516 Part 6 Computer families Section 1 The IBM 701-7094 II sequence, a family by evolution

to achieve compatibility is inexpensive when the system price is considered. Also, the incremental changes in the ISP do little to increase the Pc performance. Compared with the 704, the extensive order code of the 7094 shows an evolution in which for marketing, emotional, or analytic reasons new instructions were added. The index registers and their instructions are a good example of this trend. The 7094 has a very general set of index-register transmission instructions; if implemented properly, they are probably easier to provide than the original 704 instructions.

In the implementation of the double-precision floating-point hardware, the sense-indicator register is needed for temporary storage. Thus a user has to preserve this register when double- precision floating-point instructions are given. The reason for this undoubtedly relates to field modifications and cost. In an original design this would be inexcusable; in this case double-precision floating point is undoubtedly worth the loss of sense indicators.

All in all, the designers of the 704-7094 II provided increased generality through evolution. They gradually ran out of patching time, technology, instruction encoding space, and memory addressing bits, while exceeding compatibility constraints. It was indeed time to create the IBM System/360.