Print a string
The same program in M68K, MIPS, RISC-V, Z80.
Every program in this ladder so far has left its answer in a register or in memory, where only you can see it. This one puts its answer in the console, which means it has to go outside the program altogether, and going outside the program takes exactly one instruction.
syscall takes no operands at all, which is why each of the three blocks here is a run of mov lines
and then one bare instruction. Every part of the request is in a register before it runs: what is being
asked for, where to send it, what to send, and how much.
write sends exactly the bytes you point it at and nothing else. It does not look for a
terminator, it does not stop at one if it finds one, and it adds nothing to the end. So the line break
after Hello, world! is the , 10 you can see in the db line, and the length is whatever
$ - greeting came out as. Take the , 10 out of the first string and the console reads
Hello, world!The answer is 42 on one line.
Nothing here prints a number, either. The answer is 42 is a string that has the characters 4 and
2 in it already, and a register holding the value 42 has nothing in common with those two bytes.
Turning one into the other is a program of its own, and it is the next Example.
mov rax, 60 and syscall is the request to exit, and it is the only thing that stops the program.
Take the last three lines out and execution carries straight on into whatever bytes follow them.