Clicker Questions - 13-Assembler
The following program has been compiled to jassem assembly code. At the point where the procedure b() calls the instruction ret, the value of the frame pointer is 0x100340.
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int a(int j, int k, int l)
{
int m[2];
m[0] = l;
m[1] = j + k + l;
return m[0] + m[1];
}
int b(int x, int y, int *p)
{
int *k;
k = p+x;
y = *k;
return y;
}
int main()
{
int i;
i = a(1, 2 ,3) + b(7 ,8 ,&i);
return i;
}
Please answer the following questions about the stack at the point when b() calls ret.
You don’t have to create assembly code for this – you simply need to know how it works. When I say “What is at address x”, you should answer something like “in a(0)” or “frame pointer for main()” .
If you can’t know then answer “unknown”.
Question 1: What is value of stack pointer?
Answer
Since b() has one local variable, which is an int, the stack pointer will be four less than the frame pointer: 0x10033c.Question 2: What is at address 0x10033c?
Answer
This address is equal to the stack pointer, so it does not correspond to a variable in b(). However, since a() has the same number of parameters as b(), its stack frame started in the same place as b()'s, and it has 8 bytes of local variables rather than four. Therefore, what's there is the leftover m[0] in a().Question 3: What is at address 0x100340?
Answer
This is k in b().Question 4: What is at address 0x100344?
Answer
This is the frame pointer for main(), which is stored by the call to "jsr b".Question 5: What is at address 0x100348?
Answer
This is pc+4 for main(), which is stored by the call to "jsr b".Question 6: What is at address 0x10034c?
Answer
This is x in b(), which is pushed onto the stack by main().Question 7: What is at address 0x100350?
Answer
This is y in b(), which is pushed onto the stack by main().Question 8: What is at address 0x100354?
Answer
This is p in b(), which is pushed onto the stack by main().Question 9: What is at address 0x100358?
Answer
This is the spilled value of r2. Main() has to store the return value of a(1,2,3) in r2, so it won't be destroyed by the call to b(). Since it uses r2, it must spill it after it allocates its local variable.Question 10: What is at address 0x10035c?