Naturally, a lot of people won't understand what these opcodes are for.





In this tutorial, it is assumed you already know what those do. If not, it's best to read this tutorial first.

Lets start with the easiest, which I believe to be "NOT".




Lets say we had variable 0@ containing the value 12. In binary, this is:


Which if we were to display all 32 bits (since any bits which are off before the first bit which is on is not shown), would look like this:


That is, bit 3 (4) and 4 (8) are on (4+8=12 - again, refer to the other tutorial if you are confused). NOT simply inverts these bits, so this:


Would cause 0@ to contain the value 18446744073709551603... Oh wow, that explains everything, right?! Nah, but the binary representation does:


All NOT does is inverts the bits. The bits that are 0 will be changed to 1, the bits that are 1 will be changed to 0. Here are some more examples...






Moving swiftly on to the next simple operation.. SHL stands for "Shift To Left" (wheras NOT actually means "not")... This shifts all the bits to the left *shock*.

Lets do the example thing again...


Wow.. Something amazing happened there! Each time we shifted the bits to the left, the decimal value doubled! Now we can multiply by 2... which is a revolation that brings something we could never do before!!! Well, this is useful nontheless.. and is actually faster for a PC to do than multiplication (no faster than addition, though).

You could do SHL over and over to keep doubling.. But a more practical use nowadays would actually be to control the bits for optimisation purposes.




Even though it should be pretty obvious what this is after reading about SHL, I'll confirm it. SHR stands for "Shift To Right"... It shifts bits to the right, halves the number etc... No new concept here, really.. Just read the SHL part and think "SHR is the opposite!"




This works similar to NOT in ways. It has the capability of toggling the bits, but it takes an extra parameter:


This is where the bitwise operations start getting a bit trickier... To use them, you might want to open Windows Calculator and set it to "Scientific". If you don't have Windows Calculator, screw you.. Windows is great

The 3rd parameter of XOR is not a bit number.. it's a value containing bits (as all values do). So, a value of 255 (or 0xFF) would be the (hexa)decimal representation of 11111111. A value of 5 (or 0x5) would be the (hexa)decimal representation of 101.

Taking these numbers as they are and running them through XOR as so:


0@ would contain 250, or 0b11111010. This should explain things better:


Where the binary of the value passed to XOR was 1, the bit was switched. Lets see a few more examples:


So, it's simple enough to say "XOR switches specified bits", but hopefully you've also learned how hex/dec representations of binary are used in the rest of these bitwise operators... You'll want to open up your calculator, insert the dec or hex representation and select the radio button labeled "Bin" in Windows Calculator to see the binary and vice-versa.




Now it gets even tougher! But we've covered a lot, so I'll trust you can pick this up, too. AND is actually used primarily for clearing bits... Except we can clear more than one bit at a time, thus being more optimised than what GTA already had to offer.

The following two snippets do the same thing. Lets see if you can figure out how!



0x90 is the hex representation of 01101111. That is, the 5th and 8th bits aren't set... Unlike with XOR, in an AND operation, the bits that aren't set are used to manipulate the value.


Another way to say it is with the word. AND sets the bit in the result if the first value AND the second value have been set... Examples:


So easy, you could write it without a calculator!




OR is similar to AND, but is primarily used for setting bits. Lets go for that SCM comparison again:


No matter what the values passed to OR are, as long as one of them has a bit set, the bit will be set in the result of the operation.

OR sets the bit in the result if the first value OR the second value have been set... Examples:



Really.. REALLY easy, right? Yeah, thought so...



Ohh wait, there's one more! It's not actually a bitwise operator, it's something much greater (by greater, I mean the form of "greater" which is kinda related to "bigger", not that this is actually much of a thing to be excited by).




MOD does something completely different to the rest of the opcodes. It performs a "Modular" operation, which is a fancy way of saying "it gets the remainder of a division". Arghhh! Division!!!

No more bit examples. Simply put, if you entered:


The result would be 1. The result of a division would be 4... and 4+1 equals.. Guess what? 5!


Something I just discovered is that that MOD can represent the math of a 12-hour clock...


The result would be 4.


The result for:


Would be 0, because 18 divides by 2 perfectly.


And just one more..


Would be 3.


Now I'm just wasting time. Hopefully you got something out of this tutorial.. or it's part of my life wasted.

I didn't even know those existed lol
Well thanks ^^ But..

HUMAN ERROR!



Shouldn't it be 11010111? Well its sort of hard to verify those anyway lol

Actually, it should be 10010010, updated.

I think, that "MOD" operation will be useful also in those situations (changing "on"-"off")

Sorry this revivement in the topic...

But, I need to invert a string, so I need to invert the bytes...

I Think that this functions can help-me (or not).

Example:
CAR --> RAC
01000011 01000001 01010010 -> 01010010 01000001 01000011

One of these functions can perform this? Maybe the SHL\SHR?

This post has been edited by LINK2012: Oct 17 2011, 02:16 PM

No, these functions can't help you really here, I think. You have to do it manually.

Change state, on/off (1/0):


You have 4 active bytes, extract only one smallest byte:


This post has been edited by fastman92: Oct 17 2011, 06:36 PM

Cause i don't understand anything in it i've opened Windows Calculator as you've suggested...
Hex representation of 01101111 would be 6F (from left to right as it's written. or shoud i type it in from right to left? then it would be F6)... 0x90 in binary format would be exact opposite binaries: 10010000
is it just a "human error" or i don't even know how to use a windows calc?

Binary digits have to be written from right to left like integer ones:

So? There is an error in the main post? Just pointing it to understand it myself and to not confuse people who might look for a good description of that binary thing in the future...

Sure, he was wrong. It's likely he meant making the bitwise-AND of 0x90 (10010000) with 0xFF (11111111) will still produce 0x90 (10010000), even after swapping the 2 parameters. Doing the bitwise-XOR gives 0x6F (01101111).

This post has been edited by Wesser: Oct 18 2011, 01:42 PM