Date: Fri, 4 Mar 2011 10:57:16 +0100 From: Erik Trulsson <ertr1013@student.uu.se> To: perryh@pluto.rain.com Cc: frank@solensky.org, freebsd-questions@freebsd.org Subject: Re: Bit order == byte order?? Message-ID: <20110304095716.GA51831@owl.midgard.homeip.net> In-Reply-To: <4d70ac43.iPJOAUhHtgE0uJR%2B%perryh@pluto.rain.com> References: <910E776A-D865-4F78-8BE5-E974326636D0@solensky.org> <20110303205010.GA47653@owl.midgard.homeip.net> <4d70ac43.iPJOAUhHtgE0uJR%2B%perryh@pluto.rain.com>
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On Fri, Mar 04, 2011 at 01:09:23AM -0800, perryh@pluto.rain.com wrote: > 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. Although it certainly matters for serial I/O devices which bit goes out first on the wire, you only need to know if the I/O hardware will push out data with the most significant bit first or the least significant bit first and possibly adjust the values you write to the I/O hardware if it does not match the order you want the bits to go out. You still don't need to know anything about in which order bits are stored in a byte inside the CPU or in RAM. -- <Insert your favourite quote here.> Erik Trulsson ertr1013@student.uu.se
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