Arrays are used in assembly language to store multiple data of the same type at once. That is, you can consider it as a table or list that stores data of the same size and type. Like a variable has the address of a data. Similarly array contains address of multiple data which you can get back through index.
Assembly Arrays - Syntax
As the initial value in the variable is specified in hexadecimal, decimal or binary form.
For example, we can define a word variable 'month' in any of the following ways − MONTHS DW 12
MONTHS DW 0CH
MONTHS DW 0110B Similarly data definition instructions can also be used to define one-dimensional arrays.
Arrays define in Assembly language
An assembly language instruction is needed to define the size of each element in the array. First of all the identifier is the variable, then there is the data type and then there is the value list.
identifier: datatype item 1 , item 2 , item 3 , item 4 , We further make this one dimensional word array. Each element of which is 2 bytes long and 'AbList' is assigned as its identifier. The data type for the DW instruction word is:
Define a one dimensional array of Numbers.
AbList : DW 84, 35, 57, 27, 15, 29 The above definition declares an array of six words each starting with the numbers 84, 35, 57, 27, 15, 29. It allocates 2x6 = 12 bytes of contiguous memory space. The coded address of the first number will be AbList and the coded address of the second number will be AbList + 2 and so on.
You can do it like this also. and can initialize all values with zero, like -
INVENTORY DW 0
DW 0
DW 0
DW 0
DW 0
DW 0
DW 0
DW 0 And in this way too. Using TIMES, the Inventory array can be defined as:
INVENTORY TIMES 8 DW 0 The TIMES instruction can also be used for multiple initializations for the same value.
Getting data value from array in assembly
To access array elements, we obtain a pointer to the element we want to access. Then you can get the data back through the index.
The following snippet of code shows how to do this.
mov ecx, [esi];pointer to element at the 0th index stored in the register ecx
mov ecx, [esi+1];pointer to element at the 3rd index stored in the register ecx
mov ecx, [esi+2];pointer to element at the 2nd index stored in the register ecx
Assembly Arrays - Example
The following example demonstrates the above concepts by defining a 3-element array x that stores three values: 1, 4, and 3. This adds the values to the array and displays the sum 8
section .text
global _start ;must be declared for linker (ld)
_start:
mov eax,3 ;number bytes to be summed
mov ebx,0 ;EBX will store the sum
mov ecx, a ;ECX will point to the current element to be summed
top: add ebx, [ecx]
add ecx,1 ;move pointer to next element
dec eax ;decrement counter
jnz top ;if counter not 0, then loop again
done:
add ebx, '0'
mov [sum], ebx ;done, store result in "sum"
display:
mov edx,1 ;message length
mov ecx, sum ;message to write
mov ebx, 1 ;file descriptor (stdout)
mov eax, 4 ;system call number (sys_write)
int 0x80 ;call kernel
mov eax, 1 ;system call number (sys_exit)
int 0x80 ;call kernel
section .data
global x
a:
db 1
db 4
db 3
sum:
db 0When the above code is compiled and executed, it produces the following result −
8
The 2-dimensional array is also similar, but instead of the data address, it has the address of another 1-dimensional array. A 2 dimensional array is a set of 1 dimensional array list list. In which there is array inside array and then there is data.
