Conventional memory

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.

The 640 KB barrier is an architectural limitation of 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. In the design of the PC, the memory below 640 KB was for random-access memory on the motherboard or on expansion boards, and it was called the conventional memory area. The first memory segment (64 KB) of the conventional memory area is named lower memory or low memory area. The remaining 384 KB beyond the conventional memory area, called the upper memory area (UMA), was reserved for system use and optional devices. UMA 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 in 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 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 80286 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 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 DOS. Even the most modern Intel PCs still have the area between 640 and 1024 KB reserved. This however is invisible to programs (or even most of the operating system) on newer operating systems (such as Windows, Linux, or Mac OS X) that use virtual memory, because they have no awareness of physical memory addresses at all. Instead they operate within a virtual address space, which is defined independently of available RAM addresses.

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. Later video cards using the AGP (PCI memory space) bus can have 256 MB memory with 1 GB aperture size.

One technique used on early IBM XT computers was to install additional RAM into the video memory address range and push the limit up to the start of the Monochrome Display Adapter (MDA). Sometimes software or a custom address decoder was required for this to work. This moved the barrier to 704 KB (with MDA/HGC) or 736 KB (with CGA).

Memory managers on 386-based systems (such as QEMM or MEMMAX (+V) in DR-DOS) could achieve the same effect, adding conventional memory at 640 KB and moving the barrier to 704 KB (up to segment B000, the start of MDA/HGC) or 736 KB (up to segment B800, 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.

All Computers' piggy-back add-on memory management units AllCard for XT- and Chargecard for 286/386SX-class computers, as well as MicroWay's ECM (Extended Conventional Memory) add-on-board 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.