[ The PC Guide | Systems and Components Reference Guide | System Memory | Logical Memory Layout | Upper Memory Area (UMA) ]

Usage Map of the Upper Memory Area

The table below shows the layout of the upper memory area and the way that the various memory locations are typically used in a modern PC:

Here is what the different entries mean:

• VGA Graphics Mode Video RAM: The 64 KB block from A0000h to AFFFFh is reserved for use as VGA video graphics memory. (Original VGA is 320x200 in 256 colors, which requires 64,000 bytes). Of course modern video cards use much more memory than this, built into the card itself. This area of RAM is reserved and unavailable.
• VGA Monochrome Text Mode Video RAM: This 32 KB of memory is used when the video card is in monochrome text mode. However, in modern systems the video card is almost never in monochrome text mode; even when displaying monochrome text the color text mode is used and a white or gray color is just used for the text. Under some circumstances this block of memory can be recovered and used for drivers, to save conventional memory. See here for details.
• VGA Color Text Mode Video RAM: This 32 KB of memory is used by the video card when displaying color text. This area of RAM is reserved and unavailable.
• VGA Video BIOS ROM: C0000-C7FFFh is the normal memory location for the Video BIOS ROM, which contains the code used by your video card. (This is the code, in ROM, as opposed to the data displayed by the card, which of course changes and is stored in RAM). The RAM that underlies this ROM can be used to shadow the video BIOS.
• IDE Hard Disk BIOS ROM: C8000h is the default location for the BIOS that controls the IDE hard disk. It is referenced when the system is booted.
• Optional Adapter ROM BIOS or RAM UMBs: The blocks of memory from CC000h to DFFFFh are available for optional use. This memory can be used by hardware adapters for their ROM code (for example, some network cards, or SCSI host adapters). Or, the memory can be used as upper memory block RAM for loading drivers. Usually this memory is referred to in chunks of 16 KB (4000h) but a device does not need to use this much if it does not need it. This address space is normally empty unless you are using specialized hardware such as the devices I just mentioned.
• System BIOS Plug and Play Extended Information: The area of memory from E0000 to EFFFFh is used by Plug and Play BIOSes to hold its Extended System Configuration Data (ESCD).
• System BIOS ROM: The last 64 KB of the upper memory area holds the system BIOS code itself. RAM in this vicinity is normally used to shadow the system BIOS.

Next: Expanded Memory (EMS)

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