected mode OS, simply load up a pointer with the address you want it to point to, and dereference to your heart's content:

Code:

   void * arbitrary_ptr;

   arbitrary_ptr = 0x8123;
   for( i = 0; i < 128; i++ ){
      chksum = calc_checksum( *arbitrary_ptr++, chksum );
   }

---- rod.