361 condition codes in addition to the integer

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Unformatted text preview: nstruction 3 dereferences xp and stores the value in register %ecx, corresponding to program value x. Instruction 4 stores y at xp. Instruction 5 moves x to register %eax. By convention, any function returning an integer or pointer value does so by placing the result in register %eax, and so this instruction implements line 6 of the C code. This example illustrates how the movl instruction can be used to read from memory to a register (instructions 1 to 3), to write from a register to memory (instruction 4), and to copy from one register to another (instruction 5). Two features about this assembly code are worth noting. First, we see that what we call “pointers” in C are simply addresses. Dereferencing a pointer involves putting that pointer in a register, and then using this register in an indirect memory reference. Second, local variables such as x are often kept in registers rather than stored in memory locations. Register access is much faster than memory access. Practice Problem 3.2: You are given the following information. A function with prototype void decode1(int *xp, int *yp, int *zp); is compiled into assembly code. The body of t...
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