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variable i = 0 if (i == 0) addlw 0 - i else sublw i endif
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When the macro is expanded (or executed), the following information will be displayed in the listing le:
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00000000 Addr 3E00 M M M M M M variable i = 0 if (i == 0) addlw 0 - i else sublw i endif
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As this example Illustrates, the setting of the assembly variable (i) results in a constant value being placed somewhere between the start of the line and the middle. The instructions that
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MACRO DEVELOPMENT
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are actually added to the listing le are identi ed by the address of the instruction and its bit pattern broken up as I have shown for the addlw 0 1 instruction. In this example, only addlw 0 1 is inserted into the source code; the directives (variable, if, else, and endif), as well as the sublw i instruction, are all ignored. While directives are somewhat unusual to follow because the code is repeated within them, and there is no start and end reference information that can be easily seen. For the example
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variable i = 0 while (i < 2) movlw i i = i + 1 endw
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the following listing le information is generated:
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00000000 Addr 3000 00000001 Ad+1 3001 0000002 M M M M M M M variable i while (i < movlw i = i + 1 movlw i = i + 1 endw = 0 2) i i
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In this case, the while and endw directives are not repeated as you would expect. Again, to really understand what is happening, you have to go back and look at the instructions entered into the application code. These instructions are displayed to the left of the column of M s along with the variable updates. Probably the best way to debug a macro is to single-step in the simulator through the macro. If you are doing this in the source le, you will nd execution jumps to where the macro is de ned that can be quite disconcerting and confusing because the parameters are displayed, not what the parameters actually are. These problems can be avoided by simulating from the listing le instead of from the source le. Breakpoints cannot be placed in MPLAB at a macro invocation for a source le. When I want to set a breakpoint at a macro invocation, I will put a nop instruction before it to give the MPLAB simulator somewhere on which to hang the breakpoint. This also works with a listing le, but in this case you do not have to add the breakpoint because breakpoints can be put at instruction statements within the macro-generated code. Debugging macros is really the same as debugging a standard application, except that there is no simulator or debugger. This is an important point to remember when you are creating the macro. You may want to ag where the code is executing using messages (using the messg directive), which is the same as printf debugging in C. For example, if you have the code
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if (A == B) messg A == B ; put in code for A == B
STRUCTURED PROGRAMMING MACROS
else messg A != B ; put in code for A != B endif
the messg directives will ag the path execution takes through the macro s conditional assembly. This (and the preceding) tricks can be used with conditional code external to macros to help you follow their execution and nd any problems within them.
Structured Programming Macros
To nish off this chapter, I wanted to leave you with a few macros that should give you some ideas on how powerful macros are when you need some processing to be done when the application is being created, as well as give you a tool for making your PIC microcontroller assembly-language code easier. The structre.inc le contains nine macros that you can use for your own applications to add structured programming conditional execution. The macros are in the format
_ Test Const/Var
where Test can be do, until, while, if, else, and end, and Const/Var speci es whether or not the second parameter is a constant or a variable. The different macros are listed in Table 10.7. When conditional macros are invoked, the parameters passed to the macro include the two conditions for testing as well as the condition to test for. The standard C test conditions are used, as listed in Table 10.8. Only one condition can be accessed within a macro at any time. There is no ability to AND or OR conditions together. Using these macros in your application is quite straightforward, and if you are new to PIC microcontroller programming, you might want to take a look at using these macros for your conditional programming. Executing some code conditionally if two variables are equal would use the following macro invocations:
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