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The stack is extremely important in assembly language work, and this is doubly true in Linux work, because Linux is a C world, and in C (as in most high-level languages including Pascal) the stack has a central role The reason for this is simple: Compilers are machines that write assembly language code, and they are not human and clever like you (Although I've met some people who appear less intelligent than some of your better compilers ) This means a compiler has to use what might seem brute force methods to create its code, and most of those methods depend heavily on the use of the stack Compiler code generation is doctoral thesis stuff and I won't have much more to say about it in this book One compiler mechanism that bears on Linux assembly work is that of the stack frame Compilers depend on stack frames to create local variables in functions (in Pascal we call them procedures), and while stack frames are less useful in assembly work, you must understand them, because they provide an easy way to access command-line arguments and environment variables A stack frame is a location on the stack marked as belonging to a particular function It is basically the region between the addresses contained in two registers: base pointer EBP, and stack pointer ESP This draws better than it explains; see Figure 131
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Figure 131: A stack frame A stack frame is created by pushing the caller's copy of EBP on the stack to save it, and then copying the caller's stack pointer ESP into register EBP The first two instructions in any assembly program that honors the C calling conventions must be these: push ebp mov ebp,esp After this, you must either leave EBP alone, or else if you must use it in a serious pinch make sure you can restore it before the change violates any C library assumptions (I recommend leaving it alone!) EBP is considered the anchor of your new stack frame, which is the main reason it shouldn't be changed There are things stored on the stack above (that is, at higher addresses than) your stack frame that often need to be referenced in your code, and EBP is the only safe way to reference them (These things aren't shown in Figure 131, but I return to them later in this chapter) Less obvious is the fact that EBP is also the hidey-hole in which you stash the caller's stack pointer value, ESP This is yet another reason not to change EBP once you create your stack frame Returning control at the end of your program with a random value in ESP is the shortest path to trouble I could name Once EBP is safely anchored as one end of your stack frame, the stack pointer ESP is free to move up and down the stack as required The first things you need to put on the stack, however, are the caller's values for EBX, ESI, and EDI, as shown in Figure 131 The order in which these three are saved isn't crucial, but the order I show in Figure 131 is customary They will be popped back off the stack when the stack frame is destroyed at the end of your program, handing back to the caller (which in our case is the startup/shutdown code from the C library) the same values those registers had when the startup code called your program as the function main But once EBX, ESI, and EDI are there, you can push and pop whatever you need to for temporary storage Calling C library functions requires a fair amount of pushing and popping, as we see shortly
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