Best X86 assembler for Unix?



I am a very experienced, but rusty x86 assembly programmer
with a new AM2 Athlon64 chip that I want to code for in 64-bit
mode. Is there an alternative assembler to (G)AS for Athlon 64
that anyone can recommend? (I checked out nasm, but the one I
downloaded had no reg defs for AMD's new registers. I'm running
64-bit OpenBSD).

Thanks,
Dave Feustel

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