I have this homework from school and I do not get how to know which are valid and missing cases for this switch statement. I mean, I know that the first case is going to be 50 and I will have cases from 50 to 55 but how can I know what statement pertains to 50,51... etc. I don't get this. Thank you in advance.

This problem will give you a chance to reverse engineer a switch statement from machine code. In the following procedure, the body of the switch statement has been removed:

1 int switch_prob(int x, int n)
2 {
3     int result = x;
4
5     switch(n) {
6
7          /* Fill in code here */    
8     }    
9 
10     return result;    
11 }

Figure 3.44 shows the disassembled machine code for the procedure. We can see in lines 4 and 5 that parameters x and n are loaded into registers %eax and %edx, respectively.

The jump table resides in a different area of memory. We can see from the indirect jump on line 9 that the jump table begins at address 0x80485d0. Using the gdb debugger, we can examine the six 4-byte words of memory comprising the jump table with the command x/6w 0x80485d0. gdb prints the following:

(gdb) x/6w 0x80485d0
0x80485d0: 0x08048438 0x08048448 0x08048438 0x0804843d
0x80485e0: 0x08048442 0x08048445

Fill in the body of the switch statement with C code that will have the same behavior as the machine code.

1  08048420 <switch_prob>:
2  8048420: 55                      push %ebp
3  8048421: 89 e5                   mov %esp,%ebp
4  8048423: 8b 45 08                mov 0x8(%ebp),%eax
5  8048426: 8b 55 0c                mov 0xc(%ebp),%edx
6  8048429: 83 ea 32                sub $0x32,%edx
7  804842c: 83 fa 05                cmp $0x5,%edx
8  804842f: 77 17                ja 8048448 <switch_prob+0x28>
9  8048431: ff 24 95 d0 85 04 08    jmp *0x80485d0(,%edx,4)
10  8048438: c1 e0 02               shl $0x2,%eax
11  804843b: eb 0e                  jmp 804844b <switch_prob+0x2b>
12  804843d: c1 f8 02               sar $0x2,%eax
13  8048440: eb 09                  jmp 804844b <switch_prob+0x2b>
14  8048442: 8d 04 40               lea (%eax,%eax,2),%eax
15  8048445: 0f af c0               imul %eax,%eax
16  8048448: 83 c0 0a               add $0xa,%eax
17  804844b: 5d                     pop %ebp
18  804844c: c3                     ret

Figure 3.44