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I read out the constants from the ROM earlier and it was just a normal ROM, so I was skeptical about your comment. But what do you know, I just took a closer look at the microcode ROM and there are indeed 4 different transistor sizes! I hadn't even noticed, so thanks for mentioning this.

This will be annoying if I try to visually read out the microcode values since I'll need to examine each transistor closely. It was quick to read out the constant ROM since I just needed to glance to see if a transistor was present or not.

Here's a photo of a 7x5 region of the microcode ROM to help the discussion. The transistors (representing bits) are where the vertical polysilicon lines cross the pinkish doped silicon. Note that the "neck" is full-width, narrowed, very narrow, or entirely gone (no transistor), corresponding to the 4 values.

Image link: https://photos.app.goo.gl/pUsZSs3rx45Ry7U1A




Excellent photo. I'm surprised that the difference between the 4 levels is even visible, since I thought they would've done it by some non-obvious method like adjusting threshold voltages through implants. (The latter method was employed as a sort of obfucatory copy-protection for the Z80.)




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