Intro to Physical Computing Syllabus code, circuits, & construction

More on Variables 

As mentioned in the BX24 programming notes, variables are storage locations in the BX24's memory for changing information. Every variable has a data type. The data type of a variable determines how much memory the BX24 needs for the variable, and how it will use the data stored in the variable. You declare both the name and the Data type of the variable before you use it.
Here are the data types, broken down by memory needed: 



To understand how variables are stored in memory, it's useful to think about what memory is. a computer's memory is basically a matrix of switches, laid out in a regular grid. Each switch represents the smallest unit of memory, a bit (usually, we think in terms of bytes when talking about memory. a byte is simply eight bits). If the switch is on, the bit's value is 1. If it's off, the value is 0. Each bit has an address in the grid. The BX24 has 400 bytes of memory for variable storage. Since a byte is eight bits, that means that there are 8 x 400, or 3200 bits of memory. We can envision a grid that represents that memory like this (this grid is only a part of the whole 3200):

Note: programmers like to start with 0 as the first number. So often, as in these arrays,the first element will be numbered 0 instead of 1.  
Note that this grid is arranged in rows of 8 bits; each row represents a byte of memory in this illustration.
When we declare a variable, the BX24 picks the next available address and sets aside as many bits are needed for the data type we declare. If we declare a byte, for example, it sets aside 8 bits. An integer, 16 bits. a string, one byte (eight bits) for every character of the string. Let's say we made the following variable declarations at the top of our program: dim thisVar as byte dim biggerVar as integer dim anotherVar as byte dim reallyBigVar as long The BX24 might assign memory space for those variables something like this (the bits set aside for each variable are colorcoded):


When you refer to the variable, the BX24 checks to see what's in those bits, and gives them to you.
So if a bit can be only 0 or 1, how do we get values greater than 1? When we count normally, we count in groups of ten. This is because we have ten fingers. So to represent two groups of ten, we write "20", meaning "2 tens and 0 ones". This counting system is called base ten, or decimal notation. Each digit place in base ten represents a power of ten: 100 is 10^{2}, 1000 is 10^{3}, etc. Now, imagine we had only two fingers. We might count in groups of two. We'll call this base two, or binary notation. So two, for which we write "2" in base ten, would be "10" in base two, meaning one group of two and 0 ones. Each digit place in base two represents a power of two: 100 is 2^{2}, or 4 in base ten, 1000 is 2^{3}, or 8 in base ten, and so forth. Any number we represent in decimal notation can be converted into binary notation by simply regrouping it in groups of two. Once we've got the number in binary form, we can store it in computer memory, letting each binary digit fill a bit o memory. So if the variable myVar from above were equal to 238 (in decimal notation), it would be 11101110 in binary notation. The bits in memory used to store myVar would look like this:


There are three notation systems used in the BASIC to represent numbers: binary, decimal, and hexadecimal (base 16).In Hexadecimal notation, the letters A through F represent the decimal numbers 10 through 15. Furthermore, there is a system of notation called ASCII, which stands for American Standard Code for Information Interchange, which represents most alphanumeric characters from the romanized alphabet as number values. We'll deal with ASCII when we get to serial communication. For more, see this online table representing the decimal numbers 0 to 255 in decimal, binary, hexadecimal, and ASCII. While we'll work mostly in decimal notation, here are times when it's more convenient to represent numbers in ms other than base 10.
To represent a number in binary notation on the BX24, use this notation: myVar = bx1010_0011 ' 163 in decimal In hexadecimal, use this notation: myVar = &HA3 ' 163 in decimal Variable scope Variables can be local to a particular subroutine if they are declared in that subroutine. Local variables can't be used by subroutines outside the one that declares them, and the memory space allotted to them is released and the value lost when the subroutine ends. Variables can also be global to a module, in which case they are declared at the beginning of the module, outside all subroutines. Global variables are accessible to all subroutines in a module, and their value is maintained for the duration of the program. Usually you use global variables for values that will need to be kept in memory for future use by other subroutines, and local variables as a "scratch pad" to store values while calculating within a subroutine. Doing Arithmetic With Variables There are certain symbols you'll need to do math in BASIC. They're pretty much the same symbols as you use in other programming languages:


When you're adding, subtracting, multiplying, or dividing values stored in variables, you must make sure that the variables you're operating on are of the same type. For example:
dim someVar as byte dim anotherVar as byte dim smallVar as byte dim bigVar as integer dim yetAnotherVar as integer Sub main() call delay(0.5) ' start program with a halfsecond delay someVar = anotherVar + smallVar ' allowed, because all three ' variables are data type byte somevar = anotherVar * 3 ' allowed, because the BX24 ' interprets 3 as a byte someVar = anotherVar + bigVar ' not allowed, because ' bigVar is an integer, while ' anotherVar and someVar are bytes someVar = anotherVar  568 ' not allowed, because 568 ' is larger than a byte yetAnotherVar = bigvar + 568 ' allowed, because 568 will fit ' in an integer variable yetAnotherVar = bigvar * someVar ' not allowed, because someVar ' is a byte and the other two ' variables are integers end sub There are conversion functions to convert data types. For example: bigVar = cInt(someVar) ' sets bigVar = an integer ' of equal value to someVar's value debug.print cStr(65) ' converts value to ASCII bytes, ' the character "6" and the character "5" See the system library for all the conversion functions. 