Recitation 2 – 2/11/02 Outline Stacks & Procedures Homogenous Data –Arrays –Nested Arrays Mengzhi Wang Office Hours: Thursday 1:30 – 3:00 Wean Hall 3108 Reminders Lab 2: Tuesday, 11:59

Stacks Grows down Stores local variables that can’t fit in registers Stores arguments and return addresses %esp Stack Pointer –Points to the top value on the stack %ebp Base Pointer –Points to a function’s stack frame pushl –Decrements, then places value popl –‘Returns’ value, then increments

Stack Frames Abstract partitioning of the stack Each Frame contains the state for a single function instant Stack Pointer ( %esp ) Frame Pointer ( %ebp ) Return Addr Saved Registers Argument Build Old %ebp Local Variables Arguments Caller Frame

Procedures call: Caller Responsibilities Arguments( pushl ) –In what order? Return Address(done by call ) ret : Callee Responsibilities Save Registers (especially %ebp ) Set up Stack Frame Return value in %eax

Problem 1: Call Chain void absdiff(int *result, int x, int y) { int z; if (x >= y) z = x - y; else z = y - x; *result = z; return; } int main() { int result; int x,y; x = 5; y = -3; absdiff(&result, x, y); printf("|(%d) - (%d)| = %d\n", x, y, result); return 0; }

Problem 1: Call Chain : push %ebp mov %esp,%ebp sub $0x18,%esp mov 0xc(%ebp),%eax cmp 0x10(%ebp),%eax jl.L1 mov 0xc(%ebp),%eax mov 0x10(%ebp),%edx mov %eax,%ecx sub %edx,%ecx jmp.L2.L1 mov 0x10(%ebp),%eax mov 0xc(%ebp),%edx mov %eax,%ecx sub %edx,%ecx.L2 mov 0x8(%ebp),%eax mov %ecx,(%eax) mov %ebp,%esp pop %ebp ret : push %ebp mov %esp,%ebp sub $0x18,%esp movl $0x5,-8(%ebp) movl $0xfffffffd, -12(%ebp) add $0xfffffffc,%esp mov -12(%ebp),%eax push %eax mov -8(%ebp),%eax push %eax lea -4(%ebp),%eax push %eax call add $0x10, %esp mov -4(%ebp),%eax push %eax mov -12(%ebp),%eax push %eax mov -8(%ebp),%eax push %eax push $0x80484d8 call mov %ebp,%esp pop %ebp ret

Problem 1: Answer : push %ebp mov %esp,%ebp sub $0x18,%esp movl $0x5,-8(%ebp) movl $0xfffffffd, -12(%ebp) add $0xfffffffc,%esp mov -12(%ebp),%eax push %eax mov -8(%ebp),%eax push %eax lea -4(%ebp),%eax push %eax call Old %ebp result x = 5 y = -3 %esp -3 5 &result Rtn Address %esp %ebp

Problem 1: Answer : push %ebp mov %esp,%ebp sub $0x18,%esp mov 0xc(%ebp),%eax cmp 0x10(%ebp),%eax jl.L1 mov 0xc(%ebp),%eax mov 0x10(%ebp),%edx mov %eax,%ecx sub %edx,%ecx jmp.L2.L1 mov 0x10(%ebp),%eax mov 0xc(%ebp),%edx mov %eax,%ecx sub %edx,%ecx.L2 mov 0x8(%ebp),%eax mov %ecx,(%eax) mov %ebp,%esp pop %ebp ret %esp -3 5 &result Rtn Address %ebp Old %ebp %esp ******

Problem 1: Answer : ….. add $0x10, %esp mov -4(%ebp),%eax push %eax mov -12(%ebp),%eax push %eax mov -8(%ebp),%eax push %eax push $0x80484d8 call mov %ebp,%esp pop %ebp ret Old %ebp result 5 -3 %esp result -3 5 %ebp 0x80484d8 Rtn Address %esp

Problem 2: Recursion With the following code, what does the stack look like if we call fib(2, 1, 0) and reach the point where if(n==0) holds true? int fib(int n, int next, int result) { if(n == 0) return result; return fib(n - 1, next + result, next); }

Problem 2: Answer 0 ; third argument to fib 1 ; second 2 ; first ret ; call fib(2,1,0) oldebp ; <--- ebp of fib’s caller 1 ; <--- push next 1 ; <--- next + result 1 ; <--- n - 1 ret ; call fib (1, 1, 1) oldebp ; <--- ebp of fib’s 2 ; <--- push next 3 ; <--- push next + result 0 ; <--- push n-1 ret ; call fib (0, 3, 2)

Homogenous Data: Arrays Allocated as contiguous blocks of memory Address Computation Examples int cmu[5] = {…} cmu begins at memory address 40 cmu[0]40 + 4*0 = 40 cmu[3]40 + 4*3 = 52 cmu[-1]40 + 4*-1 = 36 cmu[15]40 + 4*15 = 100

Problem 3: Arrays get_sum: pushl %ebp movl %esp,%ebp pushl %ebx movl 8(%ebp),%ebx # ebx = 1st arg movl 12(%ebp),%ecx # ecx = 2nd arg xorl %eax,%eax # eax = 0 movl %eax,%edx # edx = 0 cmpl %ecx,%eax # jge.L4 # if (ecx >= 0) goto L4.L6: addl (%ebx,%edx,4),%eax # eax += Mem[ebx+edx*4] incl %edx # edx ++ cmpl %ecx,%edx # jl.L6 # if (edx < ecx) goto L6.L4: popl %ebx movl %ebp,%esp popl %ebp ret

Problem 3: Answer int get_sum(int * array, int size) { int sum = 0; int i=0; for (i=0; i<size; i++) sum += array[i]; return sum; } get_sum: pushl %ebp movl %esp,%ebp pushl %ebx movl 8(%ebp),%ebx movl 12(%ebp),%ecx xorl %eax,%eax movl %eax,%edx cmpl %ecx,%eax jge.L4.L6: addl (%ebx,%edx,4),%eax incl %edx cmpl %ecx,%edx jl.L6.L4: popl %ebx movl %ebp,%esp popl %ebp ret

Problem 4: Nested arrays int main(int argc, char **argv) { int i,j,r=0; for (i=0; i<argc; i++) { j=0; while(argv[i][j] != '\0') { r ^= argv[i][j]; j++; } return r; }

Problem 4: Answer main: pushl %ebp movl %esp,%ebp pushl %edi pushl %esi pushl %ebx movl 12(%ebp),%edi xorl %esi,%esi xorl %ebx,%ebx cmpl 8(%ebp),%esi jge.L4.L6: xorl %ecx,%ecx movl (%edi,%ebx,4),%eax cmpb $0,(%eax) je.L5 movl %eax,%edx.L9: movsbl (%ecx,%edx),%eax xorl %eax,%esi incl %ecx cmpb $0,(%ecx,%edx) jne.L9.L5: incl %ebx cmpl 8(%ebp),%ebx jl.L6.L4: movl %esi,%eax popl %ebx popl %esi popl %edi movl %ebp,%esp popl %ebp ret