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65 lines
2.6 KiB
TeX
65 lines
2.6 KiB
TeX
\newpage
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\subsection{Exceptions}
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Control flow is mainly dictated by program state, and manipulated using jumps, branches, calls and returns.
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To react to changes in system state, exceptional control flow is used instead.
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\textbf{Low level mechanisms}
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\begin{itemize}
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\item Hardware Exceptions
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\item Exceptions via combination of Hardware and OS software
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\end{itemize}
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\textbf{High level mechanisms}
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\begin{itemize}
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\item Process context switch
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\item Signals
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\item Nonlocal jumps
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\item Language-level exceptions (e.g. Java)
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\end{itemize}
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Generally, on an exception, control is transferred to a handler specific to the type of exception, which investigates the situation and returns control upon success.
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Mostly, this is handled via a \textit{Exception Table} which is allocated on boot. On exception, this table is indexed depending on the type of exception to locate the corresponding handler. This causes a switch to Kernel Mode.
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\inlinedef \textbf{Exception}: A control transfer to the OS in reponse to an event
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\begin{itemize}
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\item \textbf{Synchronous}: result of executing some instruction
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\item \textbf{Asynchronous}: result of an event external to the processor
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\end{itemize}
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\begin{center}
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\begin{tabular}{l|l|l}
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\textbf{Type of exception} & \textbf{Cause} & \textbf{Async/Sync} \\
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\hline
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Interrupt & Signal from I/O device & Async \\
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Trap & Intentional exception & Sync \\
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Fault & Potentially recoverable error & Sync \\
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Abort & Nonrecoverable error & Sync \\
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\end{tabular}
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\end{center}
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\subsubsection{Synchronous Exceptions}
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\inlinedef \textbf{Trap} is an intentional exception that transfers control back to the next instruction
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For example, opening a file in \verb|C| executes a trap via a system call.
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\inlinedef \textbf{Fault} is an unintentional, possibly recoverable exception. Either re-executes faulty instruction or aborts
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For example, page faults, protection faults, floating point exceptions
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\inlinedef \textbf{Abort} is unintentional and unrecoverable. Always aborts the program.
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For example, a machine error.
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\subsubsection{Asynchronous Exceptions}
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Asynchronous Exceptions are indicated by setting the processor's (physical) interrupt pin.
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For example,
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\begin{itemize}
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\item \textbf{Interrupts} are actions like network data arrival or hitting a key on the keyboard
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\item \textbf{Hard Reset Interrupts} are executed by hitting the system reset button
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\item \textbf{Soft Reset Interrupts} are caused by, for example, hitting \verb|CTRL|+\verb|ALR|+\verb|DEL| (on Windows)
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\end{itemize}
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