We focus on predefined, reusable blocks that typically lie at lower levels of logic design

hierarchies

$$ These are blocks of intermediate size that provide basic functions used in digital design

$$ They allow designers to do much of the design process above the primitive block $($i$.$e$.,$ gate level$)$

$$ We refer to these particular blocks as functional blocks

$$ A functional block is a predefined collection of interconnected gates

$$ Many of these functional blocks have been available for decades as medium$-$scale integrated $($MSI$)$

circuits that were interconnected to form larger circuits or systems

$$ Similar blocks are now, in CAD tool libraries, used for designing larger integrated circuits

$$ These functional blocks provide a catalog of digital components that are widely used in the design

and implementation of integrated circuits for computers and digital systems