FreeBSD Illegal Instruction Kernel Panic

An illegal opcode, also called an unimplemented operation, unintended opcode or undocumented instruction, is an instruction to a

CPU A central processing unit (CPU), also called a central processor, main processor or just processor, is the electronic circuitry that executes instructions comprising a computer program. The CPU performs basic arithmetic, logic, controlling, and ...

that is not mentioned in any official documentation released by the CPU's designer or manufacturer, which nevertheless has an effect. Illegal opcodes were common on older CPUs designed during the 1970s, such as the MOS Technology 6502, Intel 8086, and the

Zilog Zilog, Inc. is an American manufacturer of microprocessors and 8-bit and 16-bit microcontrollers. It is also a supplier of application-specific embedded system-on-chip (SoC) products. Its most famous product is the Z80 series of 8-bit microp ...

Z80. On these older processors, many exist as a side effect of the wiring of transistors in the CPU, and usually combine functions of the CPU that were not intended to be combined. On old and modern processors, there are also instructions intentionally included in the processor by the manufacturer, but that are not documented in any official specification. The effect of many illegal opcodes, on many processors, is just a

trap A trap is a mechanical device used to capture or restrain an animal for purposes such as hunting, pest control, or ecological research. Trap or TRAP may also refer to: Art and entertainment Films and television * ''Trap'' (2015 film), Fil ...

to an error handler. However, some processors that trap for most illegal opcodes do not do so for some illegal opcodes, and some other processors do not check for illegal opcodes, and, instead, perform an undocumented operation.

Overview

While most accidental illegal instructions have useless or even highly undesirable effects (such as crashing the computer), some can have useful functions in certain situations. Such instructions were sometimes exploited in

computer game Video games, also known as computer games, are electronic games that involves interaction with a user interface or input device such as a joystick, game controller, controller, computer keyboard, keyboard, or motion sensing device to gener ...

s of the 1970s and 1980s to speed up certain time-critical sections. Another common use was in the ongoing battle between copy protection implementations and cracking. Here, they were a form of security through obscurity, and their secrecy usually did not last very long. A danger associated with the use of illegal instructions was that, given the fact that the manufacturer does not guarantee their existence and function, they might disappear or behave differently with any change of the CPU internals or any new revision of the CPU, rendering programs that use them incompatible with the newer revisions. For example, a number of older

Apple II The Apple II (stylized as ) is an 8-bit home computer and one of the world's first highly successful mass-produced microcomputer products. It was designed primarily by Steve Wozniak; Jerry Manock developed the design of Apple II's foam-m ...

games did not work correctly on the newer

Apple IIc The Apple IIc, the fourth model in the Apple II series of personal computers, is Apple Computer's first endeavor to produce a portable computer. The result was a notebook-sized version of the Apple II that could be transported from place to pl ...

, because the latter used a newer CPU revision 65C02 that did away with illegal opcodes. More recent CPUs, such as the 80186, 80286, 68000 and its descendants, do not have illegal opcodes that are widely known/used. Ideally, the CPU will behave in a well-defined way when it finds an unknown opcode in the instruction stream, such as triggering a certain exception or fault condition. The operating system's exception or fault handler will then usually terminate the application that caused the fault, unless the program had previously established its own exception/fault handler, in which case that handler would receive control. Another, less common way of handling illegal instructions is by defining them to do nothing except taking up time and space (equivalent to the CPU's official NOP instruction); this method is used by the TMS9900 and 65C02 processors, among others. Alternatively, unknown instructions can be emulated in software (e.g. LOADALL), or even "new" pseudo-instructions can be implemented. Some

BIOS In computing, BIOS (, ; Basic Input/Output System, also known as the System BIOS, ROM BIOS, BIOS ROM or PC BIOS) is firmware used to provide runtime services for operating systems and programs and to perform hardware initialization during the ...

es, memory managers, and operating systems take advantage of this, for example, to let V86 tasks communicate with the underlying system, i.e. BOP (from "BIOS Operation") utilized by the Windows NTVDM. In spite of Intel's guarantee against such instructions, research using techniques such as fuzzing uncovered a vast number of undocumented instructions in x86 processors as late as 2018. Some of these instructions are shared across processor manufacturers, indicating that Intel and AMD are both aware of the instruction and its purpose, despite it not appearing in any official specification. Other instructions are specific to manufacturers or specific product lines. The purpose of the majority of x86 undocumented instructions is unknown. Today, the details of these instructions are mainly of interest for exact emulation of older systems.

References

External links

Christian Ludloff's site sandpile.org also contains info on undocumented opcodes
Machine code

Overview

See also

References

Further reading

External links