week5_wed - CSCC 69H3 Operating Systems Winter 2010...

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CSCC 69H3 Operating Systems Winter 2010 Professor Bianca Schroeder U of T
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Today: - Understanding system calls - Adding system calls to OS161
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Requesting OS Services OS and user programs are isolated from each other But OS provides services to user programs… So how do they communicate? User mode Application programs Privileged mode Operating system • System call • Interrupt • OS sets up registers, mode for application • Jumps to next application instruction
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Implementing a new system call User mode (application) Privileged mode (kernel) Application program: helloworld() …. sys_helloworld(): kernel routine implementing the actual functionality How does the system give user- level applications access to sys_helloworld?
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System calls: Implementing a helloworld system call User mode (application) Privileged mode (kernel) Application program: helloworld() …. sys_helloworld(): kernel routine implementing the actual functionality At the application level: helloworld C library function: Pass control to kernel Let kernel know which system call to execute How do you do that? => Raise an exception
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What is an exception? A mechanism that enables OS to regain control Interface between OS and processor Exceptions are raised for different reasons By the hardware because something went wrong E.g. division by zero, or a page fault (will see in a few weeks what this is) Or because applications asks for it E.g. through a system call … Implemented by the exception handler A carefully crafted piece of assembler
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What does the exception handler do?
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