Direct Memory Access - HP Compaq d330 DT Technical Reference Manual

Chapter 4 System Support

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 82801 I/O Controller Hub.
The 82801 ICH5 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 4-8 lists the default configuration of the DMA channels.
Table 4-8. Default DMA Channel Assignments
Default DMA Channel Assignments
DMA Channel
Controller 1 (byte transfers)
0
1
2
3
Con troller 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.
I n addition to device configuration, each chann
R egi sters) for on
♦
LPC DMA
♦ PC/PCI DMA
The LPC DMA m
im plemented for devices using DMA through the LPC47B387 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.
4-18 hp compaq d330 and d530 Series of Personal Computers
Featuring the Intel Pentium 4 Processor
Table 4-8.
Device ID
Spare
Audio subsyste
Diskette drive
Parallel port
Cascade for controller 1
Spare
Spare
Spare
e of two modes of operation:
ode uses the LPC bus to communicate DMA channel control and is
First Edition – June 2003
m
el can be configured (through PCI Configuration