How do you write comments in GAS (GNU Assembler) syntax, and what are the rules for how lines and whitespace work?

Single-line comments start with # and run to end of line; multi-line comments use /* ... */.

GAS treats the layout of an assembly file very loosely, which trips up people used to C's syntax:

.text
.globl main
/*
 * main function
 */
main:
    movl $1, %eax    # set %eax to 1
    ret              # return to caller

Key rules:

• A newline ends an instruction. There's no ; statement terminator — one instruction per line.
• Whitespace is ignored. Indentation is purely cosmetic; the assembler doesn't care if you indent or not.
• No blocks. Assembly has no { } scoping — structure comes only from labels and jumps.
• /* */ outranks #, and you cannot nest same-style comments (a /* inside a /* */ won't behave as you'd expect).

Tip: Because a .S file is run through the C preprocessor, a stray # can also look like a preprocessor directive — keep # comments clearly separated from #define lines.

Go deeper:

• Using as — the GNU Assembler manual — the manual's line, whitespace and comment syntax.
• GNU Assembler (Wikipedia) — an overview of GAS and its per-architecture quirks.