gas: initial notes

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+# gas
+
+## Frequently used directives
+- `.byte`, `.2byte`, `.4byte`, `.8byte` to define a N byte value
+  ```x86asm
+  .byte 0xaa
+  .2byte 0xaabb
+  .2byte 0xaa, 0xbb
+  .4byte 0xaabbccdd
+  .8byte 0xaabbccdd11223344
+  ```
+- `.ascii` to define an ascii string
+  ```x86asm
+  .ascii "foo"   ; allocates 3 bytes
+  ```
+- `.asciz` to define an ascii string with `'\0'` terminator
+  ```x86asm
+  .asciz "foo"   ; allocates 4 bytes (str + \0)
+  ```
+- `.macro` to define assembler macros. Arguments are accessed with the
+  `\arg` syntax.
+  ```x86asm
+  .macro defstr name str
+  \name:
+      .ascii "\str"
+  \name\()_len:
+      .8byte . - \name
+  .endm
+
+  ; use as
+  defstr foo, "foobar"
+  ```
+  > Use `\()` to concatenate macro argument and literal.
+
+
+- [GNU Assembler][gas_doc]
+- [GNU Assembler Directives][gas_directives]
+- [GNU Assembler `x86_64` dependent features][gas_x86_64]
+
+[gas_doc]: https://sourceware.org/binutils/docs/as
+[gas_directives]: https://sourceware.org/binutils/docs/as/Pseudo-Ops.html#Pseudo-Ops
+[gas_x86_64]: https://sourceware.org/binutils/docs/as/i386_002dDependent.html