What is CMOS

Complementary Metal Oxide Semiconductor.

In computing the term CMOS is often used to mean the non-volatile memory that stores motherboard BIOS information. This was called the CMOS RAM or just the CMOS because it usually used a low-power CMOS memory chip, powered by a small battery. The term remains common as nearly all modern computer components except the "CMOS memory" are constructed in CMOS technology. For the "CMOS", in modern computers, flash memory is usually used.

CMOS, short for Complementary Metal Oxide Semiconductor Is an MOS technology in which both P-channel and N-channel devices are fabricated on the same die CMOS is a widely used type of semiconductor. CMOS semiconductors use both NMOS (negative polarity) and PMOS (positive polarity) circuits. Since only one of the circuit types is on at any given time, CMOS chips require less power than chips using just one type of transistor. This makes them particularly attractive for use in battery-powered devices, such portable computers. Personal computers also contain a small amount of battery-powered CMOS memory to hold the date, time, and system setup parameters.

CMOS topology is very attractive also because circuitry logic synthesis is made automatically. So, you derive logical equation of digital circuit and don't bother about optimization and topology (not too much, maybe you have to edit place&route results). It is also a good idea to adopt CMOS topology because there will never be an high impedance node (each node will be connected to Vdd or GND), so you gain robustness and noise margin (distance between voltage of logical signals, here Vdd-0=Vdd) is always maximum.