Array allocation

• Consider the following declarations in C:

char a[12];
char *B[8];
int C[6];
double *D[5];
int P[5];
short Q[2];
int **R[9];
double *S[10];
short *T[2];

Array	Element size	Total size	Start address	Element i
A	1	12	xA	xA + i
B	8	64	xB	xB + 8i
C	4	24	xC	xC + 4i
D	8	40	xD	xD + 8i
P	4	20	xP	xP + 4i
Q	2	4	xQ	xQ + 2i
R	8	72	xR	xR + 8i
S	8	80	xS	xS + 8i
T	8	16	xT	xT + 8i

Pointer arithmetic

• Example: suppose the starting address of integer array E and integer index i are stored in registers %rdx and %rcx, respectively.

  

• TODO: exercise 3.37

Nested arrays

• In general, for an array declared as

  T D[R][C];

array element D[i][j] memory address is calculated as:

• &D[i][j] = x_D + L(C*i + j)
• where L is the size of data type T in bytes.
• C = number of columns, R = number of rows

• Example: consider this 2-D array in C

int a[5][3]; //Viewing a as an array of 5 elements, each of which is an array of 3 int's

• Suppose x_A, i and j are in registers %rdi, %rsi, %rdx respectively
• Then array element A[i][j] can be copied to register %eax by the following code:

  leaq (%rsi,%rsi,2), %rax
  leaq (%rdi,%rax,4), %rax
  movl (%rax,%rdx,4), %eax

Fixed-size arrays

Variable-size arrays