168 chapter 3 machine level representation of c

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Unformatted text preview: rictions. The compiler places directives in the assembly code indicating the desired alignment for global data. For example, the assembly code declaration of the jump table on page 131 contains the following directive on line 2: .align 4 This ensures that the data following it (in this case the start of the jump table) will start with an address that is a multiple of 4. Since each table entry is 4 bytes long, the successive elements will obey the 4-byte alignment restriction. Library routines that allocate memory, such as malloc, must be designed so that they return a pointer that satisfies the worst-case alignment restriction for the machine it is running on, typically 4 or 8. For code involving structures, the compiler may need to insert gaps in the field allocation to ensure that each structure element satisfies its alignment requirement. The structure then has some required alignment for its starting address. For example, consider the structure declaration: struct S1 { 3.10. ALIGNMENT int i; char c; int j; }; 161 Suppose the compiler used the minimal 9-by...
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This note was uploaded on 09/02/2010 for the course ELECTRICAL 360 taught by Professor Schultz during the Spring '10 term at BYU.

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