Comp Sci 251 - Arrays1

Ch. 6 Arrays

Comp Sci 251 - Arrays2

Arrays

Java: arrays are objects C++: arrays are primitive data

For now: arrays as primitive data

Comp Sci 251 - Arrays3

Array declaration and initialization

int x[100];

char y[100];

int z[3]={4,-5,7};

// c++

char s[2]= {'h','i'};

// c++

x: .space 400

y: .space 100

z: .word 4, -5, 7

s: .byte 'h', 'i'

Comp Sci 251 - Arrays4

Array element access

char a[100];

a[5] = '*';

.text

li $t0, '*'

li $t1, 5

sb $t0, a($t1)

.data

a: .space 100

Index addressing

Comp Sci 251 - Arrays5

Addressing modes

Methods of specifying instruction operands Immediate

li $t0, 5 #operand is part of instruction

Register directadd $t0, $t0, $t1 #operand is in a register

Memory direct sw $t0, x #operand is in memory at address x

Comp Sci 251 - Arrays6

Addressing modes

Indexed

sw $t0, x($t1) #addr of operand is x + $t1

Base register

sw $t0, 4($t1) #addr of operand is 4 + $t1

Register indirectsw $t0, ($t1) #addr of operand is in $t1

Comp Sci 251 - Arrays7

Examples of addressing modes:

Study base_register.a in

/shared/huen/251/ch06_array indexed.a in /shared/huen/251/ch06_array arraysP91.a in /shared/huen/251/ch06_array length.a in /shared/huen/251/ch06_array

Comp Sci 251 - Arrays8

Exercise: Initialize array elements

char x[50];

int y[100];

int i;

for(i=0; i<50; i++)

x[i] = '*';

for(i=0; i<100; i++)

y[i] = 0;

.text

.data

x: .space 50

.align 2

y: .space 400

i: .space 4

Comp Sci 251 - Arrays9

Two-dimensional arrays

Problem: map two-dimensional structure to one-dimensional memory

Most compilers use "row-major" order

0

1

2

01234

Column indexRow index

0123401234

10

0

2

21 3 4

Comp Sci 251 - Arrays10

Storage mapping function

address of a[i , j ] =

b + e(ir + j)

e element size (bytes)

b base address of array

r row size = # columns

Comp Sci 251 - Arrays11

Exercise: Translate to MIPS

int x[100][50];

int n;

x[n / 10, 2 * n + 1] = 7;

Comp Sci 251 - Arrays12

Exercise: compute row sums

int x[3][5];

int sum[3];

int i, j, total;

for(i = 0; i < 3; i++){

total = 0;

for(j = 0; j < 5; j++)

total += x[i][j];

sum[i] = total;

}