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Die photos and analysis of the revolutionary 8008 microprocessor, 45 years old

Die photos and analysis of the revolutionary 8008 microprocessor, 45 years old
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The structure of the chip

What does the die photo show?
For our purposes, the chip can be thought of as three layers.
The diagram below shows a closeup of the chip, pointing out these layers.
The topmost layer is the metal wiring. It is the most visible feature, and looks metallic (not surprisingly). In the detail below, these wires are mostly horizontal.
The polysilicon layer is below the metal and appears orange under the microscope.

A closeup of the 8008 die, showing the metal layer, the polysilicon, and the doped silicon.

A closeup of the 8008 die, showing the metal layer, the polysilicon, and the doped silicon.

The foundation of the chip is the silicon wafer, which appears purplish-gray in the photo.
Pure silicon is effectively an insulator. Regions of it are “doped” with impurities to create semiconducting silicon.
Being on the bottom, the silicon layer is difficult to distinguish, but
you can see black lines along the border between doped silicon and undoped silicon. A few vertical silicon “wires” are visible in the photo.4

Transistors are the key component of the chip, and a transistor is formed where a polysilicon wire crosses doped silicon. In the photo, the polysilicon appears as a brighter orange where it forms a transistor.

Why an 18 pin chip?

One inconvenient feature of the 8008 is it only has 18 pins, which makes the chip slower and much more difficult to use.
The 8008 uses 14 address bits and 8 data bits so with 18 pins there aren’t enough pins for each signal. Instead, the chip has 8 data pins that are reused in three cycles to transmit the low address bits, high address bits, and data bits.
A computer using the 8008 requires many support chips to interact with this inconvenient bus architecture.5



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There was no good reason to force the chip into 18 pins. Packages with 40 or 48 pins were common with other manufacturers, but 16 pins was “a religion at Intel”.6 Only with great reluctance did they move to 18 pins.
By the time the 8080 processor came out a few years later, Intel had come to terms with 40-pin chips. The 8080 was much more popular, in part because it had a simpler bus design permitted by the 40-pin package.

Power and data paths in the chip

The data bus provides data flow through the chip.
The diagram below shows the 8-bit data bus of the 8008 with rainbow colors for the 8 data lines.
The data bus connects to the 8 data pins along the outside of the upper half of the chip.
The bus runs between the ALU on the left, the instruction register (upper center), and the registers and stack on the right. The bus is split on the left with half along each side of the ALU.

Die photo of the 8008 microprocessor. The power bus is shown in red and blue. The data bus is shown with 8 rainbow colors.

Die photo of the 8008 microprocessor. The power bus is shown in red and blue. The data bus is shown with 8 rainbow colors.

The red and blue lines show power routing.
Power routing is an under-appreciated aspect of microprocessors. Power is routed in the metal layer due to its low resistance.
But since there is only one metal layer in early microprocessors, power distribution must be carefully planned so the paths don’t cross.7
The diagram above shows Vcc lines in blue and Vdd lines in red.
Power is supplied through the Vcc pin on the left and the Vdd pin on the right, then branches out into thin, interlocking wires that supply all parts of the chip.

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