On 12 Feb 99, at 20:23, Mick Fitzpatrick wrote:

> Could someone please tell me the difference between BIOS & CMOS. I
> have read various explanations but they kinda merge to me!! A simple
> definition will be appreciated.

  The CP/M operating system broke system functions into three basic
groups:  a low-level input/output interface (called BIOS), a higher-
level file-system interface (called BDOS), and a command-line user
interface (whose name escapes me...).
  At least some of the BIOS functions were implemented in ROM
(permanent Read-Only Memory), so that a CPU, on boot, could be pointed
to instructions already in memory.  These included a disk-read-by
sector-number routine, which could be used to load additional
components from the boot drive, which provided a set of disk-access-by-
filename routines and other OS services.
  MS-DOS/PC-DOS began life as a re-engineered CP/M, and retained this
terminology and basic structure.  Bigger ROM chips have allowed more
functionality to be added to the BIOS, including -- since the PC/XT --
code to scan for additional ROMs on expansion cards and include them in
the startup process.

  Some details of the startup process need to be customized to reflect
the devices actually present in the machine.  The original PC and most
XT models had two banks of DIP switches to set configuration info.  The
AT came with a special setup disk that let users enter configuration
information from the keyboard -- it stored the information in a small
package of Non-Volatile RAM.
  Non-Volatile RAM is any RAM memory that keeps its values when the
power is switched off.  Specifically, IBM chose to use a small amount
of static CMOS (Complementary Metal-Oxide Semiconductor) RAM, which
used very little power, and a battery to provide that power even when
the machine was turned off.

  So to sum up:  BIOS is code in ROM that initializes the system and
loads the operating system from disk.  CMOS is a memory technology,
chosen to implement storage of parameters needed by the BIOS, and the
term is often used to refer to this specific application.


David G

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