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Unformatted text preview: for moving data from floppy and hard drives into RAM (Figure 8-24).
Nice as it may sound, the concept of DMA as just described has a problemthere’s
only one expansion bus. What if more than one device wants to use DMA? What keeps
these devices from stomping on the external data bus all at the same time? Plus, what
if the CPU suddenly needs the data bus? How can you stop the device using DMA so
the CPU, which should have priority, can access the bus? To deal with this, IBM added
another traffic cop. ch08.indd 308 12/14/09 2:49:31 PM All-In-One / CompTIA Network+ All-in-One Exam Guide / Meyers & Jernigan / 170133-8 / Chapter 8
All-In-One Chapter 8: Expansion Bus 309 Figure 8-23 Why not talk to the chipset directly? Figure 8-24 DMA in action The DMA controller, which seasoned techs often call the 8237 after its old chip name,
controls all DMA functions. DMA is similar to IRQ handling in that the DMA controller
assigns numbers, called DMA channels, by which devices can request use of the DMA.
The DMA also handles the data passing from peripherals to RAM and vice versa. This
takes necessary but simple work away from the CPU so the CPU can spend time doing
more productive work. ch08.indd 309 12/14/09 2:49:33 PM All-In-One / CompTIA Network+ All-in-One Exam Guide / Meyers & Jernigan / 170133-8 / Chapter 8 CompTIA A+Certification All-in-One Exam Guide 310 The DMA chip sends data along the external data bus when the CPU is busy with
internal calculations and not using the external data bus. This is perfectly acceptable,
because the CPU accesses the external data bus only about five percent of the time on
a modern CPU.
The DMA just described is called classic DMA; it was the first and for a long time the
only way to do DMA. Classic DMA is dying out because it’s very slow and only supports
16-bit data transfers, a silly waste in a world of much wider buses. On most systems,
only floppy drives still use classic DMA.
All systems still support classic DMA, but most devices today that use DMA do so
without going through the...
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This note was uploaded on 04/27/2010 for the course COMPTIA 1201 taught by Professor N/a during the Spring '10 term at Galveston College.
- Spring '10