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All modern hard drives take advantage of bus

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Unformatted text preview: DMA controller. These devices are known as bus masters. Bus mastering devices have circuitry that enables them to watch for other devices accessing the external data bus; they can detect a potential conflict and get out of the way on their own. Bus mastering has become extremely popular in hard drives. All modern hard drives take advantage of bus mastering. Hard drive bus mastering is hidden under terms such as Ultra DMA, and for the most part is totally automatic and invisible. See Chapter 12, “Implementing Hard Drives,” for more details on bus mastering hard drives. NOTE Bus mastering devices ignore the DMA controller; they don’t have DMA channels. If you want to see your DMA usage, head back to the Device Manager and change the view to Resources by type. Click on Direct memory access (DMA) and you’ll see something like Figure 8-25. This system has only two DMA channels: one for the floppy drive and one for the connection to the CPU. Figure 8-25 DMA settings in the Device Manager One interesting note to DMA is that neither PCI nor PCIe supports DMA, so you’ll never find a DMA device that snaps into these expansion buses. A hard drive, floppy drive, or any other device that still wants to use DMA must do so through onboard connections. Sure, you can find hard drive and floppy drive cards, but they’re not using DMA. ch08.indd 310 12/14/09 2:49:35 PM All-In-One / CompTIA Network+ All-in-One Exam Guide / Meyers & Jernigan / 170133-8 / Chapter 8 All-In-One Chapter 8: Expansion Bus 311 Memory Addresses Some expansion cards need memory addresses, just like the system RAM. There are two reasons a card may need memory addresses. First, a card may have onboard RAM that the CPU needs to address. Second, a few cards come with an onboard ROM, the so-called adapter or option ROM you read about in Chapter 5, “Microprocessors.” In either of these situations, the RAM or ROM must steal memory addresses away from the main system RAM to enable the CPU to access the RAM o...
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