two data busses at a time, allowing input or output and switching between them by signaling with the DBIN pin on the 8080. Since only one bus was being used at a time, they could share a set of eight pins, which allowed the eight formerly dedicated to the second bus to be used as ground lines instead. Ultimately, the idea of using an external chassis was dropped. By this time the decision to use the additional lines for grounds had been made, which had the desirable side-effect of making the board easier to design. The same 50-pin concept was instead implemented in an internal expansion chassis, the Sol-BPB. This extended vertically upwards from roughly the center of the motherboard. It had five horizontal connectors, and a metal framework on either side mechanically supported the expansion cards. The chassis also had another edge connector at the top, but it is unclear whether this could be used for further expansion. The BPB retained the DBIN signaling and ground pins of the early design and this quickly became a de facto standard for S-100 cards. This change to the bus design was contentious, as it meant cards for the Altair did not work in the Sol without some adjustments. Felsenstein noted, "I take the position that Bob made me do it, and he takes the position that history will absolve him." Software Three "personality modules" were released with the original systems. CONSOL provided a simple terminal emulator function, along with a small number of additional commands to load and run programs from tape using TLOAD. SOLOS added names to the files on the cassette, the TSAVE command for saving data to the tape into a named file, and TCAT to print out the details of a named program. TXEC loaded and executed a named program in one step. SOLED included block-mode editing, used on some mainframe systems, but it is not clear if this was actually available. One commonly used software for the Sol-20 was the BASIC/5 language. This was able to run in even a minimal machine with a 4 KB expansion, but in order to fit it had only single-precision floating point numbers and lacked string variables. An Extended BASIC that ran in 8 KB added strings and other functions. Processor Technology also sold a wide variety of other programs, including many games, on cassette format for the Sol, or