barcode reader project in asp.net MACRO DEVELOPMENT in Software

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MACRO DEVELOPMENT
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requires just one more instruction for the general case (and one less if the destination is the same as one of the source variables) and requires none of the instructions used to pass data between the temporary registers and the parameters. In this example, an application written that uses this macro instead of the general subroutine always will be more ef cient. An added advantage of a macro over a subroutine is that if the macro is made available and not used, then it will not take up any program memory. If you write a subroutine, it is available within the program, whether it is called or not. One point that may not be terribly obvious is that macros execute during build time and not at run time. This means that macros execute when the source code is assembled and not when it is executing in the PIC microcontroller. If you place variables as macro parameters, the variables addresses will be used and not the contents of the variables themselves. This point is important and trips many people up when they rst start working with macros.
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The Difference Between De nes and Macros
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I consider de nes to be simple macros in their own right because of the way they operate. This perspective is slightly unusual, but I believe that it is the most accurate way to visualize how de nes work and helps you to nd good uses in a variety of different situations for which they may not be considered in the rst place. De nes are often just as exible, if not more so, as macros; along with providing a great deal of exibility, they also can add signi cantly to the ease of application programming and application readability. De nes in most languages are declared using the format
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#De ne Label String
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The Label is a standard alphanumeric text label. What makes de nes special is that when the label is encountered, it is replaced by a string. Unlike a macro text, the de ne string replaces the label that it encounters and not the entire line (or more, depending on the macro). This is a powerful concept and one that can make programming a lot easier. When used in PIC microcontroller assembly language, de nes can be used to simplify the operation setting, resetting hardware bits and ags in your application. For example, if bit 3 of the Flag variable is used to indicate the ready state of the serial port, it could be declared as
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#De ne Ser ag Flag, 3
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to reset the bit in the serial initializing routine instead of using the statement
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bsf Flag,3
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THE DIFFERENCE BETWEEN DEFINES AND MACROS
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which is not very helpful or easy to understand without a reference to how the pins are organized. In comparison, the same instruction with the Ser ag de ne, i.e.,
bsf Ser ag
is much easier to understand without the aid of documentation or even descriptive comments. The single de ne eliminates remembering (or looking up) where the bit is located and which bit it is. When reading the code, using the de ne directive enhances the readability of the purpose of the instruction over the actual source information. De nes work because they directly copy in the string and let the assembler evaluate it. This is different from equates, which evaluate a string and store it as a constant referenced to the label. This can cause some subtle differences that can be a problem if you don t know what you are doing. For example, if you had the code
variable A = 37 Test1 equal A * 5 #de ne Test2 A * 5 : A = A + 5 : movlw : movlw Test2 ; ; ; Test2 is replaced with A * 5 = 42 * 5 = 210 Test1 ; Test1 is replaced with 185 ; A = 42 ; ; ; Variable is a run time variable Test1 = 185 Test2 is Evaluated when it is used
in this case, even though Test1 and Test2 are declared at the same point in the code identically, they are evaluated differently and will be different values in different locations of the application. This is a very useful capability, but one that can be a problem elsewhere. In the sample code above, I declared the variable to be A. Variable is an MPASM directive to create a temporary storage value during the application s assembly and will be discussed in the later sections of this chapter. When it is used in Test1, the value of A when the assembler processor encounters the statement is used, multiplied by 5 and assigned to Test1. After Test1 and Test2 are de ned in the code, A is modi ed, which results in a different value being calculated for Test2 when it is used later in the application. A useful function that de nes can provide is that they can provide constant strings throughout an application. I use this ability to keep track of my version number of an
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