System Support
4.3.2 Direct Memory Access
Direct Memory Access (DMA) is a method by which a device accesses system memory without involving the microprocessor. Although the DMA method has been traditionally used to transfer blocks of data to or from an ISA I/O device, PCI devices may also use DMA operation as well. The DMA method reduces the amount of CPU interactions with memory, freeing the CPU for other processing tasks.
✎This section describes DMA in general. For detailed information regarding DMA operation, refer to the data manual for the Intel 82801 I/O Controller Hub.
The 82801 ICH8 component includes the equivalent of two 8237 DMA controllers cascaded together to provide eight DMA channels, each (excepting channel 4) configurable to a specific device. Table
Table
|
|
DMA Channel | Device ID |
Controller 1 (byte transfers) |
|
0 | Spare |
1 | Audio subsystem |
2 | Diskette drive |
3 | Parallel port |
|
|
Controller 2 (word transfers) |
|
4 | Cascade for controller 1 |
5 | Spare |
6 | Spare |
7 | Spare |
|
|
All channels in DMA controller 1 operate at a higher priority than those in controller 2. Note that channel 4 is not available for use other than its cascading function for controller 1. The DMA controller 2 can transfer words only on an even address boundary. The DMA controller and page register define a
In addition to device configuration, each channel can be configured (through PCI Configuration Registers) for one of two modes of operation:
■LPC DMA
■PC/PCI DMA
The LPC DMA mode uses the LPC bus to communicate DMA channel control and is implemented for devices using DMA through the SCH5317 I/O controller such as the diskette drive controller.
The PC/PCI DMA mode uses the REQ#/GNT# signals to communicate DMA channel control and is used by PCI expansion devices.
The DMA logic is accessed through two types of I/O mapped registers; page registers and controller registers.
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