Difference b/w system calls & user calls in linux or any OS

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http://stackoverflow.com – I am searching for answer for many days but none of them solved my query. What is difference b/w system call & user call, I want to calculate USER CPU time & Kernel CPU time for printf() function in C. User cpu time is time taken by CPU to execute user routine & Kernel cpu time is time taken by cpu to execute kernel routine. My question is some where in calling printf() , In the end system call is invoked to print characters at display. I mean to say in routine printf() which part belongs to user & which part belongs to kernel. Does simple assignment statement a=b involves syst (HowTos)