Local varible in same address

"I found the 'ch' varible is in the same memory adddress as 'dummy' varible"

This is perfectly normal & correct operation for the Keil C51 compiler.

This is called Overlaying - it is Keil's solution to the problem that the 8051 has virtually no stack to speak of.

Overlaying achieves at build-time what "normal" compilers achieve at run-time by using the stack.

"I found the 'ch' varible is in the same memory adddress as 'dummy' varible"

This is perfectly normal & correct operation for the Keil C51 compiler.

This is called Overlaying - it is Keil's solution to the problem that the 8051 has virtually no stack to speak of.

Overlaying achieves at build-time what "normal" compilers achieve at run-time by using the stack.

That is to say, it is the solution that Keil uses - it is not exclusive to Keil, and they didn't invent it.

More on Overlaying here: http://www.keil.com/forum/19231/

Just spotted this:

void a( U8 val )
{
   U8 ch=0;

   dev[ch].fun1(val); <--this is a function pointer, it will call the b function

   dev[ch].fun2(val); <--Fail here
}

The Overlaying means that you have to be very careful how you use function pointers in Keil C51 - if you use them at all.

You must read this: http://www.keil.com/appnotes/docs/apnt_129.asp
- and follow its directions carefully!

Andrew Neil ,Thanks a lot

but I still don't know how to use the key word [overlay] correctly

I just use " ?PR?A?Test!* " in BL51 Misc to fix this

In my code , Do I need to write Overlay (?PR?A?TEST ~ ?PR?B?TEST )??

Thanks again for your kindly help

Then you're going to have to keep researching & studying documentation until you do!

There's a lot of other information - App Notes, knowledgebase articles, etc - I gave some links in the thread I mentioned earlier.

Alternatively, perhaps you should take a hint that this is not something for which the 8051 is well-suited - and either restructure your application in a way that's more appropriate to the target, or choose a different target that's more appropriate to this kind of thing...