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CMU CS 15213 - Exam

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Page 1 of 11 Andrew login ID: ________________ Full Name: ________________ Section: ________________ 15-213/18-243, Summer 2011 Exam 1 Tuesday, June 28, 2011 Instructions: • Make sure that your exam is not missing any sheets, then write your Andrew login ID, full name, and section on the front. • This exam is closed book, closed notes. You may not use any electronic devices. • Write your answers in the space provided below the problem. If you make a mess, clearly indicate your final answer. • The exam has a maximum score of 100 points. • The problems are of varying difficulty. The point value of each problem is indicated. Good luck! 1 (12): 2 (12): 3 (9): 4 (20): 5 (15): 6 (20): 7 (12): TOTAL (100):Page 2 of 11 Question 1. (12 points) Bits, bytes, and floats. For the conversions below, please make sure that you show all your work and your intermediate steps. Answers that only contain end results will not be graded even if they are correct. A-) Convert 11.6875 to IEEE 32-bit floating point format. B-) Perform the following addition in the form of 8-bit, twos complement binary addition. Note if there is an overflow or not. -72 + 61 = C-) An 8-bit location in memory contains value c9 (in hex). This value would have different interpretations when it represents an 8-bit floating point number, or an 8-bit signed integer, or an 8-bit unsigned integer. Find its numerical interpretations in decimal. Again, show your work clearly. 8-bit floating point number = Signed Integer = Unsigned Integer =Page 3 of 11 Question 2. (12 points) Structs. Consider the following struct: typedef struct { char a[5]; short b[3]; double c; long double d; int* e; int f; float* g; } MYSTR; Part 1. Show how the struct above would appear on a 64-bit (“x86 64”) Linux machine. Label the bytes that belong to the various fields with their names and clearly mark the end of the struct. Use x’s to indicate bytes that are wasted in the struct. Part 2. Rearrange the above fields in the above struct such that it would consume the most space in memory. Part 3. Rearrange the above fields in the above struct such that it would consume the least space in memory.Page 4 of 11 Question 3. (9 points) Assembly to C. Consider the 64-bit assembly code for a simple sort function. We provide parts of the corresponding C code on the next page. Please fill in the missing parts of the C code. mysterysort: mov $1, %r8 jmp L1 L2: movl (%rdi, %r8, 4), %r11d mov $0, %r9 jmp L3 L4: movl (%rdi, %r9, 4), %eax cmp %eax, %r11d jge L7 mov %r8, %r10 jmp L6 L5: mov 0xfffffffffffffffc(%rdi, %r10, 4), %eax mov %eax, (%rdi, %r10, 4) sub $1, %r10d L6: cmp %r9, %r10 jg L5 movl %r11d, (%rdi, %r9, 4) jmp L1 L7: add $1, %r9 L3: cmp %r8, %r9 jl L4 add $1, %r8 L1: cmp %rsi, %r8 jl L2 retqPage 5 of 11 void mysterysort(int *arr, int len) { int i, j, k, temp; for(i = ____; i < ____; ____) { temp = ____; for(j = ____; j < ____; ____) { if(____ < ____) { for(k = ____; ____ > ____; ____) { ____ = ____; } ____ = ____; break; } } } }Page 6 of 11 Question 4. (20 points) Stacks. Consider the C code for calculating Fibonacci numbers and the corresponding 32-bit assembly code. On the chart on the next page, please document the entire state of the stack as detailed as possible just before a call to fib(4) would return, but before it has popped its stack frame, noting that higher addresses are closer to the top of the page. Your trace should begin with the arguments from the stack frame of the caller of fib(4). Please also document where the relevant registers would point on your diagram. Note that you may not find it necessary to use all of the blanks. If you find a need, you may refer to the addresses on which the marked lines of code would be with the letter with which they are marked. int fib(int n) { /* computes the nth fibonacci number */ if(n < 2) { return 1; } return fib(n-1) + fib(n-2); } fib: push %ebp mov %esp,%ebp sub $0xc,%esp mov %ebx,0xfffffff8(%ebp) mov %esi,0xfffffffc(%ebp) mov 0x8(%ebp),%esi mov $0x1,%eax cmp $0x1,%esi jle cleanup lea 0xffffffff(%esi),%eax mov %eax,(%esp) call fib # Address “A” mov %eax,%ebx # Address “B” lea 0xfffffffe(%esi),%eax mov %eax,(%esp) call fib # Address “C” add %ebx,%eax # Address “D” cleanup: mov 0xfffffff8(%ebp),%ebx mov 0xfffffffc(%ebp),%esi mov %ebp,%esp pop %ebp retPage 7 of 11 Higher addresses Lower AddressesPage 8 of 11 Question 5 (15 points) Jump Table. Consider the assembly dump for a switch and jump table given below. The switch expression and all case and break statements have been removed from the C code below it. Using what you know, fill in the switch expression, as well as case statements and break statements on the lines below, noting that you may not use every line. L1: .quad L6 .quad L4 .quad L5 .quad L3 .quad L4 .quad L4 .quad L2 .quad L6 scramble: cmpq $7, %rdi ja L4 jmp *L1(,%rdi, 8) L2: movl (%rdx), %r8d addl %r8d, (%rsi) jmp L7 L3: movl (%rsi), %r8d mov $5, %r9 movl %r8d, (%rdx, %r9, 4) jmp L7 L4: movl $15213, (%rsi) jmp L7 L5: movl (%rdx), %r8d movl %r8d, (%rsi) L6: movl (%rsi), %r8d lea (%r8, %r8, 2), %r8 movl %r8d, (%rsi) jmp L7 L7: retPage 9 of 11 void scramble(unsigned a, int * b, int * c) { switch(___) { _________________________________________ c[5] = *b; _________________________________________ _________________________________________ _________________________________________ *c += *b; _________________________________________ _________________________________________ _________________________________________ *b = *c; _________________________________________ _________________________________________ _________________________________________ *b *= 3; _________________________________________ _________________________________________ _________________________________________


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