How did Microsoft make an OS without an OS to code on?

Microsoft didn't create an operating system without an operating system to code on. The first version of Microsoft Windows, released in 1985, was actually built on top of an existing operating system called MS-DOS.

MS-DOS (short for Microsoft Disk Operating System) was an operating system that was popular in the early days of personal computing. It was a command-line interface that allowed users to interact with their computer using text-based commands. It provided basic functionality for managing files, running programs, and accessing hardware.

Microsoft saw an opportunity to improve the user experience by creating a graphical user interface (GUI) for MS-DOS. This would allow users to interact with their computer using a mouse and graphical icons, rather than typing in commands.

To build Windows, Microsoft used a development process called "rehosting." This involved taking the source code for the GUI from another operating system (in this case, Apple's Macintosh operating system), and recompiling it to run on top of MS-DOS. Microsoft did not copy Apple's source code or infringe on their patents; instead, they reverse-engineered the Macintosh GUI to create their own implementation.


Windows 1.0 was released in 1985 and included a number of applications, such as a calendar, calculator, and file manager. It was not a standalone operating system, but rather an application that ran on top of MS-DOS. Later versions of Windows, such as Windows 95, were standalone operating systems built from the ground up by Microsoft.

So, in summary, Microsoft did not create an operating system without an operating system to code on. They built the first version of Windows on top of an existing operating system, using a development process called rehosting, and later versions of Windows were standalone operating systems built from scratch by Microsoft.