An expert-system paradigm for design

level designer can become the detailed functional specification needed by a lower level. Within a design abstraction there are usually several possible alternative structures for a particular desired behavior. Each of these may exhibit differing cost performance characteristics and require different refinement and optimization techniques. These structures can be grouped into sets of similar characteristics called desrgn styles. Styles reflect varlous design approaches forced by different design constraints to achieve the same behavlor. A simple example Is the choice of ripple-carry addition versus carry-look-ahead. The ripple-carry adder is appropriate if space is at a higher premium than delay time. As each level is designed, constraints are produced which must be propagated to the designers at the lower levels. These constraints reflect design style declslons, or structural partitions of higher-level design constraints. Style decisions constrain the design styles and strategies of sub-section designers. An example is the decision to use pre-charged carry addition, forcing the use of appropriate implementation components. Structural partitioning refers to the dlvlding of global constraints such as time, power, or area into local constraints on these values. A requirement of 175nS as maximum cycle time makes demands on the critical path of operations in each cycle. As the design is implemented, this puts a partitioning constraint on the design of each functional component. Figure 1 shows the possible allocation of timing constraints in two stages of the design process. In the first case the allocation has simply divided the cycle time among the function units. In the second case (later in the design) a failure report from the multiply design task has forced a different allocation of time between the functional units. Iterative refinement of a design requires continuous performance monitoring relative to the design goals. This model assumes a simple approach similar to ‘Knobs’ and ‘Gauges’. A human operator monitoring a process closes the loop manually by reading the appropriate gauges and making adjustments to the knobs (parameters) controlling the process execution. We apply this same simple approach to controlling the design pro-