Background

The CMOS (complementary metal oxide semiconductor) memory is actually 
a 64 or 128 byte battery-backed RAM memory module that is a part of the 
system clock chip. Some IBM PS/2 models have the capability for a
2k (2048 byte) CMOS ROM Extension.

First used with clock-calender cards for the IBM PC-XT, when the PC/AT
(Advanced Technology) was introduced in 1985, the Motorola MC146818 
became a part of the motherboard. Since the clock only uses fourteen of 
the RAM bytes, the rest are available for storing system configuration data.

Interestingly, the original IBM-PC/AT (Advanced Technology) standard for 
the region 10h-3Fh is nearly universal with one notable exception: The 
IBM PS/2 systems deviate considerably (Note: AMSTRAD 8086 machines were 
among the first to actively use the CMOS memory available and since they 
*predate* the AT, do not follow the AT standard).

This is just another example of how IBM created a standard, lost control 
of it, tried to replace it, failed and lost market share in the process. 

Originally, the IBM PC/AT only made use of a small portion of CMOS memory
and was defined in the IBM PC/AT Technical Reference Manual, specifically 
bytes 10h, 12h, 14h-18h, 2Eh-33h. The balance was left undefined but was 
quickly appropriated by various BIOS manufacturers for such user-selectable 
options such as wait states, clock speeds, initial boot drive selection, and 
password storage.

Later, as CMOS memory requirements grew, newer clock chips with 128
bytes of RAM came into use. However the fact remains that once the AT 
standard was established, only IBM has tried to change the definitions 
of that first description.

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