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## PowerPoint Slideshow about ' Ch. 6 Arrays' - orla-spence

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### Ch. 6 Arrays

Comp Sci 251 - Arrays

Arrays

- Java: arrays are objects
- C++: arrays are primitive data
- For now: arrays as primitive data

Comp Sci 251 - Arrays

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'

Array declaration and initializationComp Sci 251 - Arrays

char a[100];

a[5] = '*';

.text

li $t0, '*'

li $t1, 5

sb $t0, a($t1)

.data

a: .space 100

Array element accessIndex addressing

Comp Sci 251 - Arrays

Addressing modes

Methods of specifying instruction operands

- Immediate

li $t0, 5 #operand is part of instruction

- Register direct

add $t0, $t0, $t1 #operand is in a register

- Memory direct

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

Comp Sci 251 - Arrays

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 indirect

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

Comp Sci 251 - Arrays

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 - Arrays

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

Exercise: Initialize array elementsComp Sci 251 - Arrays

Two-dimensional arrays

- Problem: map two-dimensional structure to one-dimensional memory
- Most compilers use "row-major" order

Column index

0

1

Row index

0

2

3

4

0

1

2

3

4

0

0

1

1

2

1

2

3

4

0

1

2

2

3

4

Comp Sci 251 - Arrays

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 - Arrays

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;

}

Comp Sci 251 - Arrays

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