Micro Firmware Tech Support

What is BIOS Shadowing?

Filename:  SHADOW.TXT
WWW URL:   http://www.firmware.com/support/bios/shadow.htm
FTP URL:   ftp://ftp.firmware.com/text/shadow.txt
Revision:  02/28/97  TLS  Micro Firmware Technical Support
Summary:   Explanation of BIOS ROM shadowing.

Shadowing refers to the technique of copying BIOS code from slow ROM chips into faster RAM chips during boot-up so that any access to BIOS routines will be faster. DOS and other operating systems may access BIOS routines frequently. System performance is greatly improved if the BIOS is accessed from RAM rather than from a slower ROM chip.

A PC system's ROM BIOS is stored in a ROM chip on the motherboard. On an older motherboard, the BIOS will reside in one, two or four EPROM chips. On a newer motherboard, the BIOS will probably be kept in a flash chip. In either case, the chip is accessed 8 bits at a time, while RAM on a 486 or 386DX system is accessed 32 bits at a time. Also the access time of ROM chips is slower - 150ns to 200ns compared to 60ns or 70ns for RAM.

The 64KB memory range F000-FFFF is reserved for the ROM BIOS. The ROM chip is accessed at this address. The same address range exists in RAM as well. If shadowing is enabled (in some systems a CMOS setup option, in others not optional), the BIOS is copied from the ROM chip into the same location in RAM during its boot-up process. System BIOS shadowing should normally be enabled on all PCs.

There is usually an option in CMOS setup to enable video BIOS shadowing. The video BIOS is normally in ROM chips on the video card. On motherboards with built-in video, the video BIOS may be in the same 128K chip as the system BIOS (and may be addressed at E000). The video BIOS is normally addressed at C000-C7FF. If video BIOS shadowing is enabled, the system BIOS will copy the video BIOS to RAM at C000-C7FF during its boot-up process. Video BIOS shadowing would normally be enabled on all PCs.

The BIOS may also allow for shadowing other ROMs such as on a network card. These ROMS would normally be located in the upper memory area in the range between C800 and EFFF.


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