Upper memory area

In DOS memory management, the upper memory area (UMA) is the memory between the addresses of 640 KB and 1024 KB (0xA0000–0xFFFFF) in an IBM PC or compatible. IBM reserved the uppermost 384 KB of the 8088 CPU's 1024 KB address space for BIOS ROM, Video BIOS, Option ROMs, video RAM, RAM on peripherals, memory-mapped I/O, and obsoleted ROM BASIC. However, even with video RAM, the ROM BIOS, the video BIOS, the option ROMs, and I/O ports for peripherals, much of this 384 KB of address space was unused. As the 640 KB memory restriction became ever more of an obstacle, techniques were found to fill the empty areas with RAM. These areas were referred to as upper memory blocks (UMBs).

In DOS memory management, the upper memory area (UMA) is the memory between the addresses of 640 KB and 1024 KB (0xA0000–0xFFFFF) in an IBM PC or compatible. IBM reserved the uppermost 384 KB of the 8088 CPU's 1024 KB address space for BIOS ROM, Video BIOS, Option ROMs, video RAM, RAM on peripherals, memory-mapped I/O, and obsoleted ROM BASIC. However, even with video RAM, the ROM BIOS, the video BIOS, the option ROMs, and I/O ports for peripherals, much of this 384 KB of address space was unused. As the 640 KB memory restriction became ever more of an obstacle, techniques were found to fill the empty areas with RAM. These areas were referred to as upper memory blocks (UMBs). ===Windows=== The increasing popularity of Windows 3.0 made the necessity of the upper memory area less relevant, as Windows applications were not directly affected by DOS' base memory limits, but DOS programs running under Windows (with Windows itself acting as a multitasking manager) were still thus constrained. With the release of Windows 95, it became less relevant still, as this version of Windows provides much of the functionality of the DOS device drivers to DOS applications running under Windows, such as CD, network and sound support; the memory map of Windows 95 DOS boxes was automatically optimised. However, not all DOS programs could execute in this environment. Specifically, programs that tried to directly switch from real mode to protected mode would not work as this was not allowed in the virtual 8086 mode it was running in. Also, programs that tried making the switch using the Virtual Control Program Interface (VCPI) API (which was introduced to allow DOS programs that needed protected mode to enter it from the virtual 8086 mode set up by a memory manager, as described above) didn't work in Windows 95. Only the DOS Protected Mode Interface (DPMI) API for switching to protected mode was supported. ==Implementation== ===Virtual 8086 Mode=== Upper memory blocks can be created by mapping extended memory into the upper memory area when running in virtual 8086 mode. This is similar to how expanded memory can be emulated using extended memory so this method of providing upper memory blocks is usually provided by the expanded memory manager (for example EMM386). The application programming interface for managing the upper memory blocks is specified in the eXtended Memory Specification. ===Shadow RAM=== On many systems including modern ones it is possible to use memory reserved for shadowing expansion card ROM as upper memory. Many chipsets reserve up to 384 KB RAM for this purpose and since this RAM is generally unused it may be used as real mode upper memory with a custom device driver, such as UMBPCI. ===IBM XT=== On IBM XT computers, it was possible to add more memory to the motherboard and use a custom address decoder PROM to make it appear in the upper memory area. As with the 386-based upper memory described above, the extra RAM could be used to load TSR files, or as a RAM disk. The AllCard, an add-on memory management unit for XT-class computers, allowed normal memory to be mapped into the 0xA0000-EFFFF address range, giving up to 952 KB for DOS programs. Programs such as Lotus 1-2-3, which accessed video memory directly, needed to be patched to handle this memory layout. Therefore, the 640 KB barrier was removed at the cost of software compatibility. This usage of the upper memory area is different from using upper memory blocks, which was used to free conventional memory by moving device drivers and TSRs into the upper 384 KB of the 1 MB address space, but left the amount of addressable memory (640 KB) intact. ==See also== DOS memory management Conventional memory Extended memory (XMS) Expanded memory (EMS) High Memory Area (HMA) Global EMM Import Specification (GEMMIS) LOADHIGH A20 line ==References==

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