Electronic – Why isn’t the BIOS’ ROM chip made using CMOS technology


After reading a computer hardware course on BIOS/CMOS, I'm still unable to determine the reason why the BIOS' ROM chip isn't built using CMOS technology, and why it is connected to a separate chip called "CMOS" for storing the configuration information.

This is from the lecture note:

Programs are stored on the system BIOS chip, while the changeable data
is stored on a CMOS chip

CMOS Group of Hardware: Hardware that is common, necessary but may
change – RAM, hard drives, floppy drives, serial and parallel ports

I know that the BIOS is stored in a flash memory, and that CMOS MOSFET technology dissipates less power compared to other implementations.

Why is it only the BIOS ROM that doesn't use CMOS, like other storage devices – what exactly is the advantage? And why can't the BIOS configuration information be stored in its own ROM chip instead of the "CMOS chip"?

Best Answer

You're mixing up implementation technology with colloquial terms for functionality.

CMOS - Complementary Metal Oxide Semiconductor - is a method of making logic and related circuitry using both N-Channel and P-Channel field effect transistors. One of its defining characteristics is extremely low static power consumption - power is almost only used when changing state. As a result, a CMOS static memory chip can retain its contents for years on a battery, and is a handy place to store semi-permanent information.

BIOS and related startup code has traditionally been stored in PROM or EPROM devices. EPROMs in the era of the IBM PC were typically made in pre-CMOS technologies like NMOS, but the critical distinction is that these were not typically writeable when installed in the computer, but only in a special programmer. Further while many contemporary systems and clones used EPROMs, the actual production versions of the IBM-PC used cheaper non-reprogrammable PROMs (pinouts were typically compatible in use).

The PC-AT design then added a battery backed CMOS memory for storing customizable settings, and also (perhaps in the same device) a persistent real time clock. This came to be colloquially referred to by end-users as CMOS, though it was of course only a particular usage enabled by a then rapidly spreading chip technology.

Today of course we no longer use parallel E/PROMs for BIOS, but rather use serial NOR flash and transfer the contents into faster RAM for execution. Modern FLASH chips are in fact made of CMOS-derived technologies. And they typically can be reprogrammed in circuit. It's really up to the system designer if they want to put semi-permanent configuration information in a battery backed RAM, or in reprogrammable flash - the end user or post-boot operating system may have little actual visibility into a distinction.

But even if flash is used for settings, there's still typically going to be a low power real time clock that keeps running on battery.