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[ The PC Guide | System Optimization and Enhancement Guide | System Optimizations and Enhancements | System Resource (IRQ, DMA, I/O, COM) Conservation and Optimization ]

Free Up a Serial (COM) Port by Using A PS/2-Style Mouse

There are two main types of mice used on modern PCs. The first is the serial mouse, which as the name implies uses a serial (COM) port on the PC. The second is the PS/2-style mouse which uses a dedicated port and its own interrupt setting, separate from the COM ports.

One problem that many users run into is resource conflicts related to the serial ports, or the two IRQ lines that they use. You can have up to four COM ports in a system, but by default they share only two IRQs, IRQ3 and IRQ4. This can lead to numerous problems, and they can be hard to resolve because many modems and other devices can only use the low-numbered IRQs (2 through 7). However, the PS/2-style mouse uses IRQ12, which on many systems is unused. So going to a PS/2-style mouse from a serial mouse may solve a lot of problems.

Many PCs support PS/2-style mice, but in some cases the support is not implemented or the manufacturer puts a serial mouse on the system anyway due to laziness or cheapness. Here's what you need to do if you want to move to a PS/2 mouse:

  1. Determine What You Are Using Now: This is pretty easy to do; the simplest way is to check the connection to the PC; if the mouse is plugged 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.
  2. Determine PS/2 Support: If you are currently using a serial mouse, the next thing you need to do is find out if your system supports a PS/2-style mouse. Check the back of the PC again; if you see a small, round, 6-pin connector (don't confuse it with the keyboard connector; there may be two similar connectors on some PCs) then you probably have PS/2 support. If you don't see the connector, you may still be able to use the PS/2 mouse. Some motherboard have a header for the PS/2 mouse but some manufacturers don't install the connector on the case for some reason. The reliable way to check if this situation applies to you is to check in your BIOS setup program for BIOS settings related to the PS/2 mouse. If you see them, that means your motherboard supports the PS/2 mouse. You should inspect the motherboard for the PS/2 header, and then contact the vendor or manufacturer about a PS/2 mouse connector to attach to it. Then install it, ensure that it is enabled in the BIOS setup, and you are all set. You may want to refer to this procedure, which describes how to install the internal PS/2 mouse port on AT-based systems without an integrated port.
  3. Enable PS/2 Mouse Support: Check the BIOS setup program for any settings that might exist to enable or disable PS/2 mouse support. Ensure that the port is enabled.
  4. Obtain a PS/2 Mouse: You will need a PS/2 style 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 serial mouse in the PS/2 port, with an inexpensive adapter. Otherwise, you will need to buy a PS/2 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 serial mouse.
  5. Install the PS/2 Mouse: This is of course a simple matter of plugging it into the PS/2 mouse port.
  6. 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 serial mouse driver to a PS/2 driver.
  7. Disable the COM Port in the System BIOS Setup: You must remember to disable the COM port using the appropriate BIOS setting in your BIOS setup program, in order to actually accomplish your goal of freeing up the COM port and the IRQ it uses. You'll want to do this if, for example, you want to install an internal modem that uses the same COM port number. In some cases you may not want to do this, for example if you still want to use the built-in COM port for a different external device.

Warning: If you had any devices that were using IRQ12 before you made this change, you may now have a resource conflict, since the mouse uses IRQ12. You will have to change the other device to use another IRQ. If it is a PCI device then you may be able to solve the problem by changing which PCI slot it is using..

Next: Free Up IRQ12 by Disabling the Built-in PS/2 Mouse Port and/or Moving to a Serial Mouse


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