Computer Hardware Tutorials

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CMOS ** Hardware Guide Home ** Complementary Metal Oxide Semiconductor. To perform its tasks, the BIOS need to know various parameters (hardware configuration). These are permanently saved in a little piece (64 bytes) of CMOS RAM (short: CMOS). The CMOS power is supplied by a little battery, so its contents will not be lost after the PC is turned off. Therefore, there is a battery and a small RAM memory on board, which never (should...) loses its information. The memory was in earlier times a part of the clock chip, now it's part of such a highly Integrated Circuit (IC). CMOS is the name of a technology which needs very low power so the computer's battery is not too much in use. Actually, there is not a battery on new boards, but an accumulator (Ni_Cad in most cases). It is recharged every time the computer is turned on. If your CMOS is powered by external batteries, be sure that they are in good operating condition. Also, be sure that they do not leak. That may damage the motherboard. Otherwise, your CMOS may suddenly "forget" its configuration and you may be looking for a problem elsewhere. In the monolithic PC and PC/XT, this information is supplied by setting the DIP (Dual-In- line Package) switches at the motherboard or peripheral cards. Some new motherboards have a technology named the Dallas Nov-Ram. It eliminates having an on-board battery: There is a 10 year lithium cell epoxyed into the chip Various Bus Types ** Hardware Guide Home ** Bus Types The expansion bus (where expansion cards go) is an extension of the Central Processor, so when adding cards to it, you are extending the capabilities of the CPU itself. The relevance of this regard to the BIOS is that older cards are less able to cope with modern buses running at higher speeds than the original design of 8 or so MHz. Also, when the bus is accessed, the whole computer slows down to the bus speed, so it's often worth altering the speed of the bus or the wait states between it and the CPU to speed things up. The PC actually has four buses; the processor bus connects the CPU to its support chips, the memory bus connects it to its memory, the address bus is part of both of them, and the I/O (or expansion) bus is what concerns us here.