asm_5.md

It is a fifth part of Say hello to x86_64 Assembly and here we will look at macros. It will not be blog post about x86_64, mainly it will be about nasm assembler and it's preprocessor. If you're interesting in it read next.

Macros

NASM supports two form of macro:

• single-line
• multiline

All single-line macro must start from %define directive. It form is following:

%define macro_name(parameter) value

Nasm macro behaves and looks very similar as in C. For example, we can create following single-line macro:

%define argc rsp + 8
%define cliArg1 rsp + 24

and than use it in code:

;;
;; argc will be expanded to rsp + 8
;;
mov rax, [argc]
cmp rax, 3
jne .mustBe3args

Multiline macro starts with %macro nasm directive and end with %endmacro. It general form is following:

%macro number_of_parameters
    instruction
    instruction
    instruction
%endmacro

For example:

%macro bootstrap 1
          push ebp
          mov ebp,esp
%endmacro

And we can use it:

_start:
    bootstrap

For example let's look at PRINT macro:

%macro PRINT 1
    pusha
    pushf
    jmp %%astr
%%str db %1, 0
%%strln equ $-%%str
%%astr: _syscall_write %%str, %%strln
popf
popa
%endmacro

%macro _syscall_write 2
	mov rax, 1
        mov rdi, 1
        mov rsi, %%str
        mov rdx, %%strln
        syscall
%endmacro

Let's try to go through it macro and understand how it works: At first line we defined PRINT macro with one parameter. Than we push all general registers (with pusha instruction) and flag register with (with pushf instruction). After this we jump to %%astr label. Pay attention that all labels which defined in macro must start with %%. Now we move to __syscall_write macro with 2 parameter. Let's look on __syscall_write implementation. You can remember that we use write system call in all previous posts for printing string to stdout. It looks like this:

;; write syscall number
mov rax, 1
;; file descriptor, standard output
mov rdi, 1
;; message address
mov rsi, msg
;; length of message
mov rdx, 14
;; call write syscall
syscall

In our __syscall_write macro we define first two instruction for putting 1 to rax (write system call number) and rdi (stdout file descriptor). Than we put %%str to rsi register (pointer to string), where %%str is local label to which is get first parameter of PRINT macro (pay attention that macro parameter access by $parameter_number) and end with 0 (every string must end with zero). And %%strlen which calculates string length. After this we call system call with syscall instruction and that's all.

Now we can use it:

label: PRINT "Hello World!"

Useful standard macros

NASM supports following standard macros:

STRUC

We can use STRUC and ENDSTRUC for data structure defintion. For example:

struc person
   name: resb 10
   age:  resb 1
endstruc

And now we can make instance of our structure:

section .data
    p: istruc person
      at name db "name"
      at age  db 25
    iend

section .text
_start:
    mov rax, [p + person.name]

%include

We can include other assembly files and jump to there labels or call functions with %include directive.