What are Arrays in Assembly Language?

By: Rajesh P.S.

An array in assembly language is a contiguous block of memory that contains a series of elements of the same data type. Arrays are used to store data that is related to each other, such as a list of student names or a table of numbers.

To declare an array in assembly language, the programmer uses a directive such as .data or .bss. The directive specifies the name of the array, the data type of the elements of the array, and the number of elements in the array.

Declaration and Initialization

Arrays are declared and initialized using data directives, specifying the size and type of elements.

; Example: Declaring and initializing an array of integers MyArray DWORD 10 DUP(0) ; Declare an array of 10 DWORDs, initialized to 0

The following assembly instruction declares an array called my_array that contains 100 integer elements:

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my_array: .data 100 * .word

The .data directive tells the assembler that the array my_array is located in the data section of the program. The .word directive tells the assembler that the elements of the array are 32-bit integers.

Accessing Array Elements

Once an array has been declared, the programmer can access the elements of the array using an index. The index is a number that identifies the position of the element in the array. The first element of the array has index 0, the second element has index 1, and so on. To access an element of an array in assembly language, the programmer uses the square bracket ([]) operator.

; Example: Accessing array elements MOV EAX, MyArray[2] ; Load the value of the third element (index 2) into EAX

Looping Through Arrays

Loops are commonly used to iterate through array elements, performing operations on each element.

; Example: Looping through and summing array elements MOV ECX, LENGTHOF MyArray ; Set the loop counter to the array length XOR EAX, EAX ; Clear EAX (accumulator) to store the sum MOV EDI, OFFSET MyArray ; Set EDI to point to the start of the array SumLoop: ADD EAX, [EDI] ; Add the current element to the sum ADD EDI, 4 ; Move to the next DWORD (4 bytes) in the array LOOP SumLoop ; Repeat the loop until ECX becomes zero

Multidimensional Arrays

Multidimensional arrays can be simulated using nested loops and appropriate indexing.

; Example: Accessing a 2D array MOV ECX, LENGTHOF Rows MOV EDX, LENGTHOF Cols OuterLoop: MOV ESI, ECX ; Use ESI as the row index InnerLoop: MOV EDI, EDX ; Use EDI as the column index IMUL EDI, ESI, EDX ; Calculate the linear index in a 2D array MOV EAX, My2DArray[EDI] ; Load the value from the 2D array ; Perform operations on the array element LOOP InnerLoop LOOP OuterLoop

Strings as Arrays

Strings in assembly are often treated as arrays of characters.

; Example: Loading a string and accessing individual characters MOV EDX, OFFSET MyString ; Set EDX to point to the start of the string MOV AL, [EDX] ; Load the first character of the string into AL

Addressing Modes

Various addressing modes are used to access array elements, including direct addressing and indirect addressing.

; Example: Indirect addressing with arrays MOV ESI, OFFSET MyArray ; Set ESI to point to the start of the array MOV EBX, [ESI] ; Load the value of the first element into EBX

Examples:

Here are some additional examples of how arrays are used in assembly language:

section .data student_names db "Student1", 0 db "Student2", 0 db "Student3", 0 ; ... continue for 10 students section .text global _start _start: mov ecx, 0 ; Initialize loop counter loop_students: lea rdi, [student_names + rcx] ; Get the address of the current student name mov rax, 1 ; syscall: write mov rdi, 1 ; file descriptor: STDOUT mov rdx, 20 ; length of the string syscall inc rcx ; Increment loop counter cmp rcx, 10 ; Compare with the number of students jl loop_students ; Jump back to the loop if rcx is less than 10 ; Exit the program mov rax, 60 ; syscall: exit xor rdi, rdi ; status: 0 syscall section .data extern printf

This example shows how to use an array to store a list of names. The loop_students loop iterates over the elements of the array and prints each name to the console.

section .data numbers resd 100 ; Reserve space for 100 double words (32-bit integers) section .text global _start _start: mov ecx, 0 ; Initialize loop counter loop_generate_numbers: rdrand eax ; Generate a random number into eax mov [numbers + ecx * 4], eax ; Store the random number in the array inc ecx ; Increment loop counter cmp ecx, 100 ; Compare with the number of elements jl loop_generate_numbers ; Jump back to the loop if ecx is less than 100 ; Your code continues here (after generating the numbers) ; ... ; Exit the program mov eax, 1 ; syscall: exit xor ebx, ebx ; status: 0 int 0x80 ; call kernel

This example shows how to use an array to store a list of random numbers. The loop_generate_numbers loop generates 100 random numbers and stores them in the array numbers.

Conclusion

Arrays are contiguous blocks of memory used to store and manipulate collections of data. Elements in an array are accessed through indexing, and assembly instructions are employed to iterate through the array, perform operations on its elements, and manage data storage.

Next > Strings in Assembly Language

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