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DarkBASIC Professional Discussion / Bitshift Visual C++ and DB Pro

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mahoon
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Posted: 16th Nov 2009 22:44
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Now, I don't pretend to understand this stuff, hence my request for help, but I'm trying to translate a (not terribly well written, I'm reliably informed) Visual C++ example into something I can use in DBPro

parts of the VC++ code include:

BYTE Byte[40];
Byte[0]=0;

I'm hoping the DB equivalent is:

dim bytes (40) as byte
bytes(0)=0

the next bit of VC++:
ax = Byte[0]+((Byte[1]&0x7f)<<8)

which, I hope, can be written in DBPro as:

ax= bytes(0)+((bytes(1)&&0x7f) << 8)

I wrote this little prog to just test if that syntax would at least compile. It did:

dim bytes (40) as byte

bytes(0)=0x14
bytes(1)=0x20
bytes(15)=0x0a


ax= bytes(0)+((bytes(1)&&0x7f) << 8)

for t=0 to 19
print bytes(t)
next t

print ax

wait key
end

the
bytes(0)=0x14
bytes(1)=0x20
bytes(15)=0x0a

have no significance whatsoever, I was just testing things.
The results of that prog are:

20
32
0
0
0
0
0
0
0
0
0
0
0
0
0
10
0
0
0
0
8212

Can anyone who know these things tell me if that's what I should be seeing? I feel that last number looks a little out of place and suspect it's either the && or the way I've bracketed things...
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IanM
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Location: In my moon base
Posted: 16th Nov 2009 23:51
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From the code you've given, the C and DBPro code are equivalent.

Basically, take array entry 1, mask off the high bit (which has no effect on the number 0x20), shift it left 8 bits (equivalent to multiply by 256, giving a result of 0x2000), then add array entry 0 to it togive a result of 0x2014, which is equivalent to 8212 decimal.

Utility plug-ins collection (updated 22 Oct 2009) and
http://www.matrix1.demon.co.uk
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mahoon
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Posted: 17th Nov 2009 10:00
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Thanks so much for that Ian.

Am only just beginning to understand this bitshift malarky but very pleased that bit o' code is doing what it should.
Thanks again.
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Green Gandalf
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Playing: Malevolence:Sword of Ahkranox, Skyrim, Civ6.
Posted: 17th Nov 2009 19:31
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Quote: "From the code you've given, the C and DBPro code are equivalent."


Are they? I thought the C++ declaration



declared an array with 40 elements numbered from 0 to 39, whereas the DBPro declaration



declared an array with 41 elements numbered from 0 to 40. Of course, the snippets are functionally the same in this case since the 41st element isn't accessed in either version.

Or have I got confused with arrays in HLSL ...
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IanM
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Posted: 17th Nov 2009 20:40
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No, you are correct, but it doesn't make too much difference in any event.

Utility plug-ins collection (updated 22 Oct 2009) and
http://www.matrix1.demon.co.uk
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Green Gandalf
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Posted: 17th Nov 2009 21:04
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Indeed.
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