Q12230: IRQ Settings and Mouse Installation
Article: Q12230
Product(s): See article
Version(s): 1.x 2.x 3.x 4.x 5.x 6.x 1.00
Operating System(s): MS-DOS
Keyword(s): ENDUSER | | mspl13_basic
Last Modified: 19-SEP-1988
The jumper on the bus mouse should be checked before the card is
installed in any computer, especially if the card is moved from one
machine to another. The board's jumper controls which interrupt
request (IRQ) line is used. IRQ lines are used to facilitate
information transfer from such I/O devices as disk controllers and
serial ports. The jumper avoids IRQ conflicts with other devices
already installed in the computer. Under each pair of pins on the card
is a number between 2 and 5 (inclusive). The jumper selects the IRQ
line. Microsoft ships the boards with the jumper set to IRQ 2, for
installation in a typical IBM PC or PC XT. Because IBM changed the IRQ
architecture in the PC AT by using using IRQ 2 for the second IRQ
controller, the bus mouse jumper is usually set to IRQ 5 on an AT.
While these jumper settings are correct for most installations, you
should verify which (if any) IRQ lines are being used by every device
already installed in the machine. There can be only one active device
per IRQ line. Refer to the technical manuals for each manufacturer's
product or contact the manufacturer directly for this information. The
following is an IRQ allocation table as defined in the IBM PC and AT
technical reference manuals (other manufacturers' software, hardware
and add-on boards must follow this convention in order to be IBM
compatible):
PC-AT
IRQ Line PC, PC-XT CTLR 1 CTLR 2
0 Timer Timer | IRQ8 Clock
1 Keyboard Keyboard | IRQ9 Redirected IRQ2
2 Reserved CTLR 2 <--| IRQ10 Reserved
3 COM2 COM2 | IRQ11 Reserved
4 COM1 COM1 | IRQ12 Reserved
5 Hard disk LPT2 | IRQ13 Coprocessor
6 Floppy disk Floppy disk | IRQ14 Hard disk
7 LPT1-3 LPT1 | IRQ15 Reserved
Because the mouse can be jumpered in the IRQ range of 2 through 5 and
there can be only one active device per IRQ line, the bus mouse can be
installed only if at least one of these lines is free.
For example, a bus mouse is to be installed in an IBM PC-AT with an
IBM PC-AT Serial/Parallel Adapter configured as COM1 and LPT1,
respectively; a Color Graphics Adapter; and a multi-function card with
128K of memory and a serial port configured as COM2. On this computer,
there is only one IRQ line still available on IRQ controller 1: IRQ 5.
IRQ 2 is used by the AT's second IRQ controller and IRQ lines 4 and 3
are used by COM1 and COM2. The CGA does not use an IRQ line and the
parallel port uses IRQ 7, which falls outside of the mouse's range.
The bus mouse should be jumpered for IRQ 5, thereby using the last IRQ
line in the normal IRQ range of 0 through 7 of the first IRQ controller.
The user of this computer should be aware of this for future
expansion.
The Enhanced Graphics Adapter (EGA) is now becoming a popular display
card for PCs and XTs. The EGA includes a hardware feature that allows
software to enable interrupts on IRQ2 to indicate the start of
vertical retrace. Therefore, if an EGA and bus mouse are installed in
a PC or XT, IRQ2 is no longer available for the mouse. In a full XT
with a hard disk, two serial ports, an EGA, and a bus mouse, there
will be an IRQ line overlap between two devices. Therefore, one device
will have to be sacrificed to free up an IRQ line for the bus mouse.
This is not a design deficiency of the bus mouse; it is a fundamental
design restriction in the PCs and XTs.
As discussed above, there are only eight IRQ lines in the PC and XT,
of which four are used up by the motherboard and other standard
equipment. The other four lines go quickly. Almost all expansion cards
require that a free IRQ line be available, such as the following:
1. Network cards
2. Bisync communication cards
3. Tape back-up units
4. Some clock/calendar hardware
5. Serial communication cards
6. EGAs
7. Emulation boards
8. Hard disk controllers
There are more desirable devices to install in a machine than IRQ
lines to handle them. To help relieve the crowding of IRQ lines, the
IBM AT includes a second IRQ controller with seven more lines.
Currently, hardware is evolving to take advantage of the new AT
architecture.
Once the hardware is installed, the software must be loaded. When the
mouse driver loads, using either MOUSE.SYS from CONFIG.SYS or
MOUSE.COM from a batch file or the keyboard, the file will be loaded
into memory and the driver will then install itself. This installation
requires a few seconds. Various operations are undertaken, including
mouse hardware initialization. This is the primary reason for the
delay before the mouse installation message appears.
If the error message "MOUSE: Microsoft mouse not found!" appears,
there can be a number of hardware-related causes, such as a broken
mouse; however, the problem more likely is an IRQ contention problem
either between the mouse and another device or between the serial
ports.
Typical bus mouse related problems are between the mouse and the hard
disk controller, i.e., bus mouse jumpered on IRQ 5 in an XT or IRQ 2
in an AT. The common symptom for this problem is the inability to
perform a warm boot (CTRL+ALT+DEL). If the bus mouse is jumpered on
the same line as a serial port, network card, or emulator card,
irregular and unreproducible system crashes can occur. In Windows,
with a bus mouse doubled up on the IRQ line used by a modem, the mouse
will "go away" when communications software is run.
The standard isolation procedure is to verify IRQ-line usage and if no
problems are uncovered, the next step is to remove as much hardware as
possible. In this way, the conflict should be uncovered between
particular devices and a resolution of the problem will follow.
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