Upper Memory Area
The upper memory area (UMA) refers to the address space between 640 KiB and 1024 KiB (0xA0000–0xFFFFF). Three 128 KiB regions were defined in this area. The 128 KiB region between 0xA0000 and 0xBFFFF was reserved for video adapter screen memory. The physical address space between 0xC0000 and 0xDFFFF was reserved for device BIOS ROMs, and special RAM usually shared with physical devices (for example, shared memory for a network adapter). The IBM PC reserved the uppermost 128 KiB of the address space from 0xE0000 to 0xFFFFF for the ROM BIOS and Cassette BASIC read-only memory (ROM).
For example, the monochrome video adapter memory area ran from 704 to 736 KiB (0xB0000–0xB7FFF). If only a monochrome display adapter was used, the address space between 0xA0000 and 0xAFFFF could be used for RAM, which would be contiguous with the conventional memory.
The system BIOS ROMs must be at the upper end of the address space because the CPU starting address is fixed by the design of the processor. The starting address is loaded into the program counter of the CPU after a hardware reset and must have a defined value that endures after power is interrupted to the system. On reset or power up, the CPU loads the address from the system ROM and then jumps to a defined ROM location to begin executing the system power-on self-test, and eventually load an operating system.
Since an expansion card such as a video adapter, hard drive controller, or network adapter could use allocations of memory in many of the upper memory areas, configuration of some combinations of cards required careful reading of documentation, or experimentation, to find card settings and memory mappings that worked. Mapping two devices to use the same physical memory addresses could result in a stalled or unstable system. Not all addresses in the upper memory area were used in a typical system; unused physical addresses would return undefined and system-dependent data if accessed by the processor.
Read more about this topic: DOS Memory Management
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