The 9815A was a significant improvement on the 98X0 computers. It was significantly lower cost than the previous machines and also much smaller and lighter. But, the 9815 didn't skimp on features. It was the first HP computer to incorporate the new mini-cartridge tape mass storage system, which provided 94K of storage space. The 9815 also had a built-in 16-character thermal printer and two optional I/O expansion slots. The 9815 was also the first HP desktop computer to move away from ROM-based incremental functionality; language extensions, programs, I/O capabilties and other utilities were input from the tape cartridge. It would be another six years before all new HP computers were weaned from upgrades based primarily on pluggable ROMs. The 9815S was introduced in 1978. The two I/O channels were standard in the 9815S; it also had 3800 program steps, compared to 472 in the 9815A. The 9815 had dedicated interface I/O cables; it did not share the same expansion backplane as all the other computers in this exhibit hall.