On Wed, Dec 22, 2010 at 12:24 AM, Jerry Feldman <g...@blu.org> 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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