Implementation closely follows plan9_386 version.

Change-Id: Ifb76e001fb5664e6a23541cf4768d7f11b2be68b
Reviewed-on: https://go-review.googlesource.com/18967Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>