C learning

[andri]

But a person who has used ASM would definitely know that it can be optimized...

Code: [Select]

int a, c;

a=450;
c= a>>5;


This is not really about knowing assembly, rather about general base-2 calculations.

In fact, any decent C compiler will happily do it for you. Example: I compiled with gcc 4.1.2 (with flags -m32 -march=i386 for simplicity)  the following function:

Code: [Select]

int divide (unsigned int a) {
        unsigned int b = 32;
        return a / b;
}

(I had to move it into a function, otherwise the compiler optimized the whole division away.)

With -O0 optimizations (everything turned off) it indeed compiles into rather dumb division:

Code: [Select]

divide:
        pushl   %ebp
        movl    %esp, %ebp
        subl    $16, %esp
        movl    $32, -4(%ebp)
        movl    8(%ebp), %eax
        movl    $0, %edx
        divl    -4(%ebp)
        leave
        ret

However, even with -O1 it does the optimization for us:

Code: [Select]

divide:
        pushl   %ebp
        movl    %esp, %ebp
        movl    8(%ebp), %eax
        shrl    $5, %eax
        leave
        ret

(Quick guide to i386 assembly: "shrl" is logical shift right, "divl" is unsigned division).

[migsantiago]

This is not really about knowing assembly, rather about general base-2 calculations.

Yeah, I was just looking for a simple example. Sorry. 

[joelby]

Yeah, I was just looking for a simple example. Sorry. 

Well, one that applies to the MSP430 - the processor has instructions that handle BCD numbers, but there's no standard way to represent these in C so the compiler isn't able to recognise that these instructions could be used. The solution is pretty easy - use the inline assembly code examples that TI helpfully provide.