Building External Libraries of Procedures in Visual Studio .NET

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Building External Libraries of Procedures
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You'll notice that the EAT3ASM program given at the end of the previous section had most of its bulk devoted to procedures This is as it should be, with the caution that the procedures it uses are the kind you're likely to use in any and all of your assembly language programs Keeping cursor movement and screen-clearing routines in source code form in every single program you write is a waste of space and can clutter up the program in a way that makes it less easy to understand The answer is to break the utility procedures out into an external library that you can assemble only once, and then link into every program that uses its procedures without assembling the library every time you assemble the program This is called modular programming, and it is an extremely effective tool for programming efficiently in any language, assembly language not excluded I describe this process briefly back in 4 and show it pictorially in Figures 44 and 45 A program might consist of three or four separate ASM files, each of which is assembled separately to a separate OBJ file To produce the final executable EXE file, the linker weaves all of the OBJ files together, resolving all of the references from one to the other, finally creating an EXE file Each ASM file is considered a module, and each module contains one or more procedures and possibly some data definitions When all the declarations are done correctly, all of the modules may freely call one another, and any procedure may refer to any data definition The trick, of course, is to get all the declarations right
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Public and External Declarations
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If you reference a label in your program (by, say, including a CALL instruction to that label) without defining that label anywhere in the program, the assembler will gleefully give you an error message (You've probably already experienced this if you've begun writing your own programs in assembly) In modular programming, you're frequently going to be calling procedures that don't exist anywhere in the program that you're actually working on How to get past the assembler's watchdogs The answer is to declare a procedure external This works very much like it sounds: The assembler is told that a given label will have to be found outside the program somewhere, in another module Once told that, that assembler is happy to give you a pass on an undefined label You've promised the assembler that you'll provide it later, and the assembler accepts your promise and keeps going without flagging the undefined label The promise looks like this: EXTERN ClrScr Here, you've told the assembler that the label ClrScr represents a procedure and that it will be found somewhere external to the current module That's all the assembler needs to know to withhold its error message And having done that, the assembler's part is finished It leaves in place an empty socket in your program where the external procedure may later be plugged in I sometimes think of it as an eyelet where the external procedure will later hook in Over in the other module where procedure ClrScr exists, it isn't enough just to define the procedure An eyelet needs a hook You have to warn the assembler that ClrScr will be referenced from outside the module The assembler needs to forge the hook that will hook into the eyelet You forge the hook by declaring the procedure global, meaning that other modules anywhere in the program may freely reference the procedure Declaring a procedure global is simplicity itself: GLOBAL ClrScr That done, who actually connects the hook and the eyelet The linker does that during the link operation After all, why call it a linker if it doesn't link anything At link time, the linker takes the two OBJ files generated by the assembler, one from your program and the other from the module containing ClrScr,
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and combines them into a single EXE executable file (The number of OBJ files isn't limited to two; you can have almost any number of separately assembled external modules) When the EXE file is loaded and run, the program can call ClrScr as cleanly and quickly as though both had been declared in the same source code file This process is summarized in Figure 92
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Figure 92: Connecting globals and externals What works for procedures works for data as well, and it can work in either direction Your program can declare a variable as GLOBAL, and that variable may then be used by any module in which the same variable name is declared as external with the EXTERN directive I show you how this works in the VIDLIBASM library presented a little later in this chapter Finally, procedure libraries themselves may share data and procedures in any combination, as long as the declarations are handled correctly We sometimes say that a program or module containing procedures or variables declared as public exports those items Also, we say that a program or module that uses procedures or variables that are external to it imports those items
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