IBM 5100... In September 1975, IBM announced its smallest and first portable computer (If you consider a 28 Kgs. computer portable, that is), the IBM 5100, no bigger than one of IBM's typewriters. Developed in Rochester, it used the same operating system as IBM's /370 line of main frames. Thus it could accommodate the same APL interpreter, permitting the use of APL programs. A BASIC interpreter was also available, depending of the 5100 version chosen.