Winter 2017 CS107 Midterm #2 Examination (Practice #2) Problem Points Score Grader TOTAL 30

Transcription

1 CS107 Winter 2017 CS107 Midterm #2 Examination (Practice #2) Cynthia Lee This is a closed book, closed note, closed computer exam. You have 90 minutes to complete all problems. You don t need to #include any libraries, and you needn t use assert to guard against any errors. Understand that the majority of points are awarded for concepts taught in CS107, and not prior classes. You don t get many points for for-loop syntax, but you certainly get points for proper use of &, *, and the low-level C functions introduced in the course. DO NOT ADD OR REMOVE PAGES OF THE EXAM (except optionally removing the reference page(s) at the end). If you need extra space for scratch and/or problem responses, use the blank back sides of each page. Changing the pages confuses the online grading system and will cause you to lose points. Problem Points Score Grader 1. C generics Bitwise ops and floats Assembly 10 TOTAL 30

2 Page 2 of 6 Problem 1: Implementing generic interfaces Complete the make_list function started below that takes a generic array of elements and creates a linked list of those same elements. The function's three arguments are the base address of the array, the number of elements in the array, and the size of each element in bytes. The function returns a pointer to a linked list of cells, one cell for each element in the array. The order of the elements in the list is the same as in the array. Each list cell consists of an element of elemsz bytes and a pointer to the next cell (null if this is the last cell). The element and the pointer are laid out in contiguous memory as shown below: void *make_list(const void *arr, int nelems, int elemsz) { void *list = NULL; for (int i = nelems-1; i >= 0; i--) { // prepend ith elem to front of list

3 Page 3 of 6 Problem 2: Bits a) multi_add You are to write a function that adds four 1-bit integers at the same time. These 1-bit unsigned integers are stored in every other bit of an unsigned char, like this: unsigned char ch = 0x44; /* */ /* the 1-bit numbers are: */ The least significant bit is the first 1-bit number, followed by a 0 for padding, followed by the next 1-bit number, and so on. In the above example, the 1-bit numbers are in bold and the padding digits are underlined. These 0 s for padding serve an important purpose: they will allow us to have enough space to hold the sum of two 1-bit numbers as an unsigned 2-bit integer. Without the extra bit, we could not accurately capturing the = 102 sum, because the 2 s place bit would be lost to overflow. For example: unsigned char c1 = 0x50; /* */ unsigned char c2 = 0x44; /* */ unsigned char sum = multi_add(c1, c2); /* */ So the output sum is an unsigned char with 4 2-bit unsigned integers. You could do this using addition, c1 + c2, but that is not allowed for this problem (no arithmetic operators on the bit data). Instead, you want to make creative use of the bitwise operators: &,, ^, ~, <<, >>. (Hint: Consider when certain bits of the output will be a 1. Is it when an input is 1 AND something else? or when an input bit is 1 OR something else? etc.) Avoid using loops, which is more complicated than using the bitwise operations to do the calculations in parallel (also a points deduction). unsigned char multi_add(unsigned char c1, unsigned char c2) {

4 Page 4 of 6 b) Float Write the minifloat representation of the value Recall that minifloat is a (made-up) 8-bit floating point format with 1 sign bit, 4 exponent bits, and 3 mantissa bits. Write your answer as a bit pattern in these boxes: y = Scratch work space: c) Float epsilon As we discussed in class and lab, for two float values x and y, when x is much greater than y, it may be that x + y == x. An interesting question is, how much greater does x need to be than y to make that happen? Find the smallest x greater than the y given above (2.25) such that x + y == x. Only consider positive values that are exactly representable as minifloats. Write your answer as a bit pattern in these boxes: x = 0 Scratch work space:

5 Page 5 of 6 Problem 3: x Presented below is the assembly generated as a result of feeding the ham function to gcc. Fill in the blanks in the C code below the assembly, so that ham is programmatically consistent with the assembly. Your code should refer to variables, not register names. You should write constants in the types we usually use for good C coding style, namely: write int constants in decimal (base 10), char constants as character literals, pointers/memory addresses in hexadecimal (base 16), and so on (do this even when writing constants in other formats/types would work). Note that the C code is nonsense and should just be a faithful reverse engineering of the unoptimized assembly. You may not typecast anything d <ham>: 40052d: push %rbp 40052e: push %rbx 40052f: sub $0x8,%rsp : mov %rdi,%rbx : mov %rsi,%rbp : callq <strcmp@plt> 40053e: cmp $0xffffffff,%eax : jne 40055c <ham+0x2f> : jmp <ham+0x23> : mov %eax,%edx : movb $0x0,(%rbx,%rdx,1) 40054b: add $0x1,%eax 40054e: jmp <ham+0x28> : mov $0x0,%eax : cmp $0x1b,%eax : jbe <ham+0x18> 40055a: jmp <ham+0x35> 40055c: movsbl 0x2(%rbx),%eax : jmp <ham+0x39> : movsbl 0x2(%rbp),%eax : add $0x8,%rsp 40056a: pop %rbx 40056b: pop %rbp 40056c: retq int ham(char *burr, char *aaron) { int george = ; if ( ) { for (unsigned int i = ; ; ) { ; else { return ; return ;

6 Page 6 of 6 Reference Page Misc. Functions void *memcpy(void *dest, const void *src, size_t n); void *memmove(void *dest, const void *src, size_t n); void *memset(void *ptr, int value, size_t num); void *malloc(size_t size); void *realloc(void *ptr, size_t size); void free(void *ptr); size_t strlen(const char *s); char *strcpy(char *dest, const char *src); char *strncpy(char *dest, const char *src, size_t n); char *strdup(const char *s); char *strcat(char *dest, const char *src); char *strchr(char *str, int character); char *strtok(char *str, const char *delimiters ); int sprintf ( char * str, const char * format,... ); int strcmp(const char *str1, const char *str2 ); int strncmp(const char *str1, const char *str2, size_t n); On the actual exam, this one-page x86-64 guide will also be included with the exam as a 2 nd page of reference sheet material: