Most people think of machine instructions as the fundamental steps that a computer performs.
However, many processors have another layer of software underneath: microcode.
With microcode, instead of building the processor’s control circuitry from complex logic gates, the control logic is
implemented with code known as microcode, stored in the microcode ROM.
To execute a machine instruction, the computer internally executes several simpler micro-instructions, specified by the microcode.
In this post, I examine the microcode ROM in the original Pentium, looking at the low-level circuitry.

The photo below shows the Pentium’s thumbnail-sized silicon die under a microscope.
I’ve labeled the main functional blocks.
The microcode ROM is highlighted at the right.
If you look closely, you can see that the microcode ROM consists of two rectangular banks, one above the other.

This die photo of the Pentium shows the location of the microcode ROM. Click this image (or any other) for a larger version.

The image below shows a closeup of the two microcode ROM banks.
Each bank provides 45 bits of output; together they implement a micro-instruction that is 90 bits long.
Each bank consists of a grid of transistors arranged into 288 rows and 720 columns.
The microcode ROM holds 4608 micro-instructions,
414,720 bits in total.
At this magnification, the ROM appears featureless, but it is covered with horizontal wires, each just 1.5 µm
thick.

The 90 output lines from the ROM, with a closeup of six lines exiting the ROM.

The ROM’s 90 output lines are collected into a bundle of wires between the banks, as shown above.
The detail shows how six of the bits exit from the banks and join the bundle.
This bundle exits the ROM to the left, travels to various parts of the chip, and controls the chip’s circuitry.
The output lines are in the chip’s top metal layer (M3):
the Pentium has three layers of metal wiring with M1 on the bottom, M2 in the middle, and M3 on top.

The Pentium has a large number of bits in its micro-instruction, 90 bits compared to 21 bits in the 8086.
Presumably, the Pentium has a “horizontal” microcode architecture, where the microcode bits correspond to low-level control signals,
as opposed to “vertical” microcode, where the bits are encoded into denser micro-instructions.
I don’t have any information on the Pentium’s encoding of microcode; unlike the 8086, the Pentium’s patents don’t provide any clues.
The 8086’s microcode ROM holds 512 micro-instructions, much less than the Pentium’s 4608 micro-instructions.
This makes sense, given the much greater complexity of the Pentium’s instruction set, including the floating-point unit on the chip.

The image below shows a closeup of the Pentium’s microcode ROM.
For this image, I removed the three layers of metal and the polysilicon layer
to expose the chip’s underlying silicon.
The pattern of silicon doping is visible, showing the transistors and thus the data stored in the ROM.
If you have enough time, you can extract the bits from the ROM by examining the silicon and seeing where transistors are present.

A closeup of the ROM showing how bits are encoded in the layout of transistors.

Before explaining the ROM’s circuitry, I’ll review how an NMOS transistor is constructed.
A transistor can be considered a switch between the source and drain, controlled by the gate.
The source and drain regions (green) consist of silicon doped with impurities to change its semiconductor properties, forming N+ silicon.
(These regions are visible in the photo above.)
The gate consists of a layer of polysilicon (red), separated from the silicon by a very thin insulating oxide layer. Whenever polysilicon crosses active silicon, a transistor is formed.

Diagram showing the structure of an NMOS transistor.

Bits are stored in the ROM through the pattern of transistors in the grid.
The presence or absence of a transistor stores a 0 or 1 bit.1
The closeup below shows eight bits of the microcode ROM. There are four transistors present and four gaps where transistors are
missing.
Thus, this part of the ROM holds four 0 bits and four 1 bits.
For the diagram below, I removed the three metal layers and the polysilicon to show the underlying silicon.
I colored doped (active) silicon regions green, and drew in the horizontal polysilicon lines in red.
As explained above, a transistor is created if polysilicon crosses doped silicon.
Thus, the contents of the ROM are defined by the pattern of silicon regions, which creates the transistors.

Eight bits of the microcode ROM, with four transistors present.

The horizontal silicon lines are used as wiring to provide ground to the transistors, while the horizontal polysilicon lines select one of the
rows in the ROM.
The transistors in that row will turn on, pulling the associated output lines low.
That is, the presence of a transistor in a row causes the output to be pulled low, while the absence of a transistor causes
the output line to remain high.

A schematic corresponding to the eight bits above.

The diagram below shows the silicon, polysilicon, and bottom metal (M1) layers. I removed the metal from the left to reveal the silicon and polysilicon underneath, but the pattern of vertical metal lines continues there.
As shown earlier, the silicon pattern forms transistors. Each horizontal metal line has a connection
to ground through a metal line (not shown).
The horizontal polysilicon lines select a row.
When polysilicon lines cross doped silicon, the gate of a transistor is formed.
Two transistors may share the drain, as in the transistor pair on the left.

Diagram showing the silicon, polysilicon, and M1 layers.

The vertical metal wires form the outputs. The circles are contacts between the metal wire and the silicon of a transistor.2
Short metal jumpers connect the polysilicon lines to the metal layer above, which will be described next.

The image below shows the upper left corner of the ROM. The yellowish meta