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.
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...
View Full Document
- Spring '10
- PCI Express, Computer buses, expansion bus, All-in-One Exam Guide, CompTIA Network+ All-in-One