The term Upper Memory Area (UMA) describes the reserved 384KB at the top of the first megabyte of system memory on a PC/XT and the first megabyte on an AT-type system. This memory has the addresses from A0000 through FFFFF. The way the 384KB of upper memory is used breaks down as follows:

• The first 128KB after conventional memory is called video RAM. It is reserved for use by video adapters. When text and graphics are displayed onscreen, the data bits that make up those images reside in this space. Video RAM is allotted the address range A0000BFFFF.

• The next 128KB is reserved for the adapter BIOS that resides in read-only memory chips on some adapter boards plugged into the bus slots. Most VGA-compatible video adapters use the first 32KB of this area for their onboard BIOS. Any other adapters installed can use the rest. For example, SCSI, ATA RAID, and SATA adapters with onboard BIOS chips use a portion of this area to store their BIOSes. Many network adapters also use this area for special-purpose RAM called shared memory. Adapter ROM and special-purpose RAM is allotted the address range C0000DFFFF.

• The last 128KB of memory is reserved for motherboard BIOS (the basic input/output system, which is stored in read-only RAM chips or ROM). The POST and bootstrap loader, which handles your system at boot-up until the operating system takes over, also reside in this space. Most systems use only the last 64KB (or less) of this space, leaving the first 64KB or more free for remapping with memory managers. Some systems also include the CMOS Setup program in this area. The motherboard BIOS is allotted the address range E0000FFFFF.

Not all the 384KB of reserved memory is fully used on most 16-bit and higher systems. For example, according to the PC standard, reserved video RAM begins at address A0000, which is right at the 640KB boundary. Normally, it is used for VGA graphics modes, whereas the monochrome and color text modes use B0000B7FFF and B8000BFFFF, respectively. Older non-VGA adapters used memory only in the B0000 segment. Different video adapters use varying amounts of RAM for their operations, depending mainly on the mode they are in. To the processor, however, it always appears as the same 128KB area, no matter how much RAM is really on the video card. This is managed by bank switching areas of memory on the card in and out of the A0000BFFFF segments.

Although the top 384KB of the first megabyte was originally termed reserved memory, it is possible to use previously unused regions of this memory to load 16-bit device drivers (such as the ANSI.SYS screen driver that comes with DOS) and memory-resident programs (such as MOUSE.COM, the DOS mouse driver), which frees up the conventional memory they would otherwise require. Note that this does not affect 32-bit device drivers such as those used with Windows 9x, NT/2000/XP, and so forth because they load into extended memory with no restrictions. The amount of free UMA space varies from system to system, depending mostly on the adapter cards installed on the system. For example, most video adapters, SCSI adapters, and some network adapters require some of this area for built-in ROMs or special-purpose RAM use.