Date: Fri, 04 Mar 2011 01:09:23 -0800 From: perryh@pluto.rain.com To: ertr1013@student.uu.se Cc: frank@solensky.org, freebsd-questions@freebsd.org Subject: Re: Bit order == byte order?? Message-ID: <4d70ac43.iPJOAUhHtgE0uJR%2B%perryh@pluto.rain.com> In-Reply-To: <20110303205010.GA47653@owl.midgard.homeip.net> References: <910E776A-D865-4F78-8BE5-E974326636D0@solensky.org> <20110303205010.GA47653@owl.midgard.homeip.net>
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Erik Trulsson <ertr1013@student.uu.se> wrote: > On Thu, Mar 03, 2011 at 11:26:12AM -0500, Frank Solensky wrote: > > In sys/netinet/ip.h, the first octet of the ip header structure > > tests the byte ordering to determine the ordering of the header > > length (ip_hl) and version (ip_v) fields. > > > > My question: that always works? While my reading of the > > language specification document leaves both the ordering of > > the bits within a byte and the bytes within a longer field as > > implementation choices, the two are independent of each other. > > > > I haven't run into a CPU where this assumption was proven > > incorrect ... > > Unless you have a CPU where memory is addressed bit-by-bit rather > than byte-by-byte the ordering of bits within a byte is not only > completely irrelevant, it is also pretty much impossible to > determine programatically. Agreed it is at least difficult to determine programatically, however it is quite important when dealing with hardware that converts between a sequence of bytes and a bitstream, e.g. serial ports, network interfaces, SATA ports. Driver writers had _better_ know which bit of the byte, as well as which byte of a word/longword/quadword, is going on the wire first. The O.P. is absolutely correct that bit order within a byte and byte order within a multibyte field need not, in principle, be the same.
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