Assembly - Arrays
We have already discussed that the data definition directives to the assembler are used for allocating storage for variables. The variable could also be initialized with some specific value. The initialized value could be specified in hexadecimal, decimal or binary form.
For example, we can define a word variable months in either of the following way:
MONTHS DW 12 MONTHS DW 0CH MONTHS DW 0110B
The data definition directives can also be used for defining a one-dimensional array. Let us define a one-dimensional array of numbers.
NUMBERS DW 34, 45, 56, 67, 75, 89
The above definition declares an array of six words each initialized with the numbers 34, 45, 56, 67, 75, 89. This allocates 2x6 = 12 bytes of consecutive memory space. The symbolic address of the first number will be NUMBERS and that of the second number will be NUMBERS + 2 and so on.
Let us take up another example. You can define an array named inventory of size 8, and initialize all the values with zero, as:
INVENTORY DW 0 DW 0 DW 0 DW 0 DW 0 DW 0 DW 0 DW 0
Which can be abbreviated as:
INVENTORY DW 0, 0 , 0 , 0 , 0 , 0 , 0 , 0
The TIMES directive can also be used for multiple initializations to the same value. Using TIMES, the INVENTORY array can be defined as
INVENTORY TIMES 8 DW 0
The following example demonstrates the above concepts by defining a 3-element array x, which stores three values: 2, 3 and 4. It adds the values in the array and displays the sum 9:
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, x ;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 x: db 2 db 4 db 3 sum: db 0
When the above code is compiled and executed, it produces the following result: