What is  CMOS  ? Or RTC static NVRAM       

In the dark ages of PC  30 years old  had this chip;  Soon and Later this function moved deep inside the glue logic chip)

  • Most folks land here if BIOS goes NUTs (acts crazy and unstable) Even BIOS DEAD ! is possible .
  • Beep codes or POST errors ( or caps lock blink codes) with references in the service manual to CMOS,BIOS, or NVRAM errors, of any kind replace the battery now.
  • Time of day is not correct or date wrong in BIOS screens or BIOS is dead, or PC can't find HDD to boot it. (or you correct the boot order page and it forgets it again later)
  • Or screens shows F2 errors, this happens every 7 years, due to the battery died. (F2 in this case means NVram is scrambled)
It uses a  battery the keeps it from forgetting many things ,  and  is still called NVRAM , then chip is volatile by itself , until the battery is added.
The battery is called a RTC coin cell, 99.9% of the time. (not rechargeable ever (save a rare Toshiba) "RTC = Real Time Clock" The battery  runs the  NVRAM  and the Clock  chip  , both)
The NVRAM chip has 3 sections,  BIOS configuration data, RTC time and date, and on very old PC's BIOS ADMIN password storage.
Today we don't try to look for the chip, above, it was long ago hidden  deep inside my  South Bridge chip by Intel, called ICH10  or my Z270a chip.  (this fact confuses many folks) It is still here for 3 decades.
In most cases all that matters is the COIN CELL . Do not flash your BIOS thinking the vague errors are caused by BIOS FIRMWARE, no you are wrong thinking that. Buy a battery now and end your pain.
To be accurate the BIOS passwords today are not stored elsewhere (moved first to EEPROM password chip, then later moved to TPM chip that is not hackable by DESIGN)
It is true pulling all batteries and this coin cell and AC power ,  on relic PCs will reset BIOS passwords. (but never on modern PCs 2010 and newer) (as to do Jumper clips marked CMOS_clr, PW-CLR. and the like, not seen TODAY.)
This is the coin cell do not insert it backwards ever ,never short the pins on this cell or mobo socket or the ICH chip blows to HELL ! (Intel warned you , listen !)

The is no need to read my full page on NVRAM , just measure your battery or replace it now. (below 2.9v is a bad battery)
You are here because, the PC BIOS screen,  can't hold time of day (or MM/DD/YY) or BIOS can't save configs,. or BIOS may show  checksum errors in CMOS. (or all 3)
The NVRAM holds RTC = real time clock, data and all BIOS configurations , if the battery dies,  so does NVRAM data !. (NV means non volatile but only if the battery is good.)
NVRAM is CMOS RAM plus a battery.
All you really need to know is, after about 5-7 years old, the cell will start to drop and go to 2.9vdc, (3.3 is new) and when it does this drop deal,  it is doomed, (that is the failure cliff)
The memory will fail, some where from 2.9 to 2.0vdc, this varies by who's chips you own. (in all cases) The key is NEVER LET IT GO Below 2.9Vdc.
The Energize Bunny says his battery lasts 5 to 7 years. (as does copper top)
Replace the RTC Coin Cell battery now, or it will get worse. (end story)

Warning the below is raw electronics technology.

For the curious how things work,, see below: (Do not read this, it's pure electronics  )

The correct words for this , is BIOS/RTC NVRAM  (or CMOS for short)
We call this battery backed up NVRAM using Static RAM, non-volatile RAM, the coin cell creates that magic. (NV  magic) SRAM + COIN = NVRAM. (SRAM = Static RAM)
This device is seen inside all PCs for 36 years, 1981 to NOW.
Do not read this page below, tech facts., just splurge and spend $2 on a new coin cell battery 2032s are common. (below is Electronics real) The RTC battery is sold at Walmart in the watch department. (show the lady your battery)

Its purpose is  2 fold, one is to store all BIOS settings and yours,  like for example which device boots first?, and 2nd to store the  time of day (&Calendar), this memory section is for the  RTC chip real time clock chip stores TIME in bank 2.
The reasons it's called CMOS is because, it is! (the transistors are CMOS static RAM) It uses almost no current to run and to hold data in standby.  (super super low current in standby)
Complimentary MOS transistors, This is also called a flip-flop or a bi-stable multi-vibrator. (or the basic binary memory CELL) CMOS uses floating gates. (see them)? If VDD drops too low, the gate thresholds are hit and it fails.
The transistor M1 to M4 repeats 2048 times  256x8 memory array.  In standby (PC off) the M5/6 are off, and the gates all hold there charges unless VDD falls to low.
The beauty of this cell is statically it uses almost zero power (Idd drain near zero) that magic is how the battery can last for 7 long years.  The battery drains faster  if all power to the PC is dead.  PCs never fully turns off so the coin cell lasts 7 years for  sure.
This  memory cell stores is 1 BIT.  It uses as little as 0.5uA in standby current Idd , allowing the coin cell to last a long time, PC turned off.
A nice mosfet transfer curve is here.  As you can see if the gate was at 5v charge, some where below 2vdc the flip will go nuts. (toggle in a random way) The below is the typical MOS transfer characteristics.( a large one)
This is not our transistor ,ours is very tiny and has  very unique voltage response. In fact is designed to run with weak VDD. (dain) with a weak Gate voltage VGs)
The beauty of this device,  is it can run with Vdd as low as 2.0vdc( on average), (Vdd is coin cell voltage , minus 1 diode drop) the second beauty is the super low current in standby,  really nothing beats this.
The battery has  diode (figure 3 above) that loses 0.5v so 3.3v battery is really  2.8v to the  RAM chip, or if the battery is at 2.9v  the RAM sees only 2.4v and this is very very close to failure point of RAM.
When the PC is on  VDD is 5.0v
Checkout this super low VDD version SRAM from Philips Semi. (Vdd min,  1.0v !) (unlike my PC)
If you let the coin cell go bad or below 2.9v, the memory above CELL forgets every thing it ever knew, and the BIOS ROM and the RTC goes nuts, (scrambles) loss of time, data   and CRC errors abound.
 The BIOS may tell you BIOS corruption because the NVRAM is now corrupted.  (DO NOT FLASH YOUR BIOS FOR THIS SILLY COMMON FAILURE OR RISK BRICKNG YOUR PC)
This means the battery is dying fast, below 2.9vdc on the battery , means it's on the CLASSIC  cliff of total death. Put in a new coin cell, end this minor horror.
This can happen every 5 years, buying a top makers COIN is a smart idea.  (avoid no name batteries as seen on fleabay for $1 , like the brand "SOM Ting WONG")
Buy the battery with the cute bunny rabbit? or Duracell (avoid NOS, new old stock on any battery) I like to get mine  at Walmart due to fast change over in stock ! (read FRESH) most batteries have no date codes saidly so is a crap shoot.

