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In DOS memory management, conventional memory, also called base memory, is the first 640 kilobytes () of the memory on IBM PC or compatible systems. It is the read-write memory directly addressable by the processor for use by the operating system and application programs. As memory prices rapidly declined, this design decision became a limitation in the use of large memory capacities until the introduction of operating systems and processors that made it irrelevant. ==640 KB barrier== The 640 KB barrier is an architectural limitation of IBM and IBM PC compatible PCs. The Intel 8088 CPU, used in the original IBM PC, was able to address 1 MB (220 bytes), since the chip offered 20 address lines. The first memory segment (64 KB) of the conventional memory area is named low memory. In the design of the PC, the memory below 640 KB was for random-access memory on the motherboard or on expansion boards. The 384 KB above was reserved for system use and optional devices. This upper portion of the 8088 address space was used for the ROM BIOS, additional read-only memory, BIOS extensions for fixed disk drives and video adapters, video adapter memory, and other memory-mapped input and output devices. The design of the original IBM PC placed the Color Graphics Adapter (CGA) memory map and other hardware in the 384 KB upper memory area (UMA). The need for more RAM grew faster than the needs of hardware to utilize the reserved addresses, which resulted in RAM eventually being mapped into these unused upper areas to utilize all available addressable space. This introduced a reserved "hole" (or several holes) into the set of addresses occupied by hardware that could be used for arbitrary data. Avoiding such a hole was difficult and ugly and not supported by MS-DOS or most programs that could run on it. Later, space between the holes would be used as upper memory blocks (UMBs). To maintain compatibility with older operating systems and applications, the 640 KB barrier remained part of the PC design even after the 8086/8088 had been replaced with the Intel 286 processor, which could address up to 16 MB of memory in Protected mode. The 1 MB barrier also remained as long as the 286 was running in Real mode, since MS-DOS required Real mode which uses the segment and offset registers in an overlapped manner such that addresses with more than 20 bits are not possible. It is still present in IBM PC compatibles today if they are running in Real mode such as used by MS-DOS. The most modern Windows-based PCs even still have the "hole" in RAM in the area between 640 and 1024 KBs, which however is invisible to application programs thanks to paging and virtual memory. A similar 3 GB barrier exists, which reduces 32-bit addressing from 4 GB to ~3 GB on motherboards that use memory mapped I/O. However, due to applications not assuming that the 3–4 GB range is reserved, there is no need to retain this addressing for compatibility, and thus the barrier is easily removed by using a separate address bus for hardware, and only affects a relatively small number of computers of the 386 era as contemporary Memory Controllers and Buses (e.g. Industry Standard Architecture) could only reach 16 Megabytes. (Some motherboards feature a "Memory Hole at 15 Megabytes" option required for certain VGA video cards that require exclusive access to one particular megabyte for video memory.) Newer Video cards on AGP (PCI memory space) bus can have 256MB memory with 1GB aperture size. ===Additional memory=== One technique used on early IBM XT computers was to ignore the extended video memory block and push the limit up to the start of the Monochrome Display Adapter (MDA). Sometimes software or a custom address decoder was used so that attempts to use the video card memory went instead to the standard memory. This moved the barrier to 704 KB.〔().〕 Memory managers on 386-based systems (such as QEMM or MemoryMax in DR-DOS) could achieve the same effect, adding conventional memory at 640 KB and moving the barrier to 704 KB or 736 KB (the start of the CGA). Only CGA could be used in this situation, because Enhanced Graphics Adapter (EGA) video memory was immediately adjacent to the conventional memory area below the 640 KB line; the same memory area could not be used both for the frame buffer of the video card and for transient programs. The AllCard, an add-on memory management unit for XT-class computers, allowed normal memory to be mapped into the A0000-EFFFF (hex) 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 hardware compatibility. It was also possible to use DOS's utility for console redirection, CTTY, to direct output to a dumb terminal or another computer running a terminal emulator. The video card could then be removed completely, and assuming the BIOS still permitted the machine to boot, the system could achieve a total memory of 960 KB of RAM. This also required that the system have at least 2 MB of physical memory in the machine. This procedure was tested on a 486 with IBM PC DOS 7.0. The total operating system footprint was around 20 KB, most of DOS residing in the high memory area (HMA). 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Conventional memory」の詳細全文を読む スポンサード リンク
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