written by Teresa Carrigan
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WHAT IS IT?This model demonstrates counting in binary, and gives the decimal equivalent of the bit pattern for various data representations. This demonstration will help you understand integer overflow and the problems it can cause when undetected. It can also ask you questions, to see how well you understand the concepts.
HOW IT WORKSEach time the binary number is incremented, the red arrow passes to the left. When it finds a 1, it changes the 1 to a 0 but continues to the left. When it finds a 0, it changes the 0 to a 1 and returns to the start position.
Each patch holding either a 0 or a 1 has two variables. The place-value is the decimal amount that a 1 in that position would have. The right-most place value is 1, the next is 2, the third is 4, etc. with each place-value doubling the one on its immediate right. The current-value of a patch will equal the place-value if the patch shows a 1, and equal zero otherwise.
If a bit pattern is stored using unsigned binary, then the decimal equivalent is the sum of all the current-values of the patches. For the other three representations, if the left-most bit is zero then the decimal equivalent is the same as if it were unsigned binary. When the left-most bit is one, then the decimal equivalent will be negative, and each representation gives a different result.
Assuming the left-most bit is one, the signed magnitude equivalent is calculated by adding the current-values of all the patches except the left-most one, and then making that sum negative.
If the left-most bit is one, then the one's complement equivalent is calculated by adding the current-values of all the patches (including the left-most one), and then subtracting the sum of all the place-values of the patches.
The two's complement equivalent is always one less (more negative) than the one's complement, when the left-most bit is a one.
Applets on this website were written by Teresa Carrigan in 2004, for use in computer science courses at Blackburn College, with the exception of the Fireworks applet. The applets made with NetLogo require Java 1.4.1 or higher to run. The applets made with NetBeans require Java 1.4.2 or higher to run. Applets might not run on Windows 95 or Mac OS 8 or 9. You may obtain the latest Java plugin from Sun's Java site.