Array Basics

Array Basics

• Arrays are dedicated blocks of memory for storing lists of values of the same type (ie size)
• Almost all programming languages support arrays:
• E.G in C programming: int somearray[10]; Defines a block of memory referred to as “somearray”, which holds 10 integer (32 bit) values

int somearray[10];

Using memory in ARMlite, the above could translate to a block of memory like this.

Array basics

int somearray[10];

Using memory in ARMlite, the above could translate to a block of memory like this.

“somearray” refers to an address, marking the start of the allocated block

Array basics

int somearray[10];

somearray[3] = 5;

Once declared, somarray can be bit/4 byte) integer within the array.

In above case, it is assigning the

Array basics

int somearray[10];

somearray[3] = 5;

How does this translate to ARM assembly?

Array basics

int somearray[10];

somearray[3] = 5;

How does this translate to ARM assembly? somearray:.Word

We simply define a label!

Array basics

int somearray[10];

somearray[3] = 5;

How does this translate to ARM assembly?

We simply define a label!

Array basics

int somearray[10];

somearray[3] = 5;

How does this translate to ARM assembly?

somearray:.Word 0 0 0 0 0 0 0 0 0 0

error correct: these values should be actually be one per line in actual code

Array basics

int somearray[10];

somearray[3] = 5;

How does this translate to ARM assembly?

This involved a few steps so let’s unpack it

somearray:.Word 0 0 0 0 0 0 0 0 0 0

error correct: these values should be one per line in actual code

Array basics

int somearray[10];

somearray[3] = 5;

We have the address of the array which needs to be stored in a register

MOV R0, #somearray

somearray:.Word 0 0 0 0 0 0 0 0 0 0

error correct: these values should be actually be one per line in actual code

Array basics

int somearray[10];

somearray[3] = 5;

We have an index to a 32 bit integer within the array.

Address of this value will be 3 x 4 = 12 bytes from start of array

We can store this in a register

MOV R0, #somearray
MOV R1, #12

somearray:.Word 0 0 0 0 0 0 0 0 0 0

error correct: these values should be actually be one per line in actual code

Array basics

int somearray[10];

somearray[3] = 5;

We can keep the value to store in another register

MOV R0, #somearray
MOV R1, #12
MOV R2, #5

somearray:.Word 0 0 0 0 0 0 0 0 0 0

error correct: these values should be actually be one per line in actual code

Array basics

int somearray[10];

somearray[3] = 5;

Finally, we can write the value to the array using STR

MOV R0, #somearray
MOV R1, #12
MOV R2, #5
STR R2, [R0 + R1]

somearray:.Word 0 0 0 0 0 0 0 0 0 0

error correct: these values should be actually be one per line in actual code

Array basics

int somearray[10];

somearray[3] = 5;

MOV R0, #somearray
MOV R1, #12
MOV R2, #5
STR R2, [R0 + R1]

HALT We make sure program is halted so it does not try and somearray:.Word 0 0 0 0 0 0 0 0 0 0 execute anything in the array

error correct: these values should be actually be one per line in actual code

Array Basics

• Lets have a look at the code in ARMLite

Some array definition for single byte arrays

• For a generic array of single bytes: somebytearray:.Byte 1 3 221 56
• For an array of ASCII characters (ie., a string): somestring:.ASCIZ “hello world\n”

Some array definition for single byte arrays

• For a generic array of single bytes: somebytearray:.Byte 1 3 221 56
• For an array of ASCII characters (ie., a string): somestring:.ASCIZ “hello world\n” This directive ensures a character per byte, and the appending of a NULL character at the end of the string. An alternative is.ASCII, which does not append the NULL character

Memory Alignment

• Memory Alignment is a common requirement:
• hardware often imposed alignment requirements to ensure integrity
• When defining byte addressable arrays, precede the array definitions with.Align.
• For example: somestring:.ASCIZ “hello world\n”.ALIGN N ensures the next instruction is aligned with a word address divisible by N (in general, and multiple of 4 is generally fine).