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Table Of Contents
Modern Computer Architecture and Organization - Third Edition
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Most processors support some form of interrupt handling to manage service requests from external devices. Conceptually, interrupt handling is akin to a scenario in which a person is busy working on a task and the phone rings. After answering the call and perhaps jotting a note for later action ("buy bread," for example), the individual resumes the interrupted task. Humans employ several related mechanisms, such as doorbells and alarm clocks, enabling us to interrupt lower-priority activities and respond to more urgent needs. The following sections describe how the 6502 and other processors implement this type of functionality.
processingThe 6502 integrated circuit has two input signals that allow external components to notify the processor of a need for attention. We will first discuss the interrupt request input,
. The
input is active low (meaning the signal is asserted at its low, or 0, level; this is indicated by the bar over the IRQ characters),...