Say hello to x86_64 Assembly [part 5]

http://0xax.blogspot.sg/2014/11/say-hello-to-x8664-assembly-part-5.html

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

view rawgistfile1.asm hosted with ❤ by GitHub

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

view rawgistfile1.asm hosted with ❤ by GitHub

and than use it in code: 

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

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

	%macro number_of_parameters
	instruction
	instruction
	instruction
	%endmacro

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For example: 

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

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And we can use it: 

	_start:
	bootstrap

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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

view rawgistfile1.asm hosted with ❤ by GitHub

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 writesystem 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

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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!"

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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

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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]

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%include

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

Conclusion

It was a fifth part of series 'say hello to x64 assembly', if you will have a questions/suggestions write me a comment or ping me at twitter for discussing. In next part we will talk about difference between Intel syntax and AT&T and look at gas assembly. If you're interesting in some additional themes about assembly and x86_64 programming write me a comment and I will try to write blog post about it in near time. 

All another parts you can find - here. 

All source code you can find as every time - here. 

English is not my first language, so you'll find mistakes in blog post please write me in comments or drop me email .