RAM was used differently on IBM PC computers and clones during the DOS era. Conventional memory was used for the operating system and programs, while upper memory and high memory areas were used to free up space. Digital Research DR DOS introduced a better strategy for conventional memory optimization, while Microsoft countered with their own version. With the advancement of Windows operating systems, conventional memory optimization became less important. FreeDOS is now available for those using legacy machines or running DOS programs.
RAM (Random Access Memory) was used very differently on IBM® PC computers and clones in the past, when DOS, a command-line operating system used before Windows®, dominated. The first 0-640 kilobytes (KB) of RAM were designated as conventional memory; the area where DOS is conventionally loaded at startup. The 640 KB – 1 Megabyte (MB) block made up of 384 KB was the Upper Memory Area (UMA); the first 64K beyond the 1MB limit became the high memory area (HMA or HIMEM), and everything designated as extended memory.
Conventional memory was used as a read/write area for the operating system and programs, for optimal fit. DOS also loaded routines, system drivers and system parameters into this space. To help free up space, some conventional memory residents were moved up the RAM ladder to the UMA and HMA. This was achieved by adding a couple of lines to the CONFIG.SYS file, a file which together with AUTOEXEC.BAT largely determined the memory loading parameters of the software and hardware devices used in DOS and early Windows® systems. In the late 1980s, the Quarterdeck Expanded Memory Manager® (QEMM) automated much of the tuning required by computer users of the time who were unable to do it manually.
In 1990, Digital Research® DR DOS version 5.0 introduced a better strategy for getting the most out of conventional memory, including its own built-in extended memory manager: EMM386.EXE. This version of DOS loaded almost entirely into high memory, freeing up conventional memory for programs that could only run there. Another advantage of this operating system is that it could be purchased by the public (since version 3.1), while MS DOS was only available bundled with hardware.
Microsoft® responded to Digital Research® with their own version 5.0 in 1991 which countered the benefits of DR DOS 5.0. The competition continued with DR. DOS 6.0 which included task switching and disk compression, and MS DOS 6.0 followed in nature. MS DOS 6.22 was the last independently marketed MS DOS package, while DR DOS continues to be sold through drdos.com.
As Windows® operating systems advanced, the constraints of conventional memory optimization became a thing of the past, except for those who continued to use DOS programs, out of necessity or for fun. Starting with Windows® 95, a stripped-down version of MS DOS was built in for the purpose of bootstrapping or troubleshooting. Windows® XP retains a skeleton version of MS DOS 8.0, accessible only by creating a boot diskette, as does Vista®. All versions of Windows® maintain a shell or command line interface. In earlier versions of Windows® it was COMMAND.EXE, which became CMD.EXE in the Windows-NT® family.
Starting in September 2006, a free and open source version of DOS, known as FreeDOS, also became available, although it was never developed beyond version 1.0. For those using legacy machines or running DOS programs today, there is a lot of information online for manually optimizing conventional memory to get the most out of DOS programs or games.
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