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negative value

Posted on 2011-02-17
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Last Modified: 2012-05-11


static int reg, data_varde_hex[10000];
static char data_reg_hex[10000][5]

data_varde_hex[reg] = strtol(data_reg_hex[reg], 0, 16);            

//So far it works fine, but if data_reg_hex are FFEC a expect a negative value,
//how can i do to get a negative value.
//
// Thanks
//
// Ludde72

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Question by:Ludde72
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5 Comments
 
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Expert Comment

by:imaki06
ID: 34915359
Try short instead of the int, for the data_varde_hex
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phoffric earned 2000 total points
ID: 34915376
This gives -20
static int data_varde_hex;
   static short shortVal;
   static char data_reg_hex[5] = "FFEC";

   data_varde_hex = strtol(data_reg_hex, 0, 16);  
   shortVal = (short)strtol(data_reg_hex, 0, 16);

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0
 
LVL 46

Expert Comment

by:Kent Olsen
ID: 34916002
Hi ludde,

The value 0xFFEC is not a negative value.  It is the bit stream 1111111111101100.

That value becomes negative when the hardware recognizes that the leftmost (high-order) bit is a 1.

Because the value is exactly 16 bits, placing it in a signed, 16-bit integer variable is sufficient for C to treat it as negative.  Placing it in a signed, 8-bit (char) variable will also work because the top 9 bits are 1.

But putting it in a 32-bit or 64-bit will not because C doesn't automatically sign extend.  The top be will not be carried to the left through the sign bit.  And C makes no rules on the length of short, int, long, and long long variables (except with relation to each other).

Most compilers today treat an *int* as 32 bits.  To ensure that a negative value is stored, you'll need to force a sign extended value.  And because you know that the value is 16 bits it is easy to do.

  int Value.
  unsigned int X = 0xFFEC;    // Can be any value, including the sample 16-bit value

  if (X >> 15)    // Check for the top bit
  {
    Value = -1;                       // Sets all bits in the variable;
    Value ^= (X ^ 0xFFFF);    // Merge in the value of X, sign extended
  }
  else
    Value = X;


There are certainly other ways to sign extend an integer, but this is pretty straight forward.


Good Luck,
Kent
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LVL 32

Expert Comment

by:phoffric
ID: 34916083
Here is another way to get -20 into an int.
static int data_varde_hex;
   static char data_reg_hex[5] = "FFEC";
   data_varde_hex = (short)strtol(data_reg_hex, 0, 16);  // now this give -20 also

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LVL 84

Expert Comment

by:ozo
ID: 34916245
(X ^ 0x8000) - 0x8000
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