prelab1

prelab1 - ECE 194D Prelab 1 due: 5pm 3/31/11, HFH 3120...

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Binary Number Representations and You As you begin your ECE 194D labs, you will encounter strange new data types in your Simulink models. Where did these uint8's and int32's come from, and why can't you just use doubles for everything anymore? For that matter, what is a double? And why should you care about these persnickety low level distinctions anyway? Numbers are numbers, right? Well, young student, prepare to have your interest piqued. The software we're using for the ECE 194D labs has the potential to be incredibly frustrating if you do not understand how to use these different data types. Why? When you design a Simulink model to control your robot, that block diagram gets converted into a computer programming language called C before being compiled and downloaded into your robot. In the past, when you've used Matlab or Simulink, you've probably never had to worry about variable types. Everything was handled implicitly in the background, transparently. However, C handles nothing transparently, nor in the background. Since your Simulink block diagram is getting converted into C, you need to do some hand holding to make everything work. This document will help you understand how to do this. A Brief Primer on Binary Numbers We usually represent numbers by writing out their digits. For various historical and anatomical reasons, most humans prefer base ten (decimal) numbers. This means we use ten distinct symbols (or digits), strung together with additional helper symbols like decimal points and minus signs, to represent numbers. For example: 3, 192.4, -10.2 are all base 10 numbers. Computers, however, prefer base 2 (binary) numbers. Here, the numbers are represented as a sequence of 1's and 0's. Lets say you have room to store 3 of these bi nary digi ts (bits). There are 2 3 = 8 different combinations of these three bits, listed below. Bit 2 Bit 1 Bit 0 0 0 0 0 0 1 0 1 0 0 1 1 1 0 0 1 0 1 1 1 0 1 1 1 Table 1: Every possible combination of 3 bits Lets say we want to use these three bits to represent the integers from 0 to 7. We need to come up with a one-to-one mapping from the 8 different combinations of 3 bits to the 8 integer values we're interested in. There are a huge number of possible ways 1 to choose this map, but in practice only a few are used because they have properties that make it straightforward to do binary arithmetic. 1 For n bits, there are ( 2 n ) ! distinct mappings. For n=3, this is 40,320. For n=32, you don't want to know. ECE 194D Prelab 1 due: 5pm 3/31/11, HFH 3120 (dropbox) Page 1 of 9
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Unsigned and Signed Types In the previous section, we talked about mapping a sequence of n bits to a set of 2 n integer values. I mentioned that only a few maps are commonly used. We'll talk about two of these mappings in this section. Unsigned Binary Numbers
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prelab1 - ECE 194D Prelab 1 due: 5pm 3/31/11, HFH 3120...

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