Say hello to x86_64 Assembly [part 5]

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:

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

and than use it in code:

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

For example:

And we can use it:

For example let's look at PRINT macro:

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:

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:

Useful standard macros


NASM supports following standard macros:

STRUC

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

And now we can make instance of our structure:

%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 .

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