here is some hello world function that would just add 10 to whatever number it is given
#Simple adder function in assebmly - just adds 10 the called number
.code32 #if running the code on 64 bit machine
.section .data
.section .text
.globl _start
_start: #entry point
#Assign the number to be modified here
push $9 # using immediate access mode just assign number 9
call adder10 #call the function adder10 here - basically assign a memory address name
mov %eax, %ebx #pass the return value to ebx since return value sits on eax
mov $1, %eax # call system call exit on register eax
int $0x80 #yup - interrupt it!
.type adder10, @function # like declare a function here ;)
adder10:
#save the current base pointer first
push %ebp
#load the current stack pointer to the base pointer and use the new base pointer in the function
mov %esp, %ebp
#load the paramter on register ecx
mov 8(%ebp), %ecx #access it using base index access method
add $10, %ecx #do the magic
#the return value will be on %eax
mov %ecx, %eax
#return the original values to the ebp and esp
mov %ebp, %esp
pop %ebp
ret
Save the above file as adder.s
to run this if you are on 64 bit machine use
gcc -m32 -nostartfiles -o adder adder.s
otherwise
as -o adder.o adder.s ld -o adder adder.o
and run it as
./adder
to see the result
echo $?
If all ok the above should print 19