FreeDOS

FreeCOMFreeDOS 1.0FreeDOS 1.1FreeDOS 1.2BlinkyBlinky (mascot)FreeDOS-32
FreeDOS (formerly Free-DOS and PD-DOS) is a free operating system for IBM PC compatible computers.wikipedia
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DOS

COM1AUX:LPT1
It intends to provide a complete DOS-compatible environment for running legacy software and supporting embedded systems.
Later compatible systems from other manufacturers are DR DOS (1988), ROM-DOS (1989), PTS-DOS (1993), and FreeDOS (1998).

Jim Hall (computer programmer)

Jim Hall
Jim Hall who at the time was a student posted a manifesto proposing the development of PD-DOS, a public
Jim Hall (James F. Hall) is a computer programmer and advocate of free software, best known for his work on FreeDOS.

Pat Villani

DOS-CDOS/NT
Within a few weeks, other programmers including Pat Villani and Tim Norman joined the project.
Pasquale "Pat" J. Villani (18 April 1954 – 27 August 2011) was an American computer programmer, author, and advocate of free software, best known for his creation of DOS-C, a DOS emulator written in the C language and subsequently adapted as the kernel of the FreeDOS operating system and a number of other projects including DOSEMU for Linux.

COMMAND.COM

COMMANDDOS promptMS-DOS Prompt
Between them, a kernel (by Villani), the COMMAND.COM command line interpreter (by Villani and Norman), and core utilities (by Hall) were created by pooling code they had written or found available.
The compatible command processor under FreeDOS is sometimes also named FreeCom.

4DOS

JP SoftwareNDOSDESCRIPT.ION
However, other packages that form part of the FreeDOS project include non-GPL software considered worth preserving, such as 4DOS, which is distributed under a modified MIT License.
Currently, the source code is available under a modified MIT License but it "may not be used in any commercial product without written permission from Rex C. Conn" and "may not be compiled for use on any operating system other than FreeDOS" so it does not qualify as open source as defined by Open Source Initiative.

MS-DOS

DOSMS-DOS 5.0MS-DOS 6.0
Unlike most versions of MS-DOS, FreeDOS is composed of free and open-source software, licensed under the terms of the GNU General Public License.
Other solutions include using DOS compatible alternatives, such as FreeDOS or even copying the required files and boot sector themselves.

QEMM

MagnaRAMQRAMQuarterdeck Expanded Memory Manager
Furthermore, with use of HX DOS Extender, many Windows Console applications function properly in FreeDOS, as do some rare GUI programs, like QEMM and Bochs.
QEMM provides up to 635K free conventional memory (RAM under 640K), far better than pure MS-DOS EMM386, FreeDOS JEMM386, UMBPCI and many other memory manager programs.

DOSEMU

DOSEMU, stylized as dosemu, is a compatibility layer software package that enables DOS operating systems (e.g., MS-DOS, DR-DOS, FreeDOS) and application software to run atop Linux on x86-based PCs (IBM PC compatible computers).

DOSBox

DOSBox builtin DOS
It can also boot disk images with real DOS environments (e.g. MS-DOS, PC DOS, DR-DOS or FreeDOS) as well as other operating systems.

ReactOS

FreeWin95ReactOS ExplorerReactOS kernel
The FreeDOS kernel can be booted by adding it to the Windows 2000 or XP's NT Boot Loader configuration file, boot.ini, or freeldr.ini equivalent for ReactOS.

Command-line interface

command linecommand-linecommand line interface
In many versions of DOS (MS-DOS/PC DOS 2.xx and higher, all versions of DR-DOS since 5.0, as well as PTS-DOS, Embedded DOS, FreeDOS and RxDOS) the switch character (sometimes abbreviated switchar or switchchar) to be used is defined by a value returned from a system call (INT 21h/AH=37h).

Monolithic kernel

MonolithicDynamic Extendablekernel

IBM PC compatible

PCPC compatibleIBM PC compatibles
FreeDOS (formerly Free-DOS and PD-DOS) is a free operating system for IBM PC compatible computers.

Floppy disk

floppy disk drivefloppy drivediskette
FreeDOS can be booted from a floppy disk or USB flash drive.

USB flash drive

flash driveUSB flash drivesUSB drive
FreeDOS can be booted from a floppy disk or USB flash drive.

GNU General Public License

GPLGNU GPLGPLv2
Unlike most versions of MS-DOS, FreeDOS is composed of free and open-source software, licensed under the terms of the GNU General Public License.

MIT License

MITX11 LicenseMIT Licence
However, other packages that form part of the FreeDOS project include non-GPL software considered worth preserving, such as 4DOS, which is distributed under a modified MIT License.

Microsoft

Microsoft CorporationMicrosoft Corp.MS
The FreeDOS project began 29 June 1994, after Microsoft announced it would no longer sell or support MS-DOS.