The Secret CPU Instructions Intel, AMD and ARM Keep Hidden (And Why They Matter)

Background: Where the "secret" instructions came from

In the 1970s and 1980s CPU designers faced a shortage of transistors, so they often omitted circuitry that would detect illegal opcodes. Executing an undefined opcode caused the processor to behave unpredictably, creating a class of accidental hidden instructions. As chips grew more complex, manufacturers deliberately left some undocumented debug backdoors and hidden instructions, which are far more dangerous than the accidental ones.

Historical "fossils"

SALC – Intel’s intellectual‑property trap

SALC (opcode 0xD6) copies the carry flag into the AL register. Intel never documented this instruction for almost 20 years after the 8086 launch. Hardware‑archaeologist Ken Shirriff identified it as a trap: cloning the 8086 microcode without removing SALC would constitute legal proof of copying NEC’s design, so the instruction was used to catch plagiarism.

POP CS – An "accidental benefit" for old games

POP CS never appeared in official manuals. Executing it makes program flow unpredictable, but early DOS game developers sometimes relied on it because it skipped unwanted logic, turning a bug into a feature.

LOADALL / SAVEALL – 80286’s "god mode"

LOADALL loads all CPU registers, including segment descriptor caches, directly from memory, bypassing normal security checks. Its counterpart SAVEALL (opcodes 0xF1 0x04) writes the internal CPU state to memory address 0x000800H. Researchers later proved these instructions could be used as real hardware‑exploitation paths.

AAM – A quirky way to perform modulo

The ASCII Adjust after Multiply (AAM) instruction officially accepts only the immediate 0x0A, but in practice any value 0x00‑0xFF works. Some developers use AAM 0x01 to clear the high half of AX faster than any documented instruction, a technique that would normally be rejected in code review.

Modern hidden instructions: even scarier

"God‑mode" backdoor discovered in 2018

At Black Hat 2018, researcher Christopher Domas revealed that certain x86 CPUs contain an undocumented independent RISC core used for hardware debugging. The instructions that invoke this core are absent from any official documentation.

DEF CON 31 hardware‑debug backdoor

In 2023 a security team disclosed two Intel hidden instructions—udbgrd and udbgwr—that bypass all security restrictions, allowing direct read/write of the CPU’s physical bus and peripheral registers. They also demonstrated a hidden microcode‑update mechanism that can modify the CPU’s microcode engine at runtime, enabling persistent firmware‑level backdoors.

The ultimate hidden opcode: HCF

HCF (Halt and Catch Fire) is the legendary instruction that, when executed, drives the CPU to continuously read the memory bus at maximum speed, entering a state that cannot be interrupted or reset. It was real on some CPUs in the 1970‑80s and illustrates how the consequences of hidden instructions can be completely unknown, even causing physical damage.

MystFuzz: the 2026 "hidden‑instruction hunter"

0day_ninja’s MystFuzz tool systematically searches for undocumented CPU opcodes. Traditional brute‑force enumeration is impractical on modern CPUs because the x86 instruction set is variable‑length and context‑dependent (prefix bytes, mode switches, etc.). MystFuzz leverages speculative execution and a lightweight instruction emulator to safely generate non‑standard opcodes and uncover ghost instructions that remain hidden as of 2026.

Final note

"I guess you have to call the fire department now."

In short, CPUs do contain many secret instructions—far more than most people imagine. Intel, AMD, and even ARM embed legitimate debugging backdoors and performance‑testing hooks, while a tiny fraction are so obscure that even the manufacturers have forgotten them.