Compaq W4000 - Manual (Page 74)

Chapter 4 System Support
4.4.2 IRECT MEMORY ACCESS
method by which a device accesses system memory without
volving the microprocessor. Although the DMA method has been traditionally used to transfer
r
etailed information regarding
D
Direct Memory Access (DMA) is a
in
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 fo
other processing tasks.
NOTE: This section describes DMA in general. For d
DMA operation, refer to the data manual for the Intel 82801BA I/O Controller Hub.
T
of two 8237 DMA controllers cascaded
t M channels, each (excepting channel 4) configurable to a specific
d Table 4-10 lists the d configuration of the DMA channels.
T 10. Default DMA Channel Assignments
le 4-10.
Default DMA Channel Assignments
annel ID
he 82801 ICH ncludes the equivalen
ogether to provide eight D A
evice. efault
able 4-
Tab
DMA Ch Device
Controlle
2 component i t
r 1 (byte transfers)
Spare 0
1
Audio subsystem
2
3 Diskette drive
Parallel port
Controller 2 (word transfers)
5
troller 1
Spare
4 Cascade for con
6 Spare
Spare
7
All channels in DMA controller 1 operate at a higher priority than those in controller 2. Note that
scading function for controller 1. The DMA
ess boundary. The DMA controller and page
sfers within the address space of the CPU.
ration
LPC DMA
impl he LPC47B367 I/O controller such as the diskette
The PC/PCI DMA mode uses the REQ#/GNT# signals to communicate DMA channel control and
is used by PCI expansion devices.
channel 4 is not available for use other than its ca
sfer wo controller 2 can tran rds only on an even addr
ran register define a 24-bit address that allows data t
In addition to device configuration, each channel can be configured (through PCI Configu
Registers) for one of two modes of operation:
♦
♦ PC/PCI DMA
The LPC DMA mode uses the LPC bus to communicate DMA channel control and is
emented for devices using DMA through t
drive controller.
Compaq Evo and Workstation Personal Computers
Featuring the Intel Pentium 4 Processor
Second Edition – January 2003
4-20