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