PC BIOS and CMOS

We Need to Talk! The CPU needs some method to talk to the devices to tell them what to do The devices need some way to send data to, and receive data from the CPU We need to expand our logical model from the CPU chapter

RAM Northbridge CPU Address Bus EDB

RAM Northbridge CPU Address Bus EDB Southbridge keyboard Video Card Hard Disk Drive

Chipset Address Bus Northbridge RAM CPU EDB Chipset Southbridge Keyboard Controller Video Card Hard Disk Drive

Problems Different motherboards have different keyboard controllers We need specific code to talk to the controller We need a place to store this specific programming and the programming for other devices (optical drive, floppy, etc.)

Solution Use a ROM chip Does not “forget” when power is removed, like RAM does Specific to motherboard and devices on it Read only, code does not change Can hold lots of programs, like a “chapter book” This is called “firmware”

System ROM Holds all the support programming for basic devices Is given a specific address space, or range of memory addresses for its code

Basic Hardware CPU Mouse Keyboard Video card RAM Network card Sound Card Floppy drive Hard Disk drive Zip drive USB ports CD-ROM drive

Basic Hardware CPU Mouse Keyboard Video card RAM Network card Sound Card Floppy drive Hard Disk drive Zip drive USB ports CD-ROM drive

Stored Information Stores access/write code for keyboard Stores access/write code for system speaker Stores access/write code for other chips (functions) on motherboard Does not change (more on this in a moment)

CMOS Complimentary Metal Oxide Semiconductor – how it was made Stores specific information about your system: Amount of RAM present Type of hard disk drive Date and Time Needs battery power to “remember” when power is off

Chipset Address Bus Northbridge RAM CPU EDB Chipset Southbridge BIOS And CMOS Keyboard Controller Video Card Mouse 20

BIOS - Basic Input Output System All devices need BIOS code to interoperate with CPU. Some is stored directly in System BIOS chip, most is stored elsewhere. One of the functions of BIOS is to provide the CMOS Setup Utility, or Setup, which allows us to change CMOS data.

BIOS information CMOS information

CMOS or Setup Option to use this program is early in the boot cycle – long before system loads Operating System Different key(s) to enter different Setups You should only get to Setup on purpose Program stored in BIOS, but only edits data in CMOS First CMOS was set of switches!

Who makes BIOS? Award Software Phoenix Technologies AMI – American Megatrends Inc. They write BIOS with lots of options; motherboard maker picks sections to be used Long gone are IBM’s patents on BIOS Same parent company

Setup Lots of screens and subscreens You don’t need to know about all settings on all screens Motherboard book is mildly helpful on some options You should know generally where things can be found

Date and Time and … Usually on the first screen, or first menu choice Lets you set Date, Time, floppy present (will autodetect it), memory count, hard drive(s) present (again, autodetected)

Here is where you set the boot order

When adding a modem, you might want to turn both of these off.

Chip Evolution EPROM – Erasable Programmable ROM; used ultra-violet light (sunlight) to erase EEPROM – Electrically Erasable – what we use today Flash ROM – another name for EEPROM NVRAM – Non-volatile RAM. Really not so, still needs battery power

The Three-In-One We have BIOS that stores code and access program to CMOS We have CMOS that stores changeable information about our system We have RTC (Real Time Clock) that keeps track of date and time All three are now rolled into one chip

About that Battery Should last 5 to 10 years You can find them at Savon even If your system forgets date and/or time, it’s time to replace the battery Beware the evil ESD!

More Battery Sometimes we WANT to reset CMOS: A forgotten password Too aggressive overclocking Incorrect low-level settings on some screen Either move the jumper, or pull the battery Do either for about 10 seconds If you are to pull battery, make sure you unplug system too

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More BIOS Option ROM – put a BIOS chip on the card/device Video is most common for this SCSI cards Network cards have socket, chip is extra

Power On Self Test (POST) First thing to run when you start up computer Waits for “power good” signal from power supply, then starts the CPU FFFA is (almost) at the end of 1st meg of RAM, contains a “Jump Immediate” instruction to beginning of POST code CPU instructs our “short list” items to check themselves out with own diagnostic

POST, cont. Until video card is done, we are “in the dark” and could get “Beep Codes” for errors. Don’t memorize them. Constant beeping is either memory or video error. Once the video card has checked itself out, we could get error codes/messages on the monitor as other devices report status 5

POST, cont. About this time, we get the option to run CMOS Setup routine POST is done; control passes to “Bootstrap Loader.” It’s job is to find OS. Looks for the “Boot Sector” on floppy/hard disk drive(s) A disk with functional OS is called a “Bootable Disk” or “System Disk.” Could be floppy (DOS), CD or hard disk drive.

Boot Order Somewhere in CMOS Setup is the ability to set the boot order, or the order in which the bootstrap loader will search for an OS. I set my systems to: CD/DVD drive, floppy and then hard disk drive. Usually, if a bootable CD is found, you get the option to boot from it (by pressing any key). Do nothing and loader continues the search for an OS.