CIS 2107 Computer Systems and Low-Level Programming Spring 2012 Final

Transcription

1 Spring 2012 Name: Page Points Score Total: 100 Instructions The exam is closed book, closed notes. You may not use a calculator, cell phone, etc. Unless you are told otherwise, you may use any functions in the Standard C Library. For each of the questions of this quiz, you can assume the following sizes for C data types: type bytes char 1 short 2 int 4 long 8 float 4 double 8 void* 4 i

2 1. Please place the letter of the best answer for each in the answer line. A L1 B loader C nanoseconds D cylinder E L2 F data G DRAM H BSS I SRAM J compiler K track L EAX M seconds N instruction pointer or program counter O flash P sector Q heap R magnetic disk S assembler T stack U locality V preprocessor W symbol table X CPU Y EBP Z milliseconds (1 point) (a) Translates a high level language program into assembly language. (a) (1 point) (b) Roughly one million times slower, cheaper, and larger than main memory or cache. (1 point) (c) Malloc and free manage this area of memory. (1 point) (d) Translates assembly language into machine code. (1 point) (e) Segment for area of memory allocated by malloc( ). (1 point) (f) Tendency for programs to access multiple objects in a block. (1 point) (g) Translates assembly language programs into machine language. (1 point) (h) Area of memory used for initialized global variables. (b) (c) (d) (e) (f) (g) (h) (1 point) (i) In a collection of disk platters, a set of tracks equidistant from the center of the platter. (i) (1 point) (j) Processor register that contains the address of the next instruction to be executed. (1 point) (k) Contains the return value of functions which return ints. (1 point) (l) The larger but slower cache. Still much faster than main memory. (1 point) (m) Time it takes to read from disk. (1 point) (n) Time to read from on-cpu cache. (j) (k) (l) (m) (n) 1 of 13 exam continues...

3 (2 points) 2. What is in base 2? (2 points) 3. What is 5696C1B 16 + DA in base 16? C 1 B 16 + D A Some tricky declarations. Write a very brief description in English of what is declared. For example, if the question is int func(int A[]), you d write, func is a function which is passed an array of int and returns an int. (1 point) (a) char (*p)[10]; (1 point) (b) int (*p[ ])( ); (1 point) (c) int (*p( ))[ ]; out of a possible 7 2 of 13 exam continues...

4 (6 points) 5. Encoding a floating-point number. (a) What is in fixed-point binary? (b) What is in fixed-point binary? (c) In a 32-bit C float variable, how many bits are used for the sign? (d)... how many for the exponent? (e)... how many for the mantissa? (c) (d) (f) What is the value for bias when exponents are stored using as many bits as you ve put for your answer to part d? (e) (g) How would be stored in a C float variable? out of a possible 6 3 of 13 exam continues...

5 (1 point) (a) x y 6. Some bit operations. If we have char x = 0x5C, y = 0xA9;, what is the result of the following operations? Your answer must be in the form of exactly two hex digits 1. (a) (1 point) (b) x y (b) (1 point) (c) x<<2 (c) (1 point) (d) x (d) (1 point) (e) x&0x0f (e) 1 Ignore the possibility of promotion to 32-bit ints. Behave as though we re living in the land of 8-bit arithmetic. out of a possible 5 4 of 13 question 6 continues...

6 (1 point) (f) x y (f) (1 point) (g) x&&1 (g) (1 point) (h) x-y (h) (1 point) (i)!!x (i) (1 point) (j) x&y (j)

7 (5 points) 7. If I have the following: int main(void) { int a=10; int b=20; int *p=&a; int *q=p; char *cp = (char*)q; (*p)--; q--; cp--; and memory is laid out like this: cp 1000 q 1004 p 1008 b 1012 a 1016 what do you see if you print: (a) a (b) &a (c) b (d) p (e) *p (f) &p (g) q (h) *q (i) cp (j) &cp (a) (b) (c) (d) (e) (f) (g) (h) (i) (j) out of a possible 5 6 of 13 exam continues...

