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In
computer science Computer science is the study of computation, information, and automation. Computer science spans Theoretical computer science, theoretical disciplines (such as algorithms, theory of computation, and information theory) to Applied science, ...
, self-modifying code (SMC or SMoC) is
code In communications and information processing, code is a system of rules to convert information—such as a letter, word, sound, image, or gesture—into another form, sometimes shortened or secret, for communication through a communicati ...
that alters its own instructions while it is executing – usually to reduce the
instruction path length In computer performance, the instruction path length is the number of machine code instructions required to execute a section of a computer program. The total path length for the entire program could be deemed a measure of the algorithm's perfor ...
and improve
performance A performance is an act or process of staging or presenting a play, concert, or other form of entertainment. It is also defined as the action or process of carrying out or accomplishing an action, task, or function. Performance has evolved glo ...
or simply to reduce otherwise repetitively similar code, thus simplifying
maintenance The technical meaning of maintenance involves functional checks, servicing, repairing or replacing of necessary devices, equipment, machinery, building infrastructure and supporting utilities in industrial, business, and residential installa ...
. The term is usually only applied to code where the self-modification is intentional, not in situations where code accidentally modifies itself due to an error such as a buffer overflow. Self-modifying code can involve overwriting existing instructions or generating new code at run time and transferring control to that code. Self-modification can be used as an alternative to the method of "flag setting" and conditional program branching, used primarily to reduce the number of times a condition needs to be tested. The method is frequently used for conditionally invoking test/debugging code without requiring additional computational overhead for every
input/output In computing, input/output (I/O, i/o, or informally io or IO) is the communication between an information processing system, such as a computer, and the outside world, such as another computer system, peripherals, or a human operator. Inputs a ...
cycle. The modifications may be performed: * only during initialization – based on input
parameter A parameter (), generally, is any characteristic that can help in defining or classifying a particular system (meaning an event, project, object, situation, etc.). That is, a parameter is an element of a system that is useful, or critical, when ...
s (when the process is more commonly described as software ' configuration' and is somewhat analogous, in hardware terms, to setting jumpers for
printed circuit board A printed circuit board (PCB), also called printed wiring board (PWB), is a Lamination, laminated sandwich structure of electrical conduction, conductive and Insulator (electricity), insulating layers, each with a pattern of traces, planes ...
s). Alteration of program entry pointers is an equivalent indirect method of self-modification, but requiring the co-existence of one or more alternative instruction paths, increasing the program size. * throughout execution ("on the fly") – based on particular program states that have been reached during the execution In either case, the modifications may be performed directly to the
machine code In computer programming, machine code is computer code consisting of machine language instructions, which are used to control a computer's central processing unit (CPU). For conventional binary computers, machine code is the binaryOn nonb ...
instructions themselves, by overlaying new instructions over the existing ones (for example: altering a compare and branch to an unconditional branch or alternatively a ' NOP'). In the IBM System/360 architecture, and its successors up to z/Architecture, an EXECUTE (EX) instruction ''logically'' overlays the second byte of its target instruction with the low-order 8 bits of register 1. This provides the effect of self-modification although the actual instruction in storage is not altered.


Application in low and high level languages

Self-modification can be accomplished in a variety of ways depending upon the programming language and its support for pointers and/or access to dynamic compiler or interpreter 'engines': * overlay of existing instructions (or parts of instructions such as opcode, register, flags or addresses) or * direct creation of whole instructions or sequences of instructions in memory * creation or modification of
source code In computing, source code, or simply code or source, is a plain text computer program written in a programming language. A programmer writes the human readable source code to control the behavior of a computer. Since a computer, at base, only ...
statements followed by a 'mini compile' or a dynamic interpretation (see eval statement) * creating an entire program dynamically and then executing it


