On Wed, Dec 22, 2010 at 12:24 AM, Jerry Feldman <gaf@xxxxxxx> wrote:
I learned C from K&R, from the tutorial that was included with Unix, and
the fact that I was told I had to maintain the Unix CShell. Fortunately
at the time I knew a number of languages including IBM 360/370,
AutoCoder, Burroughs, PDP-8, PDP-11, and Raytheon PTS1200/PTS100
assembly. My real learning experience was looking at the code generated
by COBOL. You can see how some compilers generate the code, but you can
also see how some decisions you make at the high level language affect
the actual code. I was taught that looping through an array by using
pointers was much faster than subscripting through the same array. BUT,
when working with the compiler group on Tru65 Unix I found that
subscripting was actually much faster when using an optimizing compiler
because the compiler must dereference every pointer, but when
subscripting, it can keep the array base in a register. I had some old C
code that I used to first test that pointers are better than
subscripting, and I then tested that in the Tru64 compiler on the Alpha,
and subscripted loops were faster.
Basically, the first rule I always use is to write code clearly and
concisely. Remember that someone (you or someone else) may have to debug
it. If you need to make the code work faster, then is the time to
analyze, and we have some really neat tools like graphical profilers
that can point out bottlenecks, and many times these surprise even the
most experienced programmers.
Basically, the CPU generally does a few things. While there are major
differences, between CISC and RISC, you load an address into a register,
perform some operation on that register, such as add, subtract,
multiply, divide, or, not, and, and a few more. Then you save the
contents into memory. Of course there are branch and conditional branch
where you go to a different part of the code based upon a true or false
condition. To really understand computer logic, an interesting approach
is to build a computer using plugboard parts. Remember, everything in a
computer is binary not decimal, not octal (3 bit representation of
binary), not hexadecimal (4 bit representation).
--
Jerry Feldman
This is much related with Assembly language.
--
Regards,
Parshwa Murdia
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