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Extended System Configuration Data (ESCD)
If the BIOS were to assign resources to each PnP device on every boot, two problems would result. First, it would take time to do something that it has already done before, each boot, for no purpose. After all, most people change their system hardware relatively infrequently. Second and more importantly, it is possible that the BIOS might not always make the same decision when deciding how to allocate resources, and you might find them changing even when the hardware remains unchanged.
ESCD is designed to overcome these problems. The ESCD area is a special part of your BIOS's CMOS memory, where BIOS settings are held. This area of memory is used to hold configuration information for the hardware in your system. At boot time the BIOS checks this area of memory and if no changes have occurred since the last bootup, it knows it doesn't need to configure anything and skips that portion of the boot process.
ESCD is also used as a communications link between the BIOS and the operating system. Both use the ESCD area to read the current status of the hardware and to record changes. Windows 95 reads the ESCD to see if hardware has been changed and react accordingly. Windows 95 also allows users to override Plug and Play resource assignments by manually changing resources in the Device Manager. This information is recorded in the ESCD area so the BIOS knows about the change at the next boot and doesn't try to change the assignment back again.
The ESCD information is stored in a non-volatile CMOS memory area, the same way that standard BIOS settings are stored.
Note: Some (relatively rare)
systems using Windows 95 can exhibit strange behavior that is caused by incompatibility
between how Windows 95 and the BIOS are using ESCD. This can cause an "Updating
ESCD" message to appear each and every time the system is booted, instead of only
when the hardware is changed. See here for