326 THE PDP-11 FAMILY
microaddress register, a single-step mode, and a microbreak function.
Software diagnostic programs are used to diagnose errors in system peripherals as well as in all CPU subsystems, such as memory management unit and cache. User mode diagnostic programs allow peripheral diagnosis to occur while the system is available for other users. Conventional standalone diagnostic programs can also be used.
Physical packaging facilitates quick repair. Hinged card cages and modular power supplies allow easy access and module change.
The design of a mid-range minicomputer has been used as a concrete illustration of tradeoffs made to effect a price/performance balance. Designers use technology advances, e.g., doubling of density on a memory chip, to produce new designs in one of two design styles: constant cost/increasing functionality or constant functionality/decreasing cost. Increased use of microprogramming, a factor of 3 in this case study, is a trend that was observed.
By choosing a less powerful cache organization, the 11/60 design obtained a factor of 5 component reduction. Cache design also illustrates how some design parameters are highly interdependent. The frequency-driven design approach used on the floating-point processor can lead to a 40 percent performance gain.
Examples of added functionality in the constant-cost style of design include greater reliability and maintainability, and user microprogramming.