Marketed as the desktop follow-on to the portable IBM 5110 Computing System, it featured two built-in 8-inch 1.2 MB floppy disk drives, an integrated 9-inch monochrome monitor, 32 KB RAM, plus an optional IBM 5114 stand-alone diskette unit with two additional 8-inch 1.2 MB floppy disk drives.

APL allowed numerous business software written on IBM minicomputers to run on the 5120.

[1] Aside from larger screen size and performance benefits over its predecessor, the IBM 5120 design incorporated several usability advantages: For its usability features and appearance, the IBM 5120 was recognized with two major industrial design awards and described with terms such as "clean, well thought out"; "subtle detailing shows great care in execution"; and "looks like quality″.

However, in 1981, Hal Prewitt, founder of Core International, Inc, invented and marketed the world's first and only hard disk subsystems and "CoreNet", a LAN used to share programs and data for the IBM 5110 and 5120 systems.

[5][6] In 1984, Core introduced PC51, software that allowed 5100 Series computer programs written in BASIC to run unmodified on the IBM PC and compatibles under PC DOS and share programs and data on CoreNet, the LAN for all these models.