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When you write a DOS assembly language program, as I explained in the earlier parts of this book, you write all of it All the code that runs is only the code that you write, or that you explicitly and optionally link into it When you run that program, DOS hands control of the machine to the program, right at the first machine instruction you wrote at the start of your program It continues to run your code until you return control back to DOS with a call to service 04CH of INT 21H This is simple and easy to understand, which is one important reason I started you off with DOS programming rather than Linux programming Linux is different Lots different And once you begin using a debugger to go inside the binary space of a program you've written, you have to understand that difference thoroughly To communicate with the Linux kernel, an assembly language program should use the C library as its communications layer It's possible to make direct kernel calls, but it's not a good idea, as the details of making those calls may change from version to version of Linux We're going to play it straight in this book and make all input and output calls through the standard C library This means we have to link the C library into an assembly language program Doing so allows us to call C library functions such as printf(), ctime(), and so on However, linking in those function calls comes with a certain amount of baggage, and the baggage consists of startup and shutdown code In truth, when you create a Linux assembly language program as I explain in this and the next chapter, you're creating a sort of a hybrid The structure of this hybrid is shown in Figure 122
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Figure 122: The structure of a Linux assembly language program Linking your assembly language program to the C library adds in all the code shown in the top bar In addition to the code containing library calls such as printf(), there is a block of code that runs before your program begins, and another block that runs after your program ends In a sense, your program is only a subroutine called by a boilerplate empty program in the C library Your program is called as though it were a subroutine (with the CALL instruction) and it returns control to the C library code using a RET instruction
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Technically, your program is a subroutine, and it helps to think of it as one That's how I've drawn it in Figure 122 When Linux begins executing your program, it actually starts, not at the top of the code you wrote, but at the beginning of the startup code When the startup code has done what it must, it executes a CALL instruction that takes execution down into your assembly code When the assembly language program you wrote returns control to its caller via RET, the shutdown code begins executing, and it's the shutdown code that actually returns control to Linux In between, you can make as many calls into the C runtime library code as you like When you link your program with gcc, the code containing the C library routines that you call is linked into the program Note well that the startup and shutdown code, as well as all the code for the library calls, are all physically present in the executable file that you generate with gcc You're not making calls into a DLL somewhere Whatever calls are made into the Linux kernel are made by the C library code The problem caused by the presence of the startup code in your executable file is that when you begin single-stepping the program, you're single-stepping through C library code That can be enlightening, and I encourage you to do it a few times while you're first getting your Linux legs However, after a while you'll be pounding on the keyboard trying to get through it so that you can figure out why your code isn't working correctly You need a way to skip past the startup code And skipping code means you need some signposts in the executable file so that you have someplace to skip to For this you need symbols in your executable file
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