While working with some memory-mapped I/O, I came across some behavior which may be construed as a bug. I declared my memory-mapped stuff as an unsized array of char's in xdata. C51 produced no errors, but the array was positioned at address 0x0000. The linker then reported a L107 error due to overlap with the normal xdata variables. I fixed the problem by specifying a size to the array, but this smells like the wrong behavior to me. Am I all wet? Remove the "//" on line 5 to fix the bug.
/* Keil v6.14 memory space bug(?) demonstration */ #define NUM // 1 char xdata test1[NUM] _at_ 0x1E00; char xdata test2[NUM] _at_ 0x1F00; char xdata test3; void main(void) { char x; x = test1[0x13]; x = test2[0x25]; x = test3; }
Note that you can explicitly specify both the location of the pointer, and the memory space which it addresses; eg,
char data * xdata str; /* ptr in xdata to data char */ int xdata * data numtab; /* ptr in data to xdata int */ long code * idata powtab; /* ptr in idata to code long */