Early IBM PC operating systems IBM released its first PC in 1981. Which operating system was most popularly associated with that machine at launch?

Introduction / Context:
The IBM PC (Model 5150) established a de facto standard platform. Understanding its original operating system clarifies many historical compatibility decisions still echoing in today’s PCs.

Given Data / Assumptions:

• Year: 1981 release of the IBM PC.
• IBM distributed its own branded operating system for this machine.
• Similar codebase was also licensed and sold under a different name by Microsoft to other OEMs.

Concept / Approach:

IBM marketed the OS as PC-DOS, licensed from Microsoft’s MS-DOS. While CP/M was a popular OS on earlier 8-bit systems, IBM’s PC-DOS rapidly became the dominant OS on the IBM PC platform. OS/360 was an IBM mainframe operating system, unrelated to the PC.

Step-by-Step Solution:

Verification / Alternative check:

IBM documentation for the 5150 references PC-DOS versions 1.x. Press materials and technical manuals from the era confirm.

Why Other Options Are Wrong:

MS-DOS is closely related but not the IBM-branded OS for the IBM PC at launch. CP/M was considered, but IBM shipped PC-DOS. OS/360 is a mainframe OS.

Common Pitfalls:

Conflating PC-DOS and MS-DOS naming; assuming CP/M carried over from earlier microcomputers to the IBM PC default.

Final Answer:

PC-DOS.