UPGRADING REPAIRING PCs

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Chapter 2—System Components and Configuration
4. Systems with COM 4 default to “sharing” COM 2’s IRQ 3. This will cause system lockups in
Windows if a serial mouse is used on COM 2 with a modem on COM 4. Use the modem, and
the IRQ conflict crashes the system. To avoid problems, set the device using COM 4 to a different
IRQ, or disable COM 2 and use COM 2 for the modem.
DMA
Direct Memory Access permits high-speed data transfer between
I/O devices and memory without CPU management. This method
of data transfer boosts performance for devices that use it, but
because there is no CPU management, the possibility of data corruption
is higher than for non-DMA transfers. Although DMA
channels can theoretically be “shared” between devices that are not
in use at the same time, this is not a recommended practice.
PCI cards don’t use these DMA channels (with the exception of
sound cards, which are emulating the ISA-based Sound Blaster
or compatibles—the major users of DMA channels today). See
Table 2.21.
Table 2.21 16/32-Bit ISA/PCI Default DMA-Channel Assignments
Standard Bus Card Recommended
DMA Function Slot Type Transfer Use
0 Available Yes 16-bit 8-bit Integrated sound
1 Available Yes 8-/16-bit 8-bit 8-bit sound
2 Floppy disk Yes 8-/16-bit 8-bit Floppy
controller controller
3 Available Yes 8-/16-bit 8-bit LPT1: in ECP mode
4 1st DMA
controller
cascade
No — 16-bit —
5 Available Yes 16-bit 16-bit 16-bit sound
6 Available Yes 16-bit 16-bit ISA SCSI adapter
7 Available Yes 16-bit 16-bit Available
Note that PCI adapters don’t use these ISA DMA channels; these are only for ISA cards.
On PC/XT systems with only 8-bit ISA slots, only DMA channels 1–3 are available. DMA channel
2 was used for the floppy controller, as it is today, but channels 1 and 3 were not assigned to
standard devices.
Determining Actual IRQ and DMA Usage
Although these tables provide the “official” guidelines for IRQ and
DMA usage, these settings might not be true for all systems at all
times.
Add-on network, sound, serial, parallel, and SCSI cards can often be
moved to different IRQ and DMA channels to work around conflicts.
Non-standard settings can be done manually with some cards
and is a virtual certainty with PnP cards used with Windows 9x and
Windows 2000. Well-designed PnP cards already installed in a