In this problem, we will explore the logic behind the code in lines 5-11 of Figure 3.43(b), where space is allocated for variable-size array p. As the annotations of the code indicate, let us let s₁ denote the address of the stack pointer after executing the subq instruction of line 4. This instruction allocates the space for local variable i. Let s₂ denote the value of the stack pointer after executing the subq instruction of line 7. This instruction allocates the storage for local array p. Finally, let p denote the value assigned to registers %r8 and %rcx in the instructions of lines 10-11. Both of these registers are used to reference array p.
The right-hand side of Figure 3.44 diagrams the positions of the locations indicated by s₁, s₂, and p. It also shows that there may be an offset of e₂ bytes between the values of s₁ and p. This space will not be used. There may also be an offset of e₁ bytes between the end of array p and the position indicated by s₁.

A. Explain, in mathematical terms, the logic in the computation of s₂ on lines 5-7. 

B. Explain, in mathematical terms, the logic in the computation of p on lines 8-10.

C. For the following values of n and s₁, trace the execution of the code to determine what the resulting values would be for s₂, p, e₁, and e₂.

D. What alignment properties does this code guarantee for the values of s₂ and p?

Figure 3.43(b)

Transcribed Image Text:

n 5 6 $1 2,065 2,064 $2 P e1 2