When a host machine sends an interrupt to a device (over say, PCI bus) by writing to a register on the device running an RTOS, is it considered a hardware or a software interrupt? Looking for some elaborate explanation.
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It is a vague and hypothetical question that perhaps indicates some misconceptions.
One system cannot typically "send" an interrupt in the sense you seem to imply or even write directly to another device's registers. A SWI requires the execution of an instruction to switch context; that is not what is happening in a device to device communication.
A device comprising of a processor and communication or bus controller peripherals (possibly on a single chip), and the communication device in question may generate a hardware interrupt to processor.
So you do not typically "send and interrupt", rather you perform some interaction or signalling that causes an interrupt to occur. Either way it is certainly a hardware interrupt.
Clifford
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