I am very new to assembly and I am wondering about the behavior of ret. If i never use a call during the code, but put a ret at the end, the code just goes back to the beginning. Is this a default behavior when there is no call used? If I used a jmp, would that affect the behavior of ret if there is still no call used?
call puts the next instruction's address into the call stack, then jumps to the target address.
ret pops the last address from the call stack, then jumps here. It seems that if the stack is empty, it returns 0x00000000, and it jumps to the start of the program. At least in some systems. You should not use this because of address space violations.
jmp has no effect on the call stack, so it won't modify the behavior of ret.
Edit: As @PeterCordes wrote; if you use push or pop, they can write stack memory, and ret would pop that.