It's a power-of-2 alignment, e.g., ALIGN(4)
for 16-byte alignment, ALIGN(5)
for 32-byte, etc. The 0x90
specifies the opcode for the NOP
instruction - to be used as the padding instructions to achieve the alignment.
There are longer instructions sequences that are also effective NOPs
. Many of the assemblers support the more flexible .p2align
directive; all recent GNU-based assemblers for example.