[ The PC Guide | System
Optimization and Enhancement Guide | System Optimizations and
Enhancements | System Resource (IRQ, DMA, I/O, COM) Conservation and
Optimization ]
Free Up IRQ12 by Disabling the Built-in PS/2 Mouse Port and/or Moving to a Serial Mouse
Many newer PCs come with an integrated PS/2-style mouse port. Using this port allows
you to conserve a serial port and serial port IRQ that would otherwise be used for your
serial mouse. However, some people do not use their PS/2 mouse port; the most usual reason
is upgrading from a previous system that didn't have one and not wanting to purchase a new
PS/2 mouse. Also, in some rare cases IRQ12 might be needed more than the serial port's
IRQ, so you may want to switch from PS/2 to a serial mouse.
Before you do anything, verify what type of mouse you are currently using. This is
pretty easy to do; the simplest way is to check the connection to the PC; if the mouse is
plugging into a small, round connector on the PC, you are already using a PS/2-style mouse
(most of the larger retail brands use them). If the mouse is going to a 9-pin or 25-pin
D-shaped connector, it is a serial mouse. You can also use the Windows 95 Device Manager
or the Windows 3.x Setup program to see what sort of mouse is installed.
If you are already using a serial mouse on a system that includes a PS/2 mouse port,
disabling the port is easy and will free up IRQ12. Go into the BIOS setup program and look
for a BIOS parameter called something like "PS/2 Mouse Enable". Disable the
parameter and the PS/2 mouse port will "disappear" from the operating system and
its IRQ will be freed up as well.
If you are currently using a PS/2 mouse and want to move to a serial mouse, here's what
you will have to do:
- Disable PS/2 Mouse Support: Check the BIOS setup program for any settings that
might exist to enable or disable PS/2 mouse support. Usually this will be called something
like "PS/2 Mouse Enable". Disable the setting.
- Obtain a Serial Mouse: You will need a serial mouse. There are some mice that can
function as either PS/2 or serial mice, but most are only one or the other; see here for more on this. If yours can do either
(called a "combo" mouse) then you should be able to use your PS/2 mouse in a
serial port, with an inexpensive adapter. Otherwise, you will need to buy a serial mouse.
If possible, buy one from a place that allows returns, in case there is a problem and you
have to go back to using the PS/2 mouse.
- Install the Serial Mouse: Install the mouse into the serial port. Normally,
serial port 1 is used for the mouse; if you have another device using it already (such as
a modem) you can use COM2 but this is non-standard.
- Switch Mouse Drivers: If you are using Windows 95 with Plug and Play, your PC
should detect the hardware change and adjust the drivers for you. Otherwise, you will have
to manually change from the PS/2 mouse driver to a serial mouse driver.
 Warning: If you had any devices
that were using COM ports before you made this change, you may theoretically now have a
resource conflict, since the mouse will be using one of the ports. In particular, if you
already have two COM ports in use, I do not recommend eliminating the PS/2 mouse, as you
will have a problem finding enough low-numbered (2 to 7) IRQs for all the serial devices.
 Next: Avoid
Resource Conflicts By Using IRQ2/IRQ9
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