Black Bag Presents. Cracking Tutorial 001 Machine Language

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*************************************** * * * Black Bag Presents... * * * * * * Cracking Tutorial 001 * * * * Machine Language * * * * * * Written By: * * The Intern of Black Bag * * Rev. 1.0 * * * *************************************** If you desire to do a bit of cracking, it would be a good idea to review (or learn) machine language. You probably have noticed most games are marked with the "B" file type when you "CATALOG" your disk, telling you that they are written in machine language. You may wonder why would some poor masochistic programmer write a game entirely in some bizzare language of ones and zeros. Well, machine language is quick -- very quick. Let's take a look at a sample program for Hi-Res graphics. We can type it in in a minute, but look to see what you're getting into: Basic ----- 10 HGR2 : HCOLOR=3 20 FOR X=0 TO 279 30 FOR Y=0 TO 191 40 HPLOT X,Y 50 NEXT Y 60 NEXT X 70 END Machine Language ---------------- 10 HGR2 : HCOLOR=3 20 HPLOT 0,0 30 CALL 62454 40 END Wait! Before you demand a refund, I realize they both look like BASIC, but the second uses a machine language routine built into your apple. It is easier to understand than a whole lot of machine language garbage. Just notice the difference in speed. Although "CALL" is a BASIC command, it is a bridge to machine language. It switches control over to machine language. Before we go any further, let's talk about what machine language is. First, there are different names for machine language. You probably used them interchangably as: Binary Machine Language Assembly Luckily, you don't have to worry much about binary. Binary is mainly for machine use only. Binary is the most primitive form and your Apple works mostly in binary because it is based on digital electronics. In digital electronics, you can have either a one or a zero. The one signifies power (voltage), the zero signifies no power. There's not enough room here to go into a deep explanation of a binary logic tree, but I can provide a short summary. A circuit in your computer will check things and receive either a one or a zero. The circuit will travel along a road. When it comes to a fork in the road, it will take the high road if it received a one, but it will take the low road if it has a zero. At the end of the journey, a specific task would have been completed. Each one or zero is a binary digit (or "bit" for short). Obviously, your 6502 can't complete every task if it uses a bit every time it comes to a junction. That is why the 6502 is referred to as an "EIGHT BIT" micro- processorr. In other words, it will take eight bits arranged in a predetermined manner for your 6502 to complete a specific task. Four bits in a row make a nybble and two nybbles (or eight bits) make a byte. One byte can be represented as a decimal number from 0 to 255. The bits in a byte are identified by thier location. In other words, a byte has 8 bits known as bit 0 through bit 7. The right-most bit is known as bit 0. To summarize: BITS: 1 1 1 1 1 1 1 1 ! ! ! ! ! ! ! ! ! !BIT!BIT!BIT!BIT!BIT!BIT!BIT!BIT! ! 7 ! 6 ! 5 ! 4 ! 3 ! 2 ! 1 ! 0 ! +---+---+---+---+---+---+---+---+ ! ! ! ! UPPER NYBBLE ! LOWER NYBBLE ! ! ! ! +---------------+---------------+ ! ! ! ENTIRE BYTE ! ! ! +-------------------------------+ Before we continue, Let's touch up a bit on your knowledge of base 2 math. Here's an example of the addition of two binary numbers: 1 1 01 01 01 + 01 ->>> + 01 ->>> + 01 ----- ----- ----- 0 10 (Step 1) (Step 2) (Step 3) :> Step 1: Start to add. :> Step 2: Add first column, there's no "2" in the binary number system, so you must carry the overflow. :> Step 3: Continue to add. Well, you may be confused. It is difficult for many people to convert a binary number into our number system, base 10 or decimal. It's really quite simple if you can visualize that each place in a binary number represents a power of two. If you had a byte like 10000000 (bie 7 = 1), you have the equivilent of two raised to the power of seven. The easiest way to demonstrate this is to use a conversion chart, such as this one: +-------------------------------------+ ! BINARY CONVERSION CHART ! +-----+---+---+---+---+---+---+---+---+ !BIT !BIT!BIT!BIT!BIT!BIT!BIT!BIT!BIT! !PLACE! 7 ! 6 ! 5 ! 4 ! 3 ! 2 ! 1 ! 0 ! +-----+---+---+---+---+---+---+---+---+ ! 2'S ! ! ! ! ! ! ! ! ! ! ! 7 ! 6 ! 5 ! 4 ! 3 ! 2 ! 1 ! 0 ! !POWER! ! ! ! ! ! ! ! ! +-----+---+---+---+---+---+---+---+---+ !DEC. !1 !6 !3 !1 ! ! ! ! ! ! ! 2 ! 4 ! 2 ! 6 ! 8 ! 4 ! 2 ! 1 ! !EQUIV! 8! ! ! ! ! ! ! ! +-----+---+---+---+---+---+---+---+---+ EXAMPLE: -------- 10001000 = 2^7+2^3 = 1*128+1*8 = 136 BINARY POWER OF MULTIPLY DECIMAL TWO Machine language is a general term generally meaning a low level language which is heavily dependant on the machine. For example, machine language on an IBM is different than the machine language on an Apple. Machine language is a very broad area. There are several different levels. Organized from lnwest to highest, they are: Translated Interactive Mini-Assembler Macro-Assembler The first refers to poking your program in from another language. This often used from within BASIC. The reason this is used is to make a program "self-contained," or to extend the limit of a machine. On a Vic-20, machine language is not supported. In order to do any machine language on a Vic-20, without purchasing an expansion module, you would have to "POKE" your program into memory from BASIC and then "CALL" it. The second, the interactive mode, usually is accomplished by a "monitor." This is not a CRT, it is a small program which contains most of the routines for running your machine (keyboard input, character output, sound, and other things) and also contains a small command processor which allows you to write machine language programs and list