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It fits because the compact version is not a conventional, readable CPU emulator: it compresses a gate-level simulation of the Intel 4004 and the supporting chips into a tightly packed C program. That simulated hardware runs the original Busicom 141-PF calculator ROM, so this is more than a program that imitates calculator answers. It is also intentionally slow: Carlini describes performance of a few hundred instructions per second, and example calculations in the repository take roughly three to ten minutes on his computer.
What the 4,004-byte program actually emulates
Nicholas Carlini’s article, published August 4, 2025, describes a gate-level emulator for the Intel 4004. Rather than implementing only the CPU’s documented instructions as higher-level software, it models the 4004 as a logic circuit and simulates that circuit. The program also models the Intel 4001, 4002, and 4003 devices used by the calculator system.
That distinction matters: the emulated machine runs the original Busicom 141-PF calculator ROM. The calculator’s behavior comes from that ROM running on the simulated hardware, rather than from a set of newly written arithmetic routines designed to return familiar answers.
How the circuit becomes compact C
The design starts with a large circuit, then compresses its representation and the surrounding logic into C. Carlini says he used miniHDL to design the circuit before reducing the generated structure to fit the byte limit. The code is deliberately difficult to read; the size target favors compact representation over maintainability.
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Micro-operations represent each instruction
The 4004 is microcoded in this implementation: an instruction can be represented by as many as 16 micro-operations. A four-bit counter tracks which micro-operation is current, while a 96-bit shift register stores the micro-operation vector.
Memory and arithmetic are modeled as hardware
The ROM is divided into program and lookup-table banks, and the arithmetic logic unit is implemented as a three-deep queue. These are parts of the emulator’s circuit model, not evidence that the program merely looks up calculator answers.
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There are two source-size figures
The 4,004-byte figure refers to the compact variant. The repository also preserves the IOCCC contest form; these are related versions, not two measurements of the same unchanged file.
Do these 3 things before closing this tab:
1Clear out junk files and repair common Windows errors2Fix the driver behind crashes, sound loss and screen glitches3Repair Windows errors before they cause bigger problems| Variant | Reported size | What the figure means |
|---|---|---|
| Compact 4004-byte variant | 4,004 bytes | The compact C program after the repository’s size-reduction tweaks. |
| IOCCC form | 4,993 bytes; 2,503 non-whitespace bytes | The repository’s contest form before the tweaks that produced the 4,004-byte program. |
What you can do with it
The calculator ROM accepts calculator-style key sequences, not modern command-line expressions. In the repository’s examples, 2+3+= returns 5 and 85*72= returns 6120. The examples also cover division, memory operations, square roots, and subtotals. In other words, the characters represent keys pressed on the original calculator interface; they are not a general-purpose expression language.
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The repository preserves the project and lists example calculations. To try it, use that repository’s build and run instructions rather than assuming a generic calculator command or input format: no specific compiler command or operating-system requirement is established here.
Why it runs slowly
A gate-level simulation does much more work than simply executing each calculator instruction in a conventional emulator. Carlini describes the result as running at a few hundred instructions per second and usable “just as long as you’re not in a hurry.” The repository estimates roughly three to ten minutes for some example calculations on Carlini’s computer; that is an approximate, machine-specific runtime, not a guarantee for every system or input.
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The original ROM can buffer only a limited number of keystrokes. The repository also warns that timing race conditions—harmless on the much faster physical calculator—can matter in the slow emulation. If a long sequence behaves unexpectedly, try shorter key sequences rather than treating the program like an interactive modern calculator that can accept an unlimited input line.
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Carlini wrote the project for the 28th International Obfuscated C Code Contest (IOCCC) in 2025. The official entry awarded it “Prize in perfect timing”; the judges specifically noted that they were impressed it simulated the drum printer. The preserved contest version and the compact 4,004-byte variant show two different aspects of the project: its IOCCC submission and a further-compressed form.
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A separate official bug page documents a zero-printing defect in a related Carlini program from 2024, where a 0 digit can print as a space. That report is about the 2024 program; it does not establish that the 4,004-byte 4004 calculator has the same defect.
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