A VLSI system sometimes is referred to as Application Specific Integrated Circuit (ASIC). There are several methodologies to implement a VLSI system. Each one having its own advantages and drawbacks. They are (1) Full custom design (2) Semi-custom design and (3) Programmable design style.
(1) Full Custom Design
In this design, the designer starts from scratch. This means that the designer abandons the approach of pre-designed and pre-tested and pre-characterized cells for all or part of the design. This might be because existing cell libraries are not fast enough, or the logic cells are not small enough or consume too much power. You may need to use full-custom design if the ASIC technology is new or so specialized that there are no existing cell libraries or because the ASIC is so specialized that some circuits must be custom designed. Fewer and fewer full-custom ICs are being designed because of the problems with these special parts of the ASIC. As a designer, you can optimize your design for speed, power, and area requirements. The full custom design approach needs more time and the cost of prototyping is higher compared to other design styles.
(2) Semi-Custom Design
In this type of design process, some part of the design is customized. That means the designer can pick up a pre-designed and tested cell from the cell library. Each cell library may contain an adder, subtractor, multiplier, flip flops, etc. For a single design, category say adder, there may be different variants with different speed and power ratings. This drastically reduces design time and cost. This is further divided into two categories: standard cell-based and gate array-based design.