Probing Memory to find size

Why does your function always return 0? I see you are reading a memory location, here and there. But how are you planning to detect the presence or absence of memory at those locations? Do you imagine that simply performing a memory reference is sufficient? If so, how does your code detect a failure?

Beyond that, you might start out attempting 16M first, fall back to a test for 4M second, if that fails. If 4M succeeds, try 8M. If 4M fails, verify the 2M.

Jon

As John pointed out, your code will likely bomb if there happens to be a memory failure (or at least return incorrect results).

In any case, what I would do is a more exhaustive test. That is writing a simple test pattern to the memory and verifying the pattern. It works for me as I did this in my homebrew Z80 sbc's firmware and it detected memory wiring errors 100%.

I am not claiming it is perfect, but it is an improvement over your suggested method.

First of all, use a simple FOR loop to generate your test addresses. Since *your* memory sizes are always a power of 2, start with 16MB and halve your address (ie. shift right by 1) each time your test fails.

for (addr=(16=1) { if (test_succeeds_just_below_this_address) break; } printk ("%d MB\n", addr>>20);

(I'm assuming your C pre-processor can do the arithmetic)

A few caveats with memory tests like this...

1. Make sure you don't have address aliasing - i.e. the addresses don't wrap at some limit and writing to a high address actually writes to a lower address.
2. In some cases you can write to 'nothing' and pre-charge the bus so that reading from 'nothing' returns the same result! You should also exercise all byte lanes and read/write 32-bit values. For a *simple* test, write different values to two sucessive addresses (eg. 16MB-4 &

And a tip on 'prettiness' - never use what I scientifically refer to as 'yucky numbers' - especially is there are a bunch of them - make the C pre-processor do the work for you!

Instead of ...

Use: #define MB(x) ((x)