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Intel's November 15, 1971 product announcement in Electronic News

Complete Artwork, Schematics, and Simulator for Intel MCS-4 (4004 family) microprocessor chip-set: "Together again after 38 years!"

4001 ROM
—Newly re-created
4002 RAM4003
I/O Expander
4004 CPU

Newly released materials (and quick links):

Lajos Kintli writes about how and why he re-created the 4001 masks and the missing 4001, 4002, and 4003 schematics

During my university years, I learned the basics of microelectronics, but it was taught entirely at the gate level. My desire to understand details of complex circuits like microprocessors was not satisfied until years later. In June of 2007, while I was searching for information on a related topic, I discovered the web site. I was thrilled to find the schematics of the 4004 microprocessor and an animated Java simulator. Seeing that the team was looking for volunteers, I did not hesitate to offer my help, and soon I became member of the Intel 4004 35th Anniversary Project team.

A challenging request arrived in June of 2008: to verify the schematics of the giant 4004 replica being built for the Intel Museum against the masks used for manufacturing the original microprocessor. With the mask artwork in-hand, and I started to develop software to do netlist extraction and comparison. Then I wanted to visualize the result. Unfortunately the schematic I had was not available in machine-readable format, so finally I decided to generate it myself. I took PDF files and put them together into a big picture. It was not as monumental a task as I thought at first, thanks to Federico Faggin's original design methodology, the schematic and layout were quite close to each other. I only had to re-draw a few of the circuit blocks (e.g. the 4 bit wide data bus buffers). Finally I "wired together" the 3 pages of the original drawings. Then I implemented a component/circuit recognition algorithm, and in parallel I refined the schematics to make them more clear to the software. When the whole circuit was recognized, the matching algorithm I developed earlier was applied, and a list of differences were found and corrected.

Here is a short list of the differences I found between the mask artwork and the schematics:

Finally I got to the point where both netlist sources were equivalent except for the "bootstrap loads." These are clever circuits for generating voltages that exceed Vdd so that powerful inverters can drive long wires. These circuits are indicated on the schematic simply as a resistor with a "B" next to it, however these are really combinations of a resistor, a capacitor and a transistor. A "regular" pull-up resistor is just a transistor with its gate tied to Vdd. I challenge the curious reader to study the diagram below and figure out how the bootstrap circuit works. (Hint: This is not a DC circuit.)

The matched netlists contain the following number of components:

Component type Layout Schematics Difference
Transistor: 1807 1741 66
Resistor: 427 427 0
Capacitor: 66 0 66
ESD protection: 8 8 0
Total: 2308 2176 132

After finishing the netlist comparison, I was motivated to simulate the extracted circuit to ensure correctness, and to see whether the whole circuit would behave same upon executing assembly code as described in the MCS-4 User Manual (PDF 29.4Mb). So I wrote a switch-level simulator. For easier testing, I gradually added more and more feature, and I replaced rough temporary solutions with more mature ones. I realized that a number of problems came from shortcomings of the simulator logic, and so I improved it until it became reliable.

Simulating the entire chip-set required re-creating the missing schematics and masks

After completing the verification work for the 4004 microprocessor, I yearned to simulate a complete microcomputer set (CPU, ROMs and RAMs) at the transistor level. In late December of 2008, Tim McNerney sent me scans of the Intel 4002 RAM mask proofs. This was directly usable for the simulator, but the schematics were not available in any form. Fortunately, I found that by reusing blocks from the CPU schematics, I could reproduce a correct schematic diagram in a relatively short time. The situation with the ROM was not as straightforward. To get me started, Tim sent me two high resolution photomicrographs of the Intel 4001 ROM, one of the unmodified chip, and one of the chip with the metal-layer etched away, revealing the transistors. Unfortunately the quality was not suitable for automatic mask extraction. The only expedient alternative was to redraw the required mask layers by tracing the photographs, and to reverse engineer the schematic. For the sake of project completeness, this is what I did.

At first only the individual chips were simulated separately by generating the necessary test patterns for the external pins of RAM/ROM (which required a clock-cycle level accurate simulator of Intel 4004 CPU too). Then I connected the individual simulators together through a TCP/IP server application. This way the CPU/RAM/ROM simulators can be run in parallel, just as they were wired in a real system. Sharing the resources of several computers was considered for providing more screens to the schematic/layout and simulator windows; but it was not necessary to get more processing power and more memory for the entire simulated environment.

In October 2009, Tim sent me the mask artwork for the 4003, a 10 bit shift register used as an I/O expander. Much like I did for the 4002 RAM, I generated the schematic, and the result was added to complete the MCS-4 simulator package.

In November 2009, Intel gave us permission to publish our work under a non-commercial Creative-Common license.

MCS-4 Composite Mask Artwork

The 4001 Mask-Programmable ROM

The 4002 RAM (External Registers)

The 4003 10-bit Shift Register (I/O Expander)

The 4004 CPU

MCS-4 Schematics

Scans of the original 4004 schematics (three drawings) are available from the Intel Corporate Archives, along with scans of the manual mentioned above and the 4004 mask proofs. These were transparencies bound together for reference purposes, not the original lithographic masks. Due to distortion and aging, we had to "clean them up" and re-verify them.

Composite Photomicrographs

Here are links to two synthetic photomicrographs, one of the 4001 and one of the 4004. Tim McNerney created them using Photoshop by layering a transparent photo of the original chip with a photo of the chip that has had its metal layer etched away using hydrofluoric acid (a really dangerous chemical--don't try this at home!). By combining the etched and unetched photos in this way, you can see the metal wires and the transistors together in the same image. It is almost like looking at an X-ray of the chip.

The MCS-4 Simulator and Chip-Set Data

Here you can download a zip file with the individual mask, schematics, documentation, and Microsoft Windows executable.

Quick start

  1. First extract it using your favorite file archiving program (mine is the free, no strings attached 7-zip)
  2. Inside the folder i400x_analyzer20091114 is an application called i400x_analyzer.exe, double-click it to run the program.
  3. The first time you run it, keep the defaults checked, and click on "OK"
  4. Four windows will appear, then a little dialog box. Click "OK" to make the dialog box go away.
  5. For casual exploration, the most interesting windows are the Layout Analyzer and the Schematic Analyzer. Move the Layout window to the right to expose the Schematic window.
  6. Click on the RM output pad. You will notice that the same pad and wire highlighted in the Schematic window.
  7. Select the Schematics window. Then click on the wire directly above the one that is highlighted in white. This will highlight the R0 output pad in the Layout window.
  8. Back in the Windows file explorer, double-click on readme.txt for more documentation. For quick access, here is a link to the same readme.txt file.

A Few Screen Shots

As you can see in this screen shot of the Layout window, the simulator uses a different-looking color scheme than Tim chose for the mask composites. In the simulator, the colors also indicate logic levels, and metal looks gray. In the mask composites, Green = Diffusion, Red = Polysilicon, and Blue = Metal. These colors were inspired by the book Introduction to VLSI Systems by Mead and Conway, as well as full-custom VLSI CAD systems that Tim used professionally, including Mentor Graphics and UCB Magic.

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