> I assume computers of the day would not have been powerful enough for a CAD tool to design a silicon or metal layer of that complexity. A mask can be stored

> I assume computers of the day would not have been powerful enough for a CAD tool to design a silicon or metal layer of that complexity. A mask can be stored as a bitmap image, at 1-bit depth, one image per layer. You only need the image large enough to resolve your smallest features. At 10,000 by 10,000 pixels, one layer only takes ~12 megabytes to store uncompressed. (It would be easily compressed, too.) A late 70s VAX minicomputer would be able to store dozens of such mask images, and could compute the difference between two masks of that size in some tens of minutes, approximately. That'll tell you if any parts overlap. Using CAD in that way in chip design, dates to at least the 1970s. From there, standard blocks for, basically, copy and paste, are an obvious improvement over scanning in cut-out designs, or setting individual pixels. Actually synthesizing the masks automatically, from high-level hardware description languages, is something that developed gradually over the 1980s.