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)

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 –Size*quantity Allocate space (in bytes) arr:.space100

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

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 x –Look at the first 4 hex char (x1001). –This is 4097 in decimal

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 0’s in the last 16 bits –ori – or immediate – logically or’s the last 16 bits of a register with the immediate operand.

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

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 arr[i]

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 (i=0; i<25; i++) arr[i]=i;

In assembler In the.data section arr:.space100 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 jloop done:

Other examples Look at the reverse string example Look at the binary search example Look at the sorting example