In the following procedure, the body of the switch statement has been removed: T
ID: 3749195 • Letter: I
Question
In the following procedure, the body of the switch statement has been removed:
The disassembled machine code for the procedure is shown below.
The jump table resides in a different area of memory.
We can see from the mov at address 0x00000000004004dc that the jump table begins at address 0x4005b8.
Using the gdb debugger, we can examine the eight 8-byte words of memory comprising the jump table with the command x/8x 0x4005b8. gdb prints the following:
(gdb) x/8x 0x4005b8
0x4005b8: 0x00000000004004e6 0x0000000000400516
0x4005c8: 0x00000000004004ed 0x0000000000400502
0x4005d8: 0x0000000000400516 0x0000000000400509
0x4005e8: 0x0000000000400516 0x000000000040050f
Dump of assembler code for function main:
0x00000000004004b6 <+0>: push %rbp
0x00000000004004b7 <+1>: mov %rsp,%rbp
0x00000000004004ba <+4>: mov %rdi,-0x18(%rbp)
0x00000000004004be <+8>: mov %rsi,-0x20(%rbp)
0x00000000004004c2 <+12>: mov %rdx,-0x28(%rbp)
0x00000000004004c6 <+16>: mov -0x18(%rbp),%rax
0x00000000004004ca <+20>: mov %rax,-0x8(%rbp)
0x00000000004004ce <+24>: mov -0x20(%rbp),%rax
0x00000000004004d2 <+28>: sub $0xd,%rax
0x00000000004004d6 <+32>: cmp $0x7,%rax
0x00000000004004da <+36>: ja 0x400516
0x00000000004004dc <+38>: mov 0x4005b8(,%rax,8),%rax
0x00000000004004e4 <+46>: jmpq *%rax
0x00000000004004e6 <+48>: addq $0xd,-0x8(%rbp)
0x00000000004004eb <+53>: jmp 0x40051e
0x00000000004004ed <+55>: mov -0x8(%rbp),%rdx
0x00000000004004f1 <+59>: mov %rdx,%rax
0x00000000004004f4 <+62>: shl $0x2,%rax
0x00000000004004f8 <+66>: add %rdx,%rax
0x00000000004004fb <+69>: add %rax,%rax
0x00000000004004fe <+72>: mov %rax,-0x8(%rbp)
0x0000000000400502 <+76>: addq $0x5,-0x8(%rbp)
0x0000000000400507 <+81>: jmp 0x40051e
0x0000000000400509 <+83>: shlq -0x8(%rbp)
0x000000000040050d <+87>: jmp 0x40051e
0x000000000040050f <+89>: shlq $0x3,-0x8(%rbp)
0x0000000000400514 <+94>: jmp 0x40051e
0x0000000000400516 <+96>: movq $0x0,-0x8(%rbp)
0x000000000040051e <+104>: mov -0x28(%rbp),%rax
0x0000000000400522 <+108>: mov -0x8(%rbp),%rdx
0x0000000000400526 <+112>: mov %rdx,(%rax)
0x0000000000400529 <+115>: pop %rbp
0x000000000040052a <+116>: retq
Questions:
(1)How many cases are specified in the switch statement?
(2)What is the smallest integral value specified for a case?
(3)What is the largest integral value specified for a case?
4) Give the address of where the code starts for the smallest integral value?
5) Specify the address of where the code starts for the largest integral value?
6) What is the address of where the code starts for the default case?
7) Give one case value that falls through to another case value.
8) List one integral value between the smallest and largest integral values (specified in 2 and 3 above) that goes to the default case.
Explanation / Answer
ANSWER :
long switch_prob(long x, long n)
{
long result = n;
switch(x){
case 60: result = n*8; break;
case 62: result = n*8; break;
case 63: result = n;
result >>= 0x3;
break;
case 64: result = n;
result <<= 0x4;
result -= n;
n = result;
n *= n;
result = n + 75;
break;
case 65: n *= n;
result = n + 75;
break;
default: result = n + 75;
}
return result;
}
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