Sign extension problems...

[Digsy]

hi guys,

I'm trying to form 4 byte values into a long value (i.e. (hex values) bytes FC 24 23 67 becomes long value FC242367) and i keep getting overflow errors which i believe are due to sign extension problems.

Here is a bit of demo code i have been using to form XX000000 (where XX is the byte value)
----
Dim MyByte As Byte
Dim MyLong As Long

MyByte = CByte("&H7F&quot ' Give byte value of '7F'
MyLong = MyByte * CLng(&H1000000) ' Want MyByte to be MS byte in long
MsgBox Hex(MyLong) ' Gives output of '7F000000'
MyByte = CByte("&H80&quot ' Now give byte value of '80'
MyLong = MyByte * CLng(&H1000000) ' Want MyByte to be MS byte in long - get overflow here
MsgBox Hex(MyLong)
----

I have actually solved the problem by converting the byte into a string and concatanating the 6 trailing 0's on but i'm sure there must be a better way.

Any ideas?

Dave

 

[johnwm]

If you have the four bytes converted to 4 strings, just concatenate the strings:
strTotal = strA & strB & strC & strD

then
msgbox(val("&H" & strTotal))

should give you your long as a hex string

You'll need to error check, of course
________________________________________________________________
If you want to get the best response to a question, please check out FAQ222-2244 first

'People who live in windowed environments shouldn't cast pointers.'

 

[johnwm]

Sorry, should read:

msgbox hex(val("&H" & strTotal))

(I'd edited the 'hex' out to prove it gave a number)
________________________________________________________________
If you want to get the best response to a question, please check out FAQ222-2244 first

'People who live in windowed environments shouldn't cast pointers.'

 

[Digsy]

Hi johnwm,

I have actually already done that but i felt that the solution wasnt particularily 'elegant' and that there must be a better way to do it....maybe there isn't.

Dave

 

[johnwm]

The problem you have, if you do it with numbers rather than strings, is that VB assumes signed Ints and Longs, so if the MSB becomes 1 then the answer goes negative (1's complement)

That's why I do it the (alright, inelegant) string method.
________________________________________________________________
If you want to get the best response to a question, please check out FAQ222-2244 first

'People who live in windowed environments shouldn't cast pointers.'