[Gambas-user] Problem with Byte datatype ?

[Doriano Blengino]

Paul Horechuk ha scritto:
> On January 29, 2009, Dominique SIMONART wrote:
>   
>> Hi evererybody,
>>
>> If I code the few lines showed below:
>>
>> DIM X AS Byte
>> DIM Texte AS String[64]
>>
>>   X = &h2E
>>   X += &hE0
>>   TextBox1.Text = Hex(X)
>>   Texte[X] = Hex(X)
>>
>> I get '10E' displayed. Is this correct for a byte?
>>     
> I think the 10E indicates an overflow (the 1 part) and thus an error, as it 
> won't fit in the 00 to FF range. The 0E would be the remainder. A question 
> would be if you wanted signed or unsigned math. With E0 as signed 
> (11100000) this is also -1F (00011111) (ones complement). Adding the 2 
> numbers would be 2E - 1F = 0F or something similar.
>
> I think there is actually a twos complement process that should be followed 
> instead of the ones complement, but I think you get the picture.
>
> Basically, byte is the wrong datatype for this.
>
>   
I think there is nothing wrong in the code; perhaps something wrong is 
in gambas.
It is common practice to use integers to do this exact kind of things. 
By definition a byte is unsigned but, even if it was signed, an overflow 
in addition should lead to one of two things: a silent loss of data 
(overflow), or an exception/runtime error or whatever. A byte cannot 
hold values larger than 255, so the value 0x10E is unreasonable - 
apparently, gambas bytes are not really bytes...


>> If you put a breakpoint on the TextBox1 line, the local variables show X
>> value as 14 (&h0E) which is correct.
>> and the last line ends wit an 'out of bounds' message.
>>
>> (as a workaround, I actually insert a' X = X MOD 256' instruction just
>> before the last line)
>>
>> If you wonder why the hell I've to use bytes, I'm working on a memory
>> capture of an Amiga program ("Captive") running inside the emulator
>> WinUAE. In those old days (twenty years ago), memory was 0.0005 Gb and
>> byte usage was quite common!
>> and more, the memory byte order was not compatible with the actual PC so
>> you have to read memory byte by byte to restore the Amiga organisation
>>
>> regards,
>> Dominique Simonart
>>
>>
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>
>
>
>   


-- 
Doriano Blengino

"Listen twice before you speak.
This is why we have two ears, but only one mouth."