THE DESIGN OF THE CONTROL UNIT USING PLA

Abstract

A digital system may be conceived of consisting of two parts, a data Processing unit, and a control unit. The two main functions of the control unit are instruction sequencing and instruction interpretation. It transfers the control of the processor from one instruction to another, and activates the control signals that cause the data processing unit to execute the instructions. The two common methods of designing the control unit are Hardwired control, and microprogrammed control. The micro-programmed control unit provides with more flexibility, some more instructions can be added to the control unit, at a later time, without any change in the design by simply adding the microinstructions to the control memory. But the main disadvan-tage with this control unit are its slower operation. Hardwired control unit is faster, but can't be progra-mraed like microprogrammed control unit. So, a method is devised, to use the hardwired control approach, but at the same time programmable or more flexible approach to design a control unit using FIA. It is also found that control unit thus obtained, is simpler, more flexible and less costlier than microprogrammed control unit. The method devised, is illustrated with certain design examples like, Twos complement multiplier using Booths multi p11-cation algorithm, fixed point binary multiplier, finally the method is applied to get the PL L for the control unit of CPU. The different techniques of optimizing the size of PLA are discussed, and illustrated by applying it to get the optimized PLA for fixed point binary multiplier, and is compared with the one that was obtained previously. The algorithm to automate the layout of the controller directly onto a chip is also mentioned.