Rom

Motherboards store the keyboard controller support programming, among other programs, on a special type of device called a read-only-memory (ROM) chip. A ROM chip stores programs, called services, exactly like RAM: that is, like an 8-bit-wide spreadsheet. But ROM differs from RAM in two important ways. First, ROM chips are nonvolatile, meaning that the information stored on ROM isn’t erased when the computer is turned off. Second, traditional ROM chips are read- only, meaning that once you store a program on one, you can’t change it. Modern motherboards use a type of ROM called flash ROM that differs from traditional ROM in that you can update and change the contents through a very specific process called “flashing the ROM,” covered later in this chapter. Shows a typical flash ROM chip on a motherboard. When the CPU wants to talk to the keyboard controller, it goes to the flash ROM chip to access the proper programming.

          Every motherboard has a flash ROM, called the system ROM chip because it contains code that enables your CPU to talk to the basic hardware of your PC. As alluded to earlier, the system ROM holds BIOS for more than just the keyboard controller. It also stores programs for communicating with the floppy drives, hard drives, CD and DVD drives, video, USB ports, and other basic devices on your motherboard.

          To talk to all of that hardware requires hundreds of little services (2 to 30 lines of code each). These hundreds of little programs stored on the system ROM chip on the motherboard are called, collectively, the system BIOS. Techs call programs stored on ROM chip of any sort firmware.

          The system ROM chip used on modern PCs store as much as 2 MB of programs, although only 65,536 bytes are used to store the system BIOS. This allows for backward compatibility with earlier systems. The rest of the ROM space is put to good use doing other jobs.