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Compiler fault? (or am I missing something?)

Paul Blackmore

Using the latest Keil C compiler (V7.00)
Executing on an Infineon XC161CJ-16F
This code appears to produce the wrong result.

At the "if" statement I expect v1 to equal v3 and for the main function to return 0 (I don't actually want to use the return value, its just a mock up piece of code). However, at the "if" statement, the value of v1 is 0x1234 and the value of v3 is 0xB1234.

Can anyone explain why?

typedef unsigned long ULONG;
typedef unsigned short USHORT;
typedef BYTE * POINTER;

POINTER v1,v2,v3,v4;
ULONG temp_ptr;
int
main(void)
{
  v1 = (POINTER)0;
  v2 = (POINTER)0x0B1234;
  v1 += (ULONG)v2;        // This assigns 0x1234 to v1, I think it should assign 0xB1234

  v3 = (POINTER)0;
  v4 = (POINTER)0x0B1234;
  temp_ptr = (ULONG)v3;
  v3 = temp_ptr+v4;      // This statement assigns 0xB1234 to v3 correctly.

  if ( v1!=v3 )
    return -1;
  else
    return 0;
}

Parents
  • 0 in reply to Paul Blackmore

    After studying the assembler I realize my original sample code is wrong. It should have been like this:

    typedef unsigned long ULONG;
typedef unsigned short USHORT;
typedef char * POINTER;

POINTER v1,v2,v3,v4;
ULONG temp_ptr;
int
main(void)
{
  v1 = (POINTER)0x0B0000;
  v2 = (POINTER)0x1234;
  v1 += (ULONG)v2;

  v3 = (POINTER)0xB0000;
  v4 = (POINTER)0x1234;
  temp_ptr = (ULONG)v4;
  v3 = v3+temp_ptr;

  if ( v1!=v3 )
    return -1;
  else
    return 0;
}

    In the code above, at no stage am I adding more than 0xFFFF to any huge pointer and it all works as expected. For the longest time in this thread, I did not "get" that I was adding more than 0xFFFF to a huge pointer, firstly because I had thought (incorrectly) I was using XLarge memory model and for a few hours after that, because I was being a little bit thick.

    So that just leaves the final question as to why this original code:

      v3 = (POINTER)0;
  v4 = (POINTER)0x0B1234;
  temp_ptr = (ULONG)v3;
  v3 = temp_ptr+v4;

    produced the expected result. Putting on my "thinking clearly" hat which I had obviously not been wearing before...

    The calculation is adding a pointer (v4) to an integer (temp_ptr) the pointer is already pointing at 0xB1234 so adding an integer of value 0 will result in a pointer of 0xB1234.

    Regards
    Paul

Reply
  • 0 in reply to Paul Blackmore

    After studying the assembler I realize my original sample code is wrong. It should have been like this:

    typedef unsigned long ULONG;
typedef unsigned short USHORT;
typedef char * POINTER;

POINTER v1,v2,v3,v4;
ULONG temp_ptr;
int
main(void)
{
  v1 = (POINTER)0x0B0000;
  v2 = (POINTER)0x1234;
  v1 += (ULONG)v2;

  v3 = (POINTER)0xB0000;
  v4 = (POINTER)0x1234;
  temp_ptr = (ULONG)v4;
  v3 = v3+temp_ptr;

  if ( v1!=v3 )
    return -1;
  else
    return 0;
}

    In the code above, at no stage am I adding more than 0xFFFF to any huge pointer and it all works as expected. For the longest time in this thread, I did not "get" that I was adding more than 0xFFFF to a huge pointer, firstly because I had thought (incorrectly) I was using XLarge memory model and for a few hours after that, because I was being a little bit thick.

    So that just leaves the final question as to why this original code:

      v3 = (POINTER)0;
  v4 = (POINTER)0x0B1234;
  temp_ptr = (ULONG)v3;
  v3 = temp_ptr+v4;

    produced the expected result. Putting on my "thinking clearly" hat which I had obviously not been wearing before...

    The calculation is adding a pointer (v4) to an integer (temp_ptr) the pointer is already pointing at 0xB1234 so adding an integer of value 0 will result in a pointer of 0xB1234.

    Regards
    Paul

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