them. When you type "CALL-151" on an Apple, you enter the "monitor" and can begin coding programs. The major drawback of the interactive and the translated method, is that you must know the numbers which correspond to commands. Machine language is much harder than BASIC simply because line numbers, variables, data, and commands are all represented by the same kind of numbers. It is very similar to Chinese. In Chinese, the same word may be an obscenity or a complement. A mini-assembler allows you to write machine language using words for commands rather than numbers. They can use line numbers (actually memory locations) such as the "mini-assembler" located within Integer BASIC, or "labels." A label allows you to name a subroutine and not have to worry about mathematically calling it. In BASIC, it would be similar to typing "GOSUB HOUSEKEEPING"" instead of "GOSUB 100." Most mini-assemblers also allow the use of variables or a name for a specific memory location that you will use for storage. When you use Apple's built-in monitor, you must always know the actual location, or a number, that you will use as a variable. Thus, mini-assemblers allow you to write programs very quickly because you need not memorize a bunch of numbers and sequences. A Macro-Assembler is the best. In BASIC, most of us have at least one typical subroutine that we use over and over again even for different programs. Each time we must re-enter it into memory. In a macro assembler, we can write this subroutine once, name it, and save it. Then, when we write our program, we need only to call it by name, and the machine will automatically insert your subroutine. Usually macro assemblers have psuedo-code options, which allow you to control the translation of text to the actual program. They can control exactly what microprocessor is installed, or how much memory is available. The last two are the easiest to use, you would simply write your machine language program in a text editor and compile them, or "assemble" them. They are very effecient because if you need to insert 1 or 100 lines of code, they will automatically refigure the program just by compiling the program again. The first two are good for short and quick applications. When you just want to test a small subroutine, or impress a your computer teacher. It is best to begin he hard way, with the translated and interactive methods, because all four forms are bound by the same rules, and any mistakes you my have made will be easy to see and even easier to change. But, if you want to begin, you must know a whole new counting system. The hexadecimal counting system, or base 16. Hexadecimal breaks up a byte into two characters. One character represents the upper nybble, the other represents the lower hybble. There is a problem though, decimal has only 10 symbols, "0" to "9", and hexadecimal system needs 16 distinct characters. To solve the problem, the symbols "A" through "F" are used to represent 10 to 15. +------------------------+ ! ! ! ! ! NYBBLE ! DECIMAL ! HEX ! ! ! ! ! +--------+---------+-----! ! ! ! ! ! 0000 ! 0 ! 0 ! ! 0001 ! 1 ! 1 ! ! 0010 ! 2 ! 2 ! ! 0011 ! 3 ! 3 ! ! 0100 ! 4 ! 4 ! ! 0101 ! 5 ! 5 ! ! 0110 ! 6 ! 6 ! ! 0111 ! 7 ! 7 ! ! 1000 ! 8 ! 8 ! ! 1001 ! 9 ! 9 ! ! 1010 ! 10 ! A ! ! 1011 ! 11 ! B ! ! 1100 ! 12 ! C ! ! 1101 ! 13 ! D ! ! 1110 ! 14 ! E ! ! 1111 ! 15 ! F ! ! ! ! ! +--------+---------+-----+ The conversion of a number to hex just takes practice, but becomes relatively easy if you take a number and break it down to the binary level, where conversion to hex is as easy as pie. Just as nybbles may be organized into groups of two, so may bytes. Two bytes together usually represent a number. The number may be expressed in two different ways. Let us use the example of the number FF00: A Two Byte Number ----------------- F F 0 0 = 0 0 F F \ / \ / \ / \ / ! ! ! ! Hi Lo Lo Hi Byte Byte When the two bytes are written together, the high byte is written first. When the two bytes are seperated, the low byte is written first. This is one difference you will notice between the different levels of machine language. An assembler will translate the standard format of two bytes written together to the machine format of "lo byte first." You can blame the designers of computers for the strange splitting of numbers, but it really makes much more sense to the computer in the lo-hi format. One term you should know is "word." The word is simply two bytes. The numbers in the paragraph above are one word long. Sixteen bit computers look at memory in increments of words, while 8 bit computers look at memory in increments of bytes. To distinguish between memory locations, values, and the different forms of number systems, some symbols are used in addition to numbers: % = Binary Location Value #% = Binary Number = Decimal Location (no symbol) # = Decimal Number $ = Hexadecimal Location Value #$ = hexadecimal Number Following the symbols would be the digits. Thus, the hexadecimal location 10 would be written $10. The binary number 0110 would be written #%0110. Again, this does not pertain to the monitor or interactive forms of writing machine language, only the forms using an assembler. In an attempt to keep things simple, I have ommited the symbol notations. This is still not true machine language, since that is a nazt. If you did not understand much of the tutorial, do not worry. For now, just understand the hexadecimal numbering system. For the next set of docs, try to find a copy of the "DOS Toolkit," and look over the following examples. Until then, good luck! The Intern of Black Bag --------------------------------------- Hex Decimal Binary FF = 255 = 1111 1111 D0 = 208 = 1101 0000 B7 = 183 = 1011 0111 A0 = 160 = 1010 0000 74 = 116 = 0111 0100 37 = 55 = 0011 0111 13 = 19 = 0000 1101 --------------------------------------

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