Looking at the low-level construction of the ARM1 chip
Getting back to the chip itself, the ARM1 chip is constructed from five layers.
If you zoom in on the chip in the simulator, you can see the components of the chip, built from these layers.
As seen below, the simulator uses a different color for each layer, and highlights circuits that are turned on.
The bottom layer is the silicon that makes up the transistors of the chip.
During manufacturing, regions of the silicon are modified (doped) by applying different impurities. Silicon can be doped positive to form a PMOS transistor (blue) or doped negative for an NMOS transistor (red). Undoped silicon is basically an insulator (black).
The ARM1 simulator uses different colors to represent the different layers of the chip.
Polysilicon wires (green) are deposited on top of the silicon. When polysilicon crosses doped silicon, it forms the gate of a transistor (yellow).
Finally, two layers of metal (gray) are on top of the polysilicon and provide wiring.[10]
Black squares are contacts that form connections between the different layers.
For our purposes, a MOS transistor can be thought of as a switch, controlled by the gate. When it is on (closed), the source and drain silicon regions are connected. When it is off (open), the source and drain are disconnected. The diagram below shows the three-dimensional structure of a MOS transistor.
Structure of a MOS transistor.
Like most modern processors, the ARM1 was built using CMOS technology, which uses two types of transistors: NMOS and PMOS.
NMOS transistors turn on when the gate is high, and pull their output towards ground.
PMOS transistors turn on when the gate is low, and pull their output towards +5 volts.
