Re: OT: kernel in C? how translated? [Was Re: Help]
At Sat, 11 Dec 2004 09:39:51 -0600,
Kent West wrote:
>
> Samuel:
>
> I've changed your subject line to something more meaningful, and have
> routed it back to the list so that others who actually know these things
> can address your questions; I'm not a programmer, and have never delved
> into the inner workings of the kernel. However, this is also somewhat
> off-topic for the Debian list so it's marked "OT"; still, some of the
> fine folks on this list might can address your question and perhaps
> steer you towards resources that are better suited for your questions.
>
> samuel prakash wrote:
>
> > I have read that kernel is the real operating system and linux is the
> > name of the kernel.
>
> The kernel is the core of the operating system. The operating system is
> composed of the kernel and any utilities that ride on top of that
> kernel. Debian/GNU Linux is a GNU operating system using the Linux kernel.
>
The kernel is the operating system itself, the thing that allows everything to
run. Everything above it is part of the distribution and not the operating
system and its whats called "user space" since it doesn't access the machine
directly but asks the kernel to do it.
>
> > can u tell me whether linux kernel is written in
> > "c" or in "assembly level language".
>
> Most of it is written in C, as that is much easier to program in than
> assembly. However, assembly might be used in some areas whenever the
> programmer needs more intimate access to the hardware than C might provide.
>
c is also much more portable, so everything that can be writen in c, is. This
way the main bulk of the kernel code can be used also on mac, irix, sun, silicon
graphixs, etc. The only thing that needs to be changed are a few platform
specific things that have to be writen in assembly, or support for hardware that
is only available on some of the platforms.
>
> > I have also read that cpu
> > understands only machine level language if so then how "c" and
> > "assembly level language" is converted in to machine level language.
>
Assemply is machine language in human readable format. It translates directly to
zeros and ones that the machine can read by a simple comverter (there are tools
that take the actual program and display it in assembly form and this is very
easy to do.
c is converted to assembly and then the actual ones and zeros using something
called a compiler. It knows how to take the c code and convert it to assembly
code.
> Assembly is just a step above machine level language; any competent
> programmer could convert assembly to machine code. However, that's
> tedious and error prone, so programmers throughout the years have
> written translation programs, called assemblers, that do the work for them.
>
> For example, an assembly instruction to add the decimal numbers 19
> (which is 13 in hex) and 43 (which is 2b in hex) (Kcalc has a mode that
> will help you convert hex numbers to decimal and/or binary and
> vice-versa) might be
>
> add 13,2B
>
> The programmer would look up the corresponding hex code for "add"; this
> code will be different depending on what CPU is being used (a Pentium,
> or a Sparc, or a PowerPC chip, etc), and the code chart is available in
> books, online, from publications by the CPU manufacturer, etc.
>
> Let's pretend the hex code for "add" in our little example is A9. So,
> the above command in pure hex would be
>
> A9 13 2B
>
> Then that converts into binary quite easily. You can see a decimal to
> hex to binary chart at the bottom of this page:
>
> http://www.computerhope.com/binhex.htm
>
> So to convert our hex instruction to binary, we convert each digit, such
> that A becomes 1010, and 9 becomes 1001, etc. The converted instruction
> looks like
>
> 1010 1001 0001 0011 0010 1011
>
> This machine code essentially means that of the 24
> transistors/capacitors/switches/magnetic dots on a tape/burned
> depressions in a CD/etc needed to store that info, the first switch/mag
> dot/etc is on, the next is off, the next is on, the next is off, then
> on, off, off, on, off, off, off, on, etc.
>
> The CPU understands switches being on and off, so it can understand this
> code.
>
> Look at the light switch on the wall of the room you're in. Just a
> simple switch; it can be on or off. Now imagine a group of four of those
> next to each other. This is equivalent to a "nybble". Now imagine a
> second set of four next to the first set of four. Now you have eight
> switches. Eight switches, or two nybbles, make a byte. You're familiar
> with the term byte; you may have 128 Mega_bytes_ of RAM in your
> computer. 128 MB of RAM is roughly 128 million groups of eight switches.
> Unlike English, which can have words of four letters or 5 or 2 or 9 or
> whatever, "words" (for the purposes of this conversation) in computer
> language are usually always 8 "letters" long; thus the "byte" (there's
> nothing particularly "magic" about this length; it's just what
> programmers agreed on throughout the years). And as each letter can only
> be on or off, just two states, rather than the couple of hundred or so
> characters used in English (26 letters, upper and lower case, plus
> punctuation and number symbols, etc), the language is referred to as
> "binary".
>
> So, if you had 6 sets of four light switches on your wall (and it'd be
> easier to "read" them if they were grouped as three sets of two sets of
> four, like so
>
> 1010 1001 0001 0011 0010 1011
>
> which is the same as above, just spaced differently), then you could
> program your lights with our instruction above. (Not that the lights
> could understand this instruction, of course, but replace your lights
> with the appropriate CPU and you've just instructed it to add two numbers.)
>
> C code is considerably "higher". Whereas a programmer could
> theoretically convert C to machine code, it would be much too difficult
> and error prone to do for any practical purpose. Instead, conversion
> programs called compilers have been written to do the translation work.
> gcc is such a compiler.
>
> To get a feel for how C code is converted into a language used by the
> kernel, here's a simple little example:
>
> http://www.faqs.org/docs/kernel/x145.html
> along with the next page after it (click on "Next" at top or bottom of
> that page)
>
> You might want to read more of this book than just the two pages I
> referenced; it might shed some light on the subject for you.
>
> Also, searching the Internet (google is your friend) might get your more
> and better information than going straight to the members of a mailing
> list. There's lots of good info out there.
>
> Hope this helps!
>
> --
> Kent
>
>
> --
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