Well,the problem lies with IO and Windows mostly.

First of all,all the math of programs are the same whatever the OS.
The problem comes when somebody wants to write data to the screen or file,or
maybe they want input from the keyboard or file.

For those things,system calls to the OS are done. However,only in Windows
do you need to include files.

In DOS,when you use int $21 you are using functions in the OS.
Because you are only calling what is in the kernel already,you keep it small.

Linux is also similar,though it's bloated as opposed to 16 bit DOS,but the
advantages make up for it with lots of memory available.

Languages like C are great in many ways as they save the trouble of depending
on a certain OS just because you can find a C compiler that implements printf
for example in a different OS while keeping the same result.

The reason I could never use only assembly is because I have uses for
printf and scanf .

Getting user input is a pain using system calls. I learned this when I wrote that 1 program
for NEWK.

I like the idea of assemblers directly producing an executable file
without any intermediate stage. It can't always be done that easily though!

You know,I wonder if anybody will,wait,I think I have a challenge that I may post somewhere!


________
There are 10 kinds of people in the world.
Those who understand binary and those that
haven't met me.