8 (10 points) 8. Use the following code to answer the questions. Data sizes are specified on the cover of the exam. 1 struct Stuff { 2 int x; 3 int *p; 4 int A[10]; 5 }; 6 7 int main(void) 8 { 9 struct Stuff s; 10 int A[10]; 11 int x, y; 12 char str[24]; x=10; 15 y=20; 16 A[0]=30; 17 strcpy(str, "almost quitting time"); 18 s.x=40; 19 s.p=&y; 20 s.a[0]=50; func01(a); 23 func02(str); 24 func03(str); 25 func04(s); return 0; 28 } void func01(int arr[]) { 31 arr[0]=3333; 32 } void func02(char *s) { 35 strcpy(s, "yeah, summer vacation"); 36 } void func03(char *s) { 39 s = malloc(40); 40 strcpy(s, "how many more pages is this thing?"); 41 } void func04(struct Stuff s) { 44 s.x=4444; 45 *(s.p)=2222; 46 s.a[0]=5555; 47 s.p=malloc(sizeof(int)); 48 *(s.p)=2020; 49 } (a) How many bytes are passed to the function func01( )? (b) How many bytes are passed to the function func02( )? (c) How many bytes are passed to the function func04( )? What is the value of each of the following after func04( ) has been called? (d) x (e) y (f) A[0] (g) s.x (h) s.a[0] (i) *(s.p) (j) str (What s the string?) (a) (b) (c) (d) (e) (f) (g) (h) (i) (j) out of a possible 10 7 of 13 exam continues...

9 (4 points) 9. For each of the following, suppose that %eax contains the value x, %ecx contains y. What s stored in %edx after the each operation? expression leal (%eax,%ecx, 4), %edx result leal 0xB(,%eax,8), %edx leal 5(%eax,%eax,8), %edx leal 0xE(%eax), %edx 10. Given the C function: int func(int x, int y) { int t;... } return x+y-t; Immediately before func( ) is called, i.e., immediately before the instruction call func, %ebp contains the value and %esp contains the value Before func( ) exits (more precisely, just before the leave and return instuctions are executed), what is: (1 point) (a) stored in %ebp? (1 point) (b) stored in %esp? (1 point) (c) the location of x? (1 point) (d) the location of y? (1 point) (e) the most likely location of t? (1 point) (f) the location of the return value? (1 point) (g) the location of the return address? (a) (b) (c) (d) (e) (f) (g) out of a possible 11 8 of 13 exam continues...

10 (5 points) 11. Write a function which is passed an unsigned int x, and an int n. The function returns the nth least significant nibble of x. For example, if x is 0x , and n is 2, the function returns 5. (5 points) 12. Write the function called slice(int A[ ], int s, int e) which returns a new array consisting of all of the elements of A[ ] from A[s] to A[e] inclusive. It is up to the caller to free the memory allocated by slice. If e<s, the function returns NULL. out of a possible 10 9 of 13 exam continues...

11 (9 points) 13. Write a C function equivalent to the following assembly (no credit for an answer containing inline assembly). 1.section.text 2.globl mystery 3.type 4 mystery: 5 pushl %ebp 6 movl %esp, %ebp 7 xorl %eax, %eax 8 xorl %ecx, %ecx 9 movl 8(%ebp), %edx 10 begin: 11 cmpl 12(%ebp), %ecx 12 jge done 13 addl (%edx, %ecx, 4), %eax 14 incl %ecx 15 jmp begin 16 done: 17 movl %ebp, %esp 18 popl %ebp 19 ret out of a possible 9 10 of 13 exam continues...

12 (8 points) 14. Implement the function void reverse(int A[ ], int len), which reverses the order of A[ ], an array of len items. Do not use the [ ] operator. No credit will be given for solutions which use the [ ] operator, or which declare len or more elements of temporary storage. void reverse(int A[ ], int len) { out of a possible 8 11 of 13 exam continues...

13 (10 points) 15. A common way of storing a spreadsheet is comma-separated text. For example, the following line in a spreadsheet: apple banana cherry some fruit beginning with d could be stored as apple, banana, cherry, some fruit beginning with d. Write the function char **split(char *s) which is passed s, which is a string of comma-separated values, and returns an array of string containing the values in the line terminated by a NULL pointer. Using our current example, we d return: w[0] w[1] w[2] w[3] w[4] apple banana cherry some fruit beginning with d NULL split( ) should return NULL on failure. Hint: if there are n commas in s, there will be n + 1 words. You may use any function in the Standard C Library. out of a possible of 13 question 15 continues...

14 (extra space) 13 of 13 end of exam