"Yellowstone" was the code name for IBM's new revolutionary computer, the IBM 5110.
About two years earlier, 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. However, it was a very primitive machine that was largely unsuccessful due to its high price tag (basic version costed $10,975) and limited expansion capabilities.
The new IBM 5110 was designed to provide much faster internal performance as well as many additional features. It supported full screen editing, 64 KB of RAM, the use of 8" disk drives and lower case characters.
The 5110 was later replaced by the 5120, an integrated system also known as the 5110 model 3.