Direct Memory Access - Compaq Evo D500 Technical Reference Manual

4.4.2 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.
NOTE: This section describes DMA in general. For detailed information regarding
DMA operation, refer to the data manual for the Intel 82801BA I/O Controller Hub.
The 82801 ICH2 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 4-10 lists the default configuration of the DMA channels.
Table 4-10. Default DMA Channel Assignments
Default DMA Channel Assignments
DMA Channel
Controller 1 (byte transfers)
0
1
2
3
Controller 2 (word transfers)
4
5
6
7
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 24-bit address that allows data transfers within the address space of the CPU.
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 LPC47B357 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.
Table 4-10.
Device ID
Spare
Audio subsystem
Diskette drive
ECP LPT1
Cascade for controller 1
Spare
Spare
Spare
Compaq Deskpro and Evo Personal Computers
Featuring Intel Celeron and Pentium III Processors
Fifth Edition - March 2002
Technical Reference Guide
4-19