Hi list, i'm working with c51v750a. My problem:
int array[ 0] _at_ 0x7600; //xdata unsigned char index = 0; //xdata printf("%d", &array[ index]); printf("%d", &array[(unsigned char)(index - 1)]); //what is wrong? // Result: // 30208 --> 0x7600 (correct) // 30206 --> 0x75FE == array[ -1] incorrect, // expected array[255] @ 0x77FE
I want to create a circular buffer with size of 256 (0-255)integers. The output is LIFO. In my case, the index-counter is decremented. The type of the index counter is unsigned char (0-255). There is no problem when i first decrement the index and then read/write the array. But when i put the result of the calculation (index - 1)
array[(unsigned char)( index - 1)] //THIS IS AN EXAMPLE
"The output is LIFO." Surely, that's a stack - not a circular buffer?
"Surely, that's a stack - not a circular buffer?" It's a mixture of both, but please, don't misinterprete the use of this code. To get the problem, you don't need to know what is the use of the code. I don't want to discuss about storage technics.
There's definitely something odd going on here. Take a look at this:
int xdata array[256] _at_ 0x7600; //xdata unsigned char index = 0; unsigned char idx; idx=(unsigned char)(index - 1); printf("%p\n", &array[(unsigned char)(index - 1)]); //x:75FE printf("%p\n", &array[idx]); //x:75FE printf("%p\n", array+(unsigned char)(index - 1)); //x:77FE printf("%p\n", array+idx); //x:75FE
Sorry, ingore my previous post. It's full of typos. This is hopefully correct:
int xdata array[256] _at_ 0x7600; //xdata unsigned char index = 0; unsigned char idx; idx=(unsigned char)(index - 1); printf("%p\n", &array[(unsigned char)(index - 1)]); //x:75FE printf("%p\n", &array[idx]); //x:77FE printf("%p\n", array+(unsigned char)(index - 1)); //x:75FE printf("%p\n", array+idx); //x:77FE
Stefan, did you compile with c51v750a ? BTW, the software-versions which are displayed in the uV2 are: C-Compiler C51.exe v7.50 Assembler A51.exe v7.10 Linker BL51.exe v5.12 Librarian LIB51.exe v4.24 I'm sure i've updated to C51v750a. I will compile the example code with an older version and post the result here.
"did you compile with c51v750a ?" No, C51 v7.01.
"No, C51 v7.01." ok, then i don't need test it with my older version V7.04 I will contact Keil for informations, thanks.
"I will contact Keil for informations, thanks." Please post the result when you get one.
Today i received a mail from Keil: Mr. Schneebauer wrote: "prinzipiell haben Sie Recht, daß es sich hier um einen Fehler handelt. Normalerweise versucht der Compiler das '-1' in die Konstanten zu verpacken um das Programm besser zu optimieren. Wir haben diesen Fehler nun auch behoben. Da Änderungen an zentralen Funktionen gemacht wurden, muß dieser Compiler noch ausführlich getestet werden bevor wir ihn ausliefern können." Basicly, what he writes is: Yes, its a bug. The bug is fixed and the new compiler-version will be tested. Stefan, thank you for verifying this problem.