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Since writing the rst edition, I have discovered that some of the biggest questions people have about developing PIC microcontroller software applications are about how macros work and how they are implemented. This was surprising to me because I have always felt that macros are a very simple tool to understand and use. They are a feature of most assemblers (including MPASM), and they can be used to make application development much easier. A macro can be de ned as a function that is invoked from within an application s code. The invocation statements are replaced by the code within the macro. Macros can be thought of as being similar to subroutines, but they have one big difference: Instead of calling a central routine, the routine s code is embedded into the source, as shown in Fig. 10.1.
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PIC Microcontroller Assembly-Language Macros
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When macros are invoked (which is the term used to indicated their use, like calls for subroutines), parameters or arguments are used that replace labels within the macro. These parameters are not similar to the arguments of a function call the macro parameters are used to replace the macro arguments, and their contents are not copied from a variable into another one, as is done in a function call. As will be shown later in this chapter, the invocation parameters do not have to be variables; instead, they are strings, which allows for some interesting capabilities. For example, instead of repeatedly writing the code
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incf btfsc incf A, f STATUS, Z A + 1, f
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Figure 10.1 Macros differ from subroutine calls by inserting the code to be executed at the current location in the application code rather than jumping to a new address, executing the required instructions, and then returning.
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every time a 16-bit increment is required, the following macro could be used:
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inc16 incf btfsc incf endm macro Variable Variable, f STATUS, Z Variable + 1, f
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Four strings have been added to the three 16-bit incrementing codes. After the identifying label inc16, macro indicates that the following code is to be placed in line each time the identifying string is encountered until the endm directive is encountered. The Variable is a parameter that can be used to customize the macro code that is inserted into the application in this macro the parameter Variable will be replaced by whatever string you speci ed for it. If the macro was invoked with the statement
inc16 B
the instructions
incf btfsc incf B, f STATUS, Z B + 1, f
would be inserted into the application. The advantages of using macros are that the overhead of subroutines can be avoided. The immediate overhead that comes to mind is the call and return instructions, but parameters that are passed to the routine do not have to be saved. This leads to some interesting situations where macros can create much more ef cient application code than calling a subroutine. For example, to add two 16-bit numbers together, if a subroutine were used, the
PIC MICROCONTROLLER ASSEMBLY-LANGUAGE MACROS
parameters have to be saved in temporary variables that are processed by the subroutine, and then the result is returned in a temporary variable and then passed to the nal destination. In mid-range PIC microcontrollers, the code to do this could be
; C = A + B movf movwf movf movwf movf movwf movf movwf call movf movwf movf movwf : Add16Bits ; 16 movf Temp2 + 1, addwf Temp1 + 1, movf Temp2, w addwf Temp1, f btfsc STATUS, C incf Temp1 + 1, return Bit Addition Subroutine w ; Add the High Byte First f ; Add the Low Byte A, w Temp1 A + 1, w Temp1 + 1 B, w Temp2 B + 1, w Temp2 + 1 Add16Bits Temp1, w C Temp1 + 1, w C + 1 ; Save A in a Temporary Register
Save B in a Temporary Register
Call the 16 Bit Addition Subroutine Put the Result in C
If Low Byte Carry, Increment High Byte Return to Caller
In this example, the actual number of instructions required for supporting the routine is almost twice what the subroutine uses! In comparison, a macro for the same function, i.e.,
Add16BitsMacro macro VarA, VarB, VarC movf VarA + 1, w ; Add the High Byte First addwf VarB + 1, w movwf VarC + 1 ; Save the Result movf VarA, w ; Add the Low Byte addwf VarB, w movwf VarC btfsc STATUS, C ; If low byte carry, Increment ; High Byte incf VarC + 1, f endm
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