When you look at the die shots, do you get any idea of how the masks were drawn in those days? 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. Were they designed in smaller parts and then stitched together into a final design? -- perhaps evident by different parts having different design styles, or not quite using all the space between functional units. Were all parts done on a computer, or some pieces hand-drawn?
There's a detailed article "Recollections of Early Chip Development at Intel". For the first 10 years, Intel drew schematics and layout by hand. In 1974 they started digitizing on a Calma GDS I system. For the 8086, they spent two weeks manually matching the schematics against the drawn plots looking for errors and found 20 errors.
They cut out the drawings on sheets of Rubylith to create the masks. The earliest chips used a "Coordinatograph", a tool where you'd manually enter the coordinates from the drawing and it would move the cutter appropriately. Then they moved to a Xynetics plotter with a knife. The Rubylith sheets would be sent to the mask vendor, who would photographically reduce it down to mask size.
Intel had simulation tools, but they could only simulate 5-20 transistors at a time. So they could only simulate small, critical chunks like parts of the ALU.
In case you haven't seen it (although I'm sure you have), Now The Chips Are Down had a rare look inside an Intel fab in the 1970s: https://en.wikipedia.org/wiki/Now_the_Chips_Are_Down There are copies of the film on the internet.
> 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.
I can perhaps give some insight into this. One step in the mask making process was hand cutting the design into a masking film called Rubylith. Rubylith is made of a backing sheet coated in a UV resistant red film. The design was hand cut into this film on a large scale then used as a “master” to expose smaller copies.
