Arrays and Pointers

1
Arrays and Pointers

• Variables are just names of memory locations
• An array is a variable (just with many memory
  locations all contiguous)
• Address pointer (in function, not C syntax)
• Address pointer in assembler (no data typing)

2
Allocating an Array

• You have to know the size in bytes of each
  element of the array
• Int 4 bytes
• Float 4 bytes
• Char 1 byte
• Determine the number of bytes needed
• Sizequantity
• Allocate space (in bytes) arr .space 100

3
Memory Access

• Remember, the REAL assembly language can only
  access memory via number(R)
• Or disp(base) where disp is a number and base is
  a register
• The address is the value in the base register
  added to the displacement

4
The la command

• The la (load address) macro command is very
  helpful (especially with arrays)
• We will load the address of the data section.
• The data section always starts at memory location
  x10010000.
• Look at the first 4 hex char (x1001).
• This is 4097 in decimal

5
The lui command

• Used by the assembler a lot
• Some instructions only have a 16 bit operand
• To get a 32 bit value into a register, we need
  two instruction
• lui load upper immediate
• Loads the upper 2 bytes (16 bits) of the register
  with the specified value and places 0s in the
  last 16 bits
• ori or immediate logically ors the last 16
  bits of a register with the immediate operand.

6
Back to the la

• So, to load the 32 bit address of a variable into
  t1, the assembler uses
• lui t1,4097 now t1 has the addr of the
  data section
• ori t1,offset the distance the var is from
  the beginning of the .data
• This replaces the la t1,var instruction

7
Working with Arrays

• First, get the address of the array into a
  register
• la t1,arr
• To get to element i of an array, you multiply i
  by the size of an element and add that to the
  base address of the array.
• This gives the address of arri

8
Processing an entire array

• If you are looping through an array, rather than
  multiply, you could just add the size of an
  element to the current address to get the
  address of the next element
• Example for (i0 ilt25 i) arrii

9
In assembler

• In the .data section
• arr .space 100
• In the .text section la t1,arr li t2,0
  i li t3,25 limit
• loop beq t2,t3,done sw t2,0(t1) addi t1,4
  address increases by 4
• addi t2,1 counter increases by 1 j loop
• done

10
Other examples

• Look at the reverse string example
• Look at the binary search example
• Look at the sorting example