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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