Activity18 Soln - SOLUTION In-Class Activity #18 3/27/2001...

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ECSE-2660 Computer Architecture, Networks, & Operating Systems, Spring 2001 Page 1 of 2 SOLUTION In-Class Activity #18 3/27/2001 9:19 AM Activity 18.1 (First Hour) 1. (25 points) What is the purpose of system calls? Using an example, describe how are they different from ordinary subroutines? System calls provide the programmer’s interface to the operating system. They are usually available as assembly- language instructions, and/or a format that closely resembles subroutines. System calls are fundamentally different from subroutines. They run in privileged mode, and can access I/O devices. Only the OS is allowed to run them. When a user places a system call in his program (e.g., a file seek call), the user process is suspended, and control is transferred to the OS. The OS then executes the system call on the user’s behalf. Before doing this, it is able to make a variety of checks for errors and authorization. It is also able to do a variety of other things, such as controlled sharing of I/O devices, etc. 2.
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Activity18 Soln - SOLUTION In-Class Activity #18 3/27/2001...

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