The 6561 VIC chip is an NMOS IC. This means that it uses MOS transistors and that it only uses n-type for the diffusion areas. PMOS is another type of process that uses only p-type diffusion, and CMOS is a process that uses both n-type and p-type diffusion. For the VIC chip, we are interested only in NMOS.
For an introduction to NMOS VLSI, I fully recommend reading "Introduction to VLSI Systems" by Carver Mead and Lynn Conway (Addison-Wesley, December 1979). We can begin to understand this book's relevance when we consider that 1977 was when the VIC chip was designed, and 1980 was when the VIC 20 was released, which means that this book describes the NMOS design techniques in use at that time. The first drafts of the book were in fact written in 1977.
The following web site has links to the drafts of the book in PDF format:
http://ai.eecs.umich.edu/people/conway/VLSI/VLSIText/VLSIText.html
Chapter 1 introduces the MOS transistor at the bottom of page 1, with a diagram at the top of page 2. By page 6 it has introduced the inverter and shown how this would appear at the silicon level. It has also discussed the depletion mode pull-up transistor and why it is used in preference to a resistive line. At this early point of the book, most of the basics have been covered. For the purposes of reversing the circuit from the die shot, the formulas and graphs discussed are of little importance, since they relate more to the design of an integrated circuit rather than to the analysis of an already designed IC.
The important concepts to grasp from the book about NMOS ICs are:
They have three separate layers of conducting material called metal, polysilicon, and diffusion that are separated by intervening layers of insulating material.
A MOS transistor is formed wherever a polysilicon path crosses over a diffusion area (actually, it only appears that the polysilicon passes over a diffusion line; there is never any diffusion under a transistor gate, but rather the diffusion comes up to each side of the gate. Only oxide and the substrate exists under the gate).
A polysilicon line can connect to a diffusion line rather than pass over it. A transistor is not formed when they connect in this way.
A metal line can pass over both polysilicon and diffusion without affecting them.
A contact must be made between metal and polysilicon or diffusion in order for the metal line to connect with areas of those types.
A depletion mode MOS transistor is used as a pull-up in place of a resistor (primarily due to a resistor in NMOS requiring a much greater surface area).
The sub pages below show examples of each of the features identifiable within the 6561 die shot and explains how to recognise them.