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arga
Joined: 09 Sep 2003
Posts: 22
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| Pointer Address |
 Posted: Tue Jan 13, 2004 9:47 pm |
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How do you ABSOLUTELY make sure that the pointer holds the correct address if it is pointing to an array.
Sample of this code below presents a problem with addressing. The pointers seems to be confused even though the pointer is initialised by calling it inside main().
Assume that the array aX is allocated in BOTH Bank 1 and Bank 2. Therefore, the address is 14 bits wide but i seems the pointer SOMETIMES only copies the first 8 bits.
//declarations
int aX[60];
int *pA;
void initPointer()
{
pA = aX;
}
main()
{
initPointer();
//more code here to handle timer, processing of ata in aX, so, so...
}
Any help? Or should I just AVOID using pointers at all times. |
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PCM programmer
Joined: 06 Sep 2003
Posts: 21708
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| Posted: Wed Jan 14, 2004 1:20 am |
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Is this a theoretical problem, or is this a problem that you have
definitely found ?
What PIC are you using ?
What is the version of the compiler ?
Because you mentioned a 14-bit address, I'll assume that you
are using the PCM compiler. I'll assume the PIC is a 16F877.
| Quote: | Assume that the array aX is allocated in BOTH Bank 1 and Bank 2
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The compiler will never do this. Each array must not exceed
the size of a RAM bank.
Remember that in CCS, an "int" is only 8-bits. Therefore, this
array only uses 60 bytes. Since a RAM bank on a 16F877 is at least
80 bytes, the array will fit within one RAM bank. If it doesn't fit, the
compiler will give you an error messsage.
| Quote: | | Therefore, the address is 14 bits wide but it seems the pointer SOMETIMES only copies the first 8 bits. |
If the compiler is using 8-bit pointers, it will only copy the lower 8-bits.
To enable 16-bit pointers, you need to do this:
| Code: | #include <16F877.H>
#device *=16 // This line enables 16-bit pointers |
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arga
Joined: 09 Sep 2003
Posts: 22
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| Posted: Wed Jan 14, 2004 3:30 am |
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The problem I have is that the array is in bank 1.
The array is type int, and so is the pointer.
#device *=16 is included.
I'm using the pic16f877, compiler ver. 3.173 |
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Ttelmah
Guest
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| Posted: Wed Jan 14, 2004 8:19 am |
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| arga wrote: | The problem I have is that the array is in bank 1.
The array is type int, and so is the pointer.
#device *=16 is included.
I'm using the pic16f877, compiler ver. 3.173 |
I hope you mean that the pointer, is a pointer to an int, not an 'int' as such. Remember that an 'int', will only hold an 8bit value on a PIC, and an address (with 16bit RAM addressing), needs a 16bit storage location.
I have run into problems with the compiler truncating pointers in the past, but have not seen this on the more recent versions. When I ran into this, I found that I could 'bodeg' round the problem, by using a forced cast. I found that if you specifically tell the compiler to treat the pointers as being to 'long' types, it prevented the behaviour.
However this fault was definately fixed a long time ago.
Best Wishes |
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Mark
Joined: 07 Sep 2003
Posts: 2838
Location: Atlanta, GA
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| Posted: Wed Jan 14, 2004 8:21 am |
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| So what is the problem? The upper byte shouldn't need to be changed if the array is in a bank. It would just be wasted instructions. Are you having a problem with the address being incorrect? |
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