How BIOS And Device Drivers Work

How BIOS And Device Drivers Work

The ROM BIOS Startup Program organizes hardware, configuration information, and BIOS information, as well as stores vital information in the I/O address table in preparation for the operating system to take over.

The operating system continues the process of initialization by loading device drivers into memory. All this takes place before the first application software is executed. It’s important to separate the BIOS used to initialize the system from the BIOS stored in memory and referred to in the I/O address table that is used to manage hardware devices.

One of the most important procedures in the BIOS Startup Program is POST (power on self test). During POST, when an expansion card such as the video controller card (sometimes called the display adapter) is accessed, the BIOS permanently stored on the card’s ROM chips tells the computer that BIOS software is present on the card.

POST assigns the BIOS software addresses in memory. The area of memory most often used for BIOS programs working under DOS and Windows 9x is upper memory, the range of memory addresses from 640K to 1024K. Recall that the location of the program is stored in the I/O address table, which will later be accessed by an IRQ request.

During the execution of an application program, a device that needs to interrupt the CPU for servicing is assigned a number called an IRQ.

The CPU uses this IRQ to identify which peripheral device needs servicing. This interrupt request number is associated with an address in the I/O address table that allows the CPU to locate the System BIOS Program for the peripheral device. For the keyboard, for example, the process works like this:

You press a key on the keyboard.

The keyboard controller sends its assigned IRQ to the CPU, thus saying, “I need attention.”

The CPU sees the IRQ and turns its attention to servicing it.

The CPU uses the IRQ to point to a row in the I/O address table assigned to that IRQ.

From the I/O address table, the CPU locates the memory addresses in which the instructions to service the keyboard are stored.

The CPU follows the instructions stored at these upper memory addresses to process the keystroke.

Sometimes data from the device is temporarily stored in the I/O address table with the locating address of the device driver or ROM BIOS needed to service the device. The same action happens when data is read from a disk. The disk drive sends the data along with the correct IRQ for the drive to the CPU. The disk drive instructions used by the CPU are stored in System BIOS. The data is handed over to the operating system, which can pass it along to an application.

Video monitors do not require an IRQ because they only receive data from the CPU and never send data back to the CPU. (One exception is the touch screen monitor, which serves as an input as well as an output device.) However, monitors do have software that must be accessed by the CPU when the CPU wants to use the monitor. Video cards control monitors; software to control monitors, also a part of BIOS, is found on the video card together with some video RAM that holds data being passed to the monitor.