cpu-architecture

I've learned the basics about CPUs/ASM/C and don't understand why we need to compile C code differently for different OS targets. What the compiler does is create Assembler code that then gets assembled to binary machine code. The ASM code of course is different per CPU architecture (e.g. ARM) as the instruction set architecture is different. But as Linux and Windows run on the same CPU, the machine operations like MOVE/ADD/... should be identical. While I do know that there are OS-specific functions like printing to a terminal, this functionality could be provided by different implementations

this is my makefile: task0 : main.o numbers.o add.o gcc -m32 -g -Wall -o task0 main.o numbers.o add.o main.o : main.c gcc -g -Wall -m32 -ansi -c -o main.c numbers.o : numbers.c gcc -g -Wall -m32 -ansi -c -o numbers.c add.o: add.s nasm -g -f elf -w+all -o add.o add.s clean : rm -f *.o task0 and this is the terminal output: gcc -m32 -g -Wall -o task0 main.o numbers.o add.o /usr/bin/ld: skipping incompatible /usr/lib/gcc/x86_64-linux-gnu/4.8/libgcc.a when searching for -lgcc /usr/bin/ld: cannot find -lgcc /usr/bin/ld: skipping incompatible /usr/lib/gcc/x86_64-linux-gnu/4.8/libgcc_s.so when

This question already has answers here : INC instruction vs ADD 1: Does it matter? (2 answers) I've read many times over the years that you should do XOR ax, ax because it is faster... or when programming in C use counter++ or counter+=1 because they would INC or ADD... Or that in the Netburst Pentium 4 the INC was slower than ADD 1 so the compiler had to be warned that your target was a Netburst so it would translate all var++ to ADD 1... My question is: Why INC and ADD have different performances? Why for example INC was claimed to be slower on Netburst while faster than ADD in other