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[ The PC Guide | Systems and Components Reference Guide | Motherboard and System Devices | System BIOS | BIOS Settings | Advanced Features ]

C8000-CBFFF Shadow, CC000-CFFFF Shadow, etc.

Most BIOSes have several settings for shadowing each of the 16K blocks of RAM from C8000h through DFFFFh. These settings show up as something like "C8000-CBFFF Shadow", "CC000-CFFFF Shadow", etc., up to "DC000-DFFFF Shadow". Some systems have settings for ROM shadowing in 32K blocks instead of 16K, so you will see "C8000-CFFFF" instead of "C8000-CBFFF" and "CC000-CFFFF". (Some systems leave off the last digit in their notation, calling the blocks "C800-CBFF", etc. It's the same thing.)

When enabled, the setting selected turns on adapter ROM shadowing for that 16K block of memory. See here for a full description of what ROM shadowing does; in short, it speeds up your system by copying the contents of any BIOS code found in adapters using this memory space, from the slow ROM in which it resides into faster RAM.

The areas of memory from C8000 to DFFFFh are normally used by expansion cards such as network adapters. Turning on shadowing would speed these adapters up in the same way that shadowing the system BIOS speeds up the system BIOS code. However, things are much more tricky here, because some adapters use RAM as well as ROM, and map this RAM into this address space as well. If they do, and you enable shadowing, the adapter will malfunction because shadowing write-protects the RAM it uses (since it thinks it is emulating a ROM only, which cannot be rewritten). This can cause spurious results when using these cards, and can be very difficult to diagnose. In addition, normally unused areas of memory in this region are used as UMBs for loading drivers via the EMM386 driver, and enabling shadowing will cause this to malfunction.

For this reason, the default for shadowing these adapter ROM areas is normally "Disabled" and I recommend that it be left that way in most cases. If you know all the details on the card whose ROM you are trying to shadow, enabling this can in theory increase performance, but it is not going to be anything very substantial in most cases. Incidentally, on most IDE/ATA systems, the block from C8000 to CBFFFh is reserved by the IDE hard disk BIOS.

Next: Advanced Chipset Features

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