The 6502 chip explained down to the silicon

The 6502 CPU’s overflow flag explained at the silicon level

 In this article, I show how overflow is computed in the 6502 microprocessor at the transistor and silicon level. I’ve discussed the mathematics of the 6502 overflow flag earlier and thought it would be interesting to look at the actual chip-level implementation. Even though the overflow flag is a slightly obscure feature, its circuit is simple enough that it can be explained at the silicon level.

The 6502 microprocessor chip

The 6502 CPU’s overflow flag explained at the silicon level
The 6502 is an 8-bit microprocessor that was very popular in the 1970s and 1980s, powering popular home computers such as the Apple II, Commodore PET, and Atari 400/800. The following photograph shows the die of a 6502 processor. Looking at the photograph, it seems impossibly complex, but it turns out that it actually can be understood, using the Visual 6502 group’s reverse engineered 6502. The red box shows that part of the chip that will be explained in this article. The 6502 chip is made up of 4528 transistors (3510 enhancement transistors and 1018 depletion pull up transistors). (By comparison, a modern Xeon processor has over 2.5 billion transistors, which would be almost hopeless to try to understand.)As a rough overview of the above photograph, the edge of the die shows the wires going to the pins. Approximately top fifth of the chip (with the regular rectangular pattern) is the PLA that decodes instructions. The middle third is a bunch of logic, mostly to do additional decoding of instructions. The bottom half has the registers, ALU (arithmetic-logic unit), and main busses. They are all 8 bits, with each bit in a horizontal layer. The high-order bit is at the bottom of the photo, and this is where the overflow logic lies.
Read More: (A small part of) The 6502 chip explained down to the silicon

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