ucexperiment.wordpress.com has published a series of tutorials on how to use assembly language on Arduino. This can be pretty helpful if you want to do some advantaged coding on Arduino.
An inline assembly statement is a string which specifies assembler code. The string can contain any instructions recognized by the assembler, including directives (we will not discuss assembler directives here). GCC does not parse the assembler instructions and does not know what they mean or even whether they are valid. Multiple assembler instructions can be placed together in a single asm string.
Arduino Inline Assembly Tutorial – [Link]