Compaq 850 manual Direct Memory Access, DMA Channel Assignments And Register Ports

Technical Reference Guide

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)		Control registers 000h-00Eh
0	Unused	Page register 087h
1	Audio subsystem	Page register 083h
2	Diskette drive	Page register 081h
3	Parallel port (ECP or EPP mode)	Page register 082h
Controller 2 (word transfers)		Control registers 0C0h-0DEh
4	Cascade for controller 1	n/a
5	Unused	Page register 08Bh
6	Unused	Page register 089h
7	Unused	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.

Compaq Deskpro EXS and Workstation 300 Personal Computers 4-19

Featuring the Intel Pentium 4 Processor

First Edition - December 2000