In this article I will describe the IDE configuration process, writing the first Hello World assembler x86_64 for Ubuntu Linux operating system.
Let’s start with IDE SASM plant assembler nasm:
sudo apt install sasm nasm
Next, invoke SASM and write Hello World:
global main section .text main: mov rbp, rsp ; for correct debugging mov rax, 1 ; write( mov rdi, 1 ; STDOUT_FILENO, mov rsi, msg ; "Hello, world!\n", mov rdx, msglen ; sizeof("Hello, world!\n") syscall ; ); mov rax, 60 ; exit( mov rdi, 0 ; EXIT_SUCCESS syscall ; ); section .rodata msg: db "Hello, world!" msglen: equ $-msg
Hello World code is taken from the blog James Fisher, Adapted for assembling and debugging SASM. In SASM documentation states that the entry point must be a function named main, otherwise debug and compile code is incorrect.
What we did in this code? Made the call syscall – an appeal to the Linux operating system kernel with the correct arguments in registers, a pointer to a string in the data section.
Consider the code details:
global – assembler directive allows you to set global symbols with string names. A good analogy – interface header files C / C ++ languages. In this case, we ask the main character for the input function.
section – assembler directive allows define sections (segments) of code. Section directive or a segment equal. The .text section is placed code.
Announces the beginning of the main function. The assembler function called subroutines (subroutine)
mov rbp, rsp
The first machine instruction mov – puts the value of the argument 1 to argument 2. In this case, we transfer the register value in rbp rsp. Of comments you can understand that this line added SASM to simplify debugging. Apparently that is a personal affair between SASM and debugger gdb.
Next, look at the code to .rodata data segment, two call syscall, first outputs Hello World string exits from the second application with the correct code 0.
Let us imagine that the registers are variables with names rax, rdi, rsi, rdx, r10, r8, r9. By analogy with the high-level language, turn from vertical to horizontal view of the assembly, then the call syscall will look like this:
syscall(rax, rdi, rsi, rdx, r10, r8, r9)
Then the call to print text:
syscall(1, 1, msg, msglen)
Calling the exit with the correct code 0:
Consider the arguments in more detail in the header asm/unistd_64.h file find function __NR_write – 1, then look in the documentation for the arguments write:
ssize_t write (int fd, const void * buf, size_t count);
The first argument – the file descriptor, the second – the buffer with the data, the third – the counter bytes to write to a file handle. We are looking for the number of file descriptor for standard output, in the manual on stdout find the code 1. Then the case for small, to pass a pointer to the Hello World string buffer from the data section .rodata – msg, byte count – msglen, transfer registers rax, rdi, rsi, rdx correct has argument and call syscall.
Designation constant length lines and is described in manual nasm:
message db 'hello, world' msglen equ $-message
Simple enough right?