Q. What do you mean by SSI chips?

The 3^rd generation computers mostly used SSI chips. One of the key concepts that were brought forward for the duration of this time was concept of family of compatible computers. IBM majorly began this theory with its system/360 family.A family of computers comprises different models. Every model is assigned a model number e.g. IBM system/360 family has Model 30, 40, 50, 65 and 75. The processing speed, memory capacity as well as cost increases as we go up the ladder. But a lower model is compatible to higher model which means program written on a lower model can be executed on a higher model without requiring any change. Only time of execution is decreased as we go in the direction of higher model and also a higher model has greater number of instructions. The major advantage of this family system was flexibility in selection of model.

E.g. if you had a limited budget as well as processing requirements then you might probably start with relatively moderate model. As your business grows up and your processing requirements rise you may upgrade your computer with higher models dependent on your need. Though please note that as you've gone for computer of same family you won't be sacrificing investment on already developed software as they can still be used on newer machines too.