free barcode reader c# 6: Understanding the Expansion Bus in Software

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6: Understanding the Expansion Bus
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All I/O addresses only use the last 16 bits (they all start with 16 0000) Sixteen bits makes 2 = 65,536 I/O address ranges plenty for even the most modern PCs Should PCs begin to need more I/O addresses in the future, the current I/O addressing system is ready!
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1011 = B 1100 = C 1101 = D 1110 = E 1111 = F Let s pick an arbitrary string of ones and zeroes: 00000000000000000000000111110000 To convert to hex, just chop them into chunks of four: 0000 0000 0000 0000 0000 0001 1111 0000 Then use the key above to convert: 000001F0 Then push the hex values together: 000001F0 You now understand what those values mean in Device Manager Scroll down until you find the [000001F0 - 000001F7] Primary IDE Channel setting Notice that there are two I/O addresses listed These show the entire range of I/O addresses for this device; the more complex the device, the more I/O addresses it uses Address ranges are generally referred to by the first value in the range, commonly known as the I/O base address Here are the most important items to remember about I/O addresses First, every device on your PC has an I/O address Without it, the CPU wouldn t have a way to send a device commands! Second, I/O addresses are configured automatically you just plug in a device and it works Third, no two devices should share I/O addresses The system handles configuration, so this is done automatically
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Interrupt Requests
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Between the standardized expansion bus connections and BIOS using I/O addressing, the CPU can now communicate with all of the devices inside the computer, but a third and final hurdle remains I/O addressing enables the CPU to talk to devices, but how does a device tell the CPU it needs attention How does the mouse tell the CPU that it has moved, for example, or how does the keyboard tell the CPU that somebody just pressed the J key The PC needs some kind of mechanism to tell the CPU to How do devices tell the CPU they need attention stop doing whatever it is doing and talk to a particular device (Figure 616) This mechanism is called interruption Every CPU in the PC world has an INT (interrupt) wire, shown in Figure 617 If a device puts voltage on this wire, the CPU will stop what it is doing and deal with the interrupting device Suppose you have a PC with only one peripheral, a keyboard that directly connects to the INT wire If the user presses the J key, the keyboard charges the INT wire The CPU temporarily stops running the browser (or whatever program is active) and runs the necessary BIOS routine to query the keyboard
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Figure 616
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Figure 617
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The INT wire
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This would be fine if the computer had only one device As you know, however, PCs have many devices, and almost all of them need to interrupt IOAPIC functions are usually the CPU at some point So the PC needs some kind of traffic cop to act as built into the Southbridge an intermediary between all the devices and the CPU s INT wire This traffic cop chip, called the I/O Advanced Programmable Interrupt Controller (IOAPIC) , uses special interrupt wires that run to all devices on the expansion bus (Figure 618) If a device wants to get the CPU s attention, it lights the interrupt wires with a special pattern of ones and zeroes just for that device The IOAPIC then interrupts the CPU The CPU queries the IOAPIC to see which device interrupted, and then it begins to communicate with the device over the address bus (Figure 619) These unique patterns of ones and zeroes manifest themselves as something called interrupt requests (IRQs) Before IOAPICs, IRQs were actual wires leading to the previous generation of traffic cops, called PICs It s easy to see if your system has a PIC or an IOAPIC Go into De- Figure 618 Eight interrupt wires (IRQs) run from the expansion bus to the IOAPIC vice Manager and select Interrupt request (IRQ) Figure 620 shows 24 IRQs, numbered 0 through 23, making this an IOAPIC system IRQ 9 is special this IRQ is assigned to the controller itself and is the IOAPIC s connection to the CPU If you look closely, you ll also notice that some IRQs aren t listed These are unused or open IRQs If you add another device to the system,
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