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Technical Reference Guide
Compaq Deskpro EXS and Workstation 300 Personal Computers
Featuring the Intel Pentium 4 Processor
First Edition - December 2000
4-19
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 4-9 lists the default configuration of the DMA channels.
Table 4-9. Default DMA Channel Assignments And Register I/O Ports
Table 4-9
DMA Channel Assignments And Register Ports
DMA Channel Function I/O Port
Controller 1 (byte transfers)
0
1
2
3
Unused
Audio subsystem
Diskette drive
Parallel port (ECP or EPP mode)
Control registers 000h-00Eh
Page register 087h
Page register 083h
Page register 081h
Page register 082h
Controller 2 (word transfers)
4
5
6
7
Cascade for controller 1
Unused
Unused
Unused
Control registers 0C0h-0DEh
n/a
Page register 08Bh
Page register 089h
Page register 08Ah
Refresh 08Fh [see note]
NOTE:
The DMA memory page register for the refresh channel must be
programmed with 00h for proper operation.
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 Mode - Uses the LPC bus to communicate DMA channel control and is
implemented for devices using DMA through the I/O controller such as the diskette drive
controller.
♦ PC/PCI DMA Mode - Uses the REQ#/GNT# signals to communicate DMA channel control
and is used by PCI expansion devices.