CMOS RAM using  tiny chip standby power.
That lets the COIN last for 5-7 years  (some can go 10 years, but is not ever for sure){ others from China, with no famous makers name , can fail in months}
RTC = Real time clock, it works just like your wrist watch clock chip, I runs and stores the time of day,M/D/Year in the NVRAM.
In modern times we don't use  a simple single chip NVRAM now, we use a huge glue chip, below. It's still there, just hidden deeper, like in  the huge Intel ICH10 chip or newer...

THE Intel, ICH8 BGA chip, Glue logic complex.  I/O Controller Hub 8 family, All my laptops work just like this only each year newer more powerful chips and USB 3 added.  AKA the SOUTH BRIDGE CHIP.
This chip has the RTC modules inside and the NVRAM bank inside called BIOS CMOS NVRAM, there tool and  coin cell battery, yes Intel even uses the word COIN too.
The Intel chip VCCRTC seen here has voltage spec of 2.0v to 3.465vdc.  We all know less that 2.9v is a bad battery. See my marks in red, my HP has the same setup exactly see those low Vf drop diodes.
The D2 diode will have like 0.5v Vf drop so 2.5vdc is Vbatt  min.
The 2 Diodes are current steering devices,  the lower diode prevent the PC power rails from blowing up the coin cell ,its not rechargeable at all .
Diode keeps the COIN cell from draining dead , on the VCCSUS3_3 buss as it surely would fast.
The RTCRST pin resets the ICH8 if the coin is removed, but all other power needs to be removed first. (Main batteries, and PSU /Power packs turned off.) and let stand awhile to discharge all CAP.'s.
The BIOS ROM connects directly to this same chip via direct way (relic PCs) or SPI serial way as all new PC's do. 
This CHIP saves the BIOS settings to the NVRAM inside the ICH8.  As you can see the battery runs the RTC clock all the time if the PC has no power (no main battery ,no AC power)
The Schottly diodes have 0.5vdc voltage drop (Vforward biased)  Intel warns us never short these pins or ICH blows up;. The 1k resistor below protects ICH.

Intel  ICH data sheet, snap shot...

Intel told you , "Never short these pins, Vbat, or VCCRTC ever. " or this bad act will wreck the nice ICH chip .
No pins on this  page can be shorted.  (remove the coin cell, and the Laptop battery and the line cord and then push power, button, now  it is reset)  (same on desktop no huge battery there)
See power timing here) the this ICH8,

Desktop MOBO. The cell here is an industry standard part.

Below is a Laptop COIN cell , in shrinkwrap, other laptops use same socket as see directly above.  The new battery measure 3.3vdc., below 2.9v is DANGER ZONE of death soon. (on the cliff of deeth)
If you are savvy you can solder this battery to the 2 wire below (red to +) using this 3vdc  cell  with tabs.
The Pink Bunny says in there offical data sheets , max life is 7 years, so noodle that a bit first, if the battery reads low, and you ignore it how fun that will be on vacation off the grid.
Learn that Pink Bunny's never lie, but Pink Elephants do ( LOL)! Duracell shows same 7 year rule.

The laptop has this coin plugged in to any where on the MOBO, varies by all laptops made , exact spot. The connector is unique to all brands of Laptops.

The laptop coin cell can be pure hell to  get to , some are easy other the whole laptops needs to be dissembled, totally. (a royal pain some are); Desktops are a cake walk easy.
  • Under the keyboard , keyboard out.
  • Under a lid on the bottom
  • Under the lid, on the bottom for ram but ram pulled to reach it. see mine below photo.
  • Under or near the HDD bay lid off.
  • Under the blue tooth hatch.
  • Under the Wifi lid hatch.
  • Now the bad one,  remove the screen, top of case and mobo, and only now can you reach it. (sux no?)

Slange used here : (some are mine others are industry standards , vast in scope)
Hack or hackable.  (hacks are possible sure, ,but I  mean: if you do you risk bricking the whole motherboard,  a $200+ huge mistake for most owners) (do not listen to basement goon web posters having you find and short out chips) OK?
RAM , random access memory that forgets with lost power.
NVram , RAM that does not forget if the battery is still good) NV mean non-volatile. (means does not forget)
CMOS, (Is a transistor (MOS) configuration that uses almost no power, if static. NVram uses CMOS memory cells do to that 1 fact.
Coin, means small as a coin, some near quarter sized, or tad less. see photos, above.
Exceptions, some rare PCs by Toshiba use a rechargable Battery for the NVRAM that is a pain to unsolder and source new. ROYAL PAIN it  is. Avoid this maker of PC, buy HP or Dell .

version 8.  5-9-2017