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Patent Title: Exception handling method and apparatus for a microkernel data processing system

Assignee: IBM
Patent Number: US5606696
Issue Date: 02-25-1997
Application Number:
File Date:09-25-1995

Abstract: Floating point hardware register set is not given to any user level thread unless it is required to perform floating point operations. Thus, for any non-floating thread, its context does not include the floating point hardware state. This effectively reduces the amount of information to be handled when threads are swapped in the processor. During the course of a thread's execution, at the first instance of an attempt by the thread to execute a floating point instruction, the "float-unavailable" exception occurs. This, in turn, invokes the microkernel's floating point exception handler. The function of this exception handler is to make floating point available to the thread that requires it. The exception handler dynamically allocates space for saving the thread's floating point registers, initializes the registers, and turns on the "float-available" bit in its machine state register. Once a thread obtains floating point context, it continues to have it for the remainder of its life.

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