2's complement conversion. Is this right?

[Ross Ridge]

Bob Greschke  <bob at passcal.nmt.edu> wrote:
>I'm reading 3-byte numbers from a file and they are signed (+8 to 
>-8million).  This seems to work, but I'm not sure it's right.
>
># Convert the 3-characters into a number.
>    Value1, Value2, Value3 = unpack(">BBB", Buffer[s:s+3])
>    Value = (Value1*65536)+(Value2*256)+Value3
>        if Value >= 0x800000:
>            Value -= 0x1000000
>    print Value
>
>For example:
>16682720 = -94496
>
>Should it be Value -= 0x1000001 so that I get -94497, instead?

Your first case is correct, "Value -= 0x1000000".  The value 0xFFFFFFF
should be -1 and 0xFFFFFFF - 0x1000000 == -1.

An alternative way of doing this:

	Value = unpack(">l", Buffer[s:s+3] + "\0")[0] >> 8

					Ross Ridge

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