🚀 go-pugleaf

I'm trying to learn how to make TSRs. One of the biggest problem I have, is
how to use DOS-functions in my TSR (e.i INT 21h). I've learned that because
DOS is not reentrant you cannot call any DOS-function at anytime.
How can I determine if it's safe to call DOS? 

One way to (I`ve heard) is to check the INDOS-flag, returned by a 
DOS-function (!). But I havent made I work yet. Should I get the pointer for the
flag everytime I wish to check it, or is it sufficient to call the Get INDOS-flag
function once? Are there any other ways to determine what DOS is up to.
The DOS-idle interrupt has appearently something to tell me, but what?

Bye!
Fredrik Gunne