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RAM for data across a reset

 
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stevech



Joined: 23 Feb 2006
Posts: 656
Posted: 22 September 2006, 2:47 AM    Post subject: RAM for data across a reset Reply with quote
Is there a way to hide some data in RAM so that (as a hack) I can use it after the micro is reset (without loss of power)?

such as, some kludgey way to put data where I (hope/know) that the heap won't go?
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dkinzer
Site Admin


Joined: 03 Sep 2005
Posts: 2493
Location: Portland, OR
Posted: 22 September 2006, 3:10 AM    Post subject: Reply with quote
It would be cleaner to use Persistent Memory or Program Memory.

That said, depending on how you're going to use it, the following technique may work:
Code:
Sub Main()
   Dim addr as UnsignedInteger
   Dim val as UnsignedInteger Based addr   

   addr = Register.RamStart + Register.RamUsed + 100
   val = val + 1
   Debug.Print CStrHex(addr)
   Debug.Print CStr(val)
End Sub

This code relies on the fact that the RAM that is not statically allocated is not initialized (an implementation detail that could change in the future). Note that the stack for Main() begins at Register.RamStart + Register.RamUsed. The offset of 100 puts the address well past what might be used in this simple case.

If you compile and run this code, the value displayed should be incremented on each reset.
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stevech



Joined: 23 Feb 2006
Posts: 656
Posted: 22 September 2006, 5:00 AM    Post subject: Reply with quote
thanks. I'll look at that technique.
I tried using system.alloc to see if that memory was altered by a reset, and it is.

use of persistent memory is a concern because I'd have to re-write it too frequently.

One issue is the ATN signal. I need to arrange a terminal program that doesn't cause DTR to toggle when the COM port is opened. Seems like Windows wants to do so. This of course causes the ZX24 to reset each time a terminal program opens the serial port.

Also, the debug window in the ZBasic IDE always toggles DTR when the IDE opens the port.
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