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Unformatted text preview: d the testb instruction to perform the AND operation. Problem 3.11 Solution: [Pg. 125] This is another chance to practice deciphering loop code. The C compiler has done some interesting optimizations. A. The register usage can be determined by looking at how the arguments get fetched, and how registers are initialized. Register Usage Register Variable Initially %eax a a %ebx b b %ecx i 0 %edx result a B. The test-expr occurs on line 5 of the C code and on line 10 and the jump condition of line 11 in the assembly code. The body-statement occurs on lines 6 through 8 of the C code and on lines 7 to 9 of the assembly code. The compiler has detected that the initial test of the while loop will always be true, since i is initialized to 0, which is clearly less than 256. C. The annotated code is as follows
1 2 3 4 5 movl 8(%ebp),%eax movl 12(%ebp),%ebx xorl %ecx,%ecx movl %eax,%edx .p2align 4,,7
Put a in %eax Put b in %ebx i=0 result = a a in %eax, b in %ebx, i in %ecx, result in %edx B.3. MACHINE LEVEL REPRESENTATION OF C PROGRAMS
6 7 8 9 10 11 12 705 .L5: addl %eax,%edx subl %ebx,%eax addl %ebx,%ecx cmpl $255,%ecx j...
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