C语言的RAM校验和

Question

I need to check the RAM of a MCU at startup using a checkerboard-like algorithm. I do not want to lose whatever data is already in RAM, and also i do not know how not to affect the variables im using to perform this algorithm. I was thinking something like:

for (position=0; position< 4096; position++)
 {
     *Temporal = 0x5555;
     if(*Temporal != 0x5555) Error = TRUE;
     *Temporal  = 0xAAAA;
     if(*Temporal != 0xAAAA) Error= TRUE;

    Temporal +=1;

 }

should i modify the linker to know where Temporal and Error are being placed?

Answer 1

Use the modifier "register" when declaring your pointers (as in "register int *"). Registers are a special segment of memory inside the processor core that (usually) does not count as part of RAM, so any changes to them don't count as RAM read/writes. Some exceptions exist; for instance, in AVR microcontroles, the first 32 bytes of RAM are "faked" into registers.

Your question probably got a downvote for the lack of clarity, and the question about retaining whatever was in RAM being easily solved by most beginner C programmers (just copy the contents of the pointer into a temp variable before testing it, and copying back after the end of the test). Also, you're not performing a Checksum: you're just making a Memory Test. These are very different, separate things.

Answer 2

You need to ensure the memory that you are testing is in a different location than the memory containing the program you are running. You may be able to do this by running directly out of flash, or whatever other permanent storage you have connected. If not, you need to do something with the link map to ensure proper memory segmentation.

Inside the test function, ussing register as MVittiS suggests is a good idea. Another alternate is to use a global variable mapped to a different segment than the one under test.

You may want to read this article about memory testing to understand the limitations of the test you propose, how memory can fail, and what you should be testing for.

Answer 3

I don't know (not enough details) but your memory test may be a cache test and might not test any RAM at all.

Your memory test (if it does test memory) is also very badly designed. For example, you could chop (or short out) all of the address lines and all data lines except for the least significant 2 data lines, and even though there'd be many extremely serious faults the test will still pass.

My advice would be to read something like this web page about memory testing, just to get some ideas and background information: http://www.esacademy.com/en/library/technical-articles-and-documents/miscellaneous/software-based-memory-testing.html

In general, for non-destructive tests you'd copy whatever you want to keep somewhere else, then do the test, then copy the data back. It's very important that "somewhere else" is tested first (you don't want to copy everything into faulty RAM and then copy it back).

I would also recommend using assembly (instead of C) to ensure no unwanted memory accesses are made; including your stack.

If your code is in RAM that needs to be tested then you will probably need 2 copies of your RAM testing code. You'd use the second copy of your code when you're testing the RAM that contains the first copy of your code. If your code is in ROM then it's much easier (but you still need to worry about your stack).