Assembly language

Self-modifying code is quite straightforward to implement when using
assembly language In computing, assembly language (alternatively assembler language or symbolic machine code), often referred to simply as assembly and commonly abbreviated as ASM or asm, is any low-level programming language with a very strong correspondence bet ...
. Instructions can be dynamically created in
memory Memory is the faculty of the mind by which data or information is encoded, stored, and retrieved when needed. It is the retention of information over time for the purpose of influencing future action. If past events could not be remembe ...
(or else overlaid over existing code in non-protected program storage), in a sequence equivalent to the ones that a standard compiler may generate as the
object code In computing, object code or object module is the product of an assembler or compiler In computing, a compiler is a computer program that Translator (computing), translates computer code written in one programming language (the ''source'' ...
. With modern processors, there can be unintended
side effect In medicine, a side effect is an effect of the use of a medicinal drug or other treatment, usually adverse but sometimes beneficial, that is unintended. Herbal and traditional medicines also have side effects. A drug or procedure usually use ...
s on the
CPU cache A CPU cache is a hardware cache used by the central processing unit (CPU) of a computer to reduce the average cost (time or energy) to access data from the main memory. A cache is a smaller, faster memory, located closer to a processor core, whi ...
that must be considered. The method was frequently used for testing 'first time' conditions, as in this suitably commented IBM/360 assembler example. It uses instruction overlay to reduce the
instruction path length In computer performance, the instruction path length is the number of machine code instructions required to execute a section of a computer program. The total path length for the entire program could be deemed a measure of the algorithm's perfor ...
by (N×1)−1 where N is the number of records on the file (−1 being the overhead to perform the overlay). SUBRTN NOP OPENED FIRST TIME HERE? * The NOP is x'4700' OI SUBRTN+1,X'F0' YES, CHANGE NOP TO UNCONDITIONAL BRANCH (47F0...) OPEN INPUT AND OPEN THE INPUT FILE SINCE IT'S THE FIRST TIME THRU OPENED GET INPUT NORMAL PROCESSING RESUMES HERE ... Alternative code might involve testing a "flag" each time through. The unconditional branch is slightly faster than a compare instruction, as well as reducing the overall path length. In later operating systems for programs residing in protected storage this technique could not be used and so changing the pointer to the
subroutine In computer programming, a function (also procedure, method, subroutine, routine, or subprogram) is a callable unit of software logic that has a well-defined interface and behavior and can be invoked multiple times. Callable units provide a ...
would be used instead. The pointer would reside in dynamic storage and could be altered at will after the first pass to bypass the OPEN (having to load a pointer first instead of a direct branch & link to the subroutine would add N instructions to the path length – but there would be a corresponding reduction of N for the unconditional branch that would no longer be required). Below is an example in
Zilog Z80 The Zilog Z80 is an 8-bit computing, 8-bit microprocessor designed by Zilog that played an important role in the evolution of early personal computing. Launched in 1976, it was designed to be Backward compatibility, software-compatible with the ...
assembly language. The code increments register "B" in range ,5 The "CP" compare instruction is modified on each loop. ;

ORG 0H CALL FUNC00 HALT ;

FUNC00: LD A,6 LD HL,label01+1 LD B,(HL) label00: INC B LD (HL),B label01: CP $0 JP NZ,label00 RET ;

Self-modifying code is sometimes used to overcome limitations in a machine's instruction set. For example, in the Intel 8080 instruction set, one cannot input a byte from an input port that is specified in a register. The input port is statically encoded in the instruction itself, as the second byte of a two byte instruction. Using self-modifying code, it is possible to store a register's contents into the second byte of the instruction, then execute the modified instruction in order to achieve the desired effect.


High-level languages

Some compiled languages explicitly permit self-modifying code. For example, the ALTER verb in
COBOL COBOL (; an acronym for "common business-oriented language") is a compiled English-like computer programming language designed for business use. It is an imperative, procedural, and, since 2002, object-oriented language. COBOL is primarily ...
may be implemented as a branch instruction that is modified during execution. Some batch programming techniques involve the use of self-modifying code. Clipper and SPITBOL also provide facilities for explicit self-modification. The Algol compiler on B6700 systems offered an interface to the operating system whereby executing code could pass a text string or a named disc file to the Algol compiler and was then able to invoke the new version of a procedure. With interpreted languages, the "machine code" is the source text and may be susceptible to editing on-the-fly: in
SNOBOL SNOBOL ("StriNg Oriented and symBOlic Language") is a series of programming languages developed between 1962 and 1967 at AT&T Bell Laboratories by David J. Farber, Ralph Griswold and Ivan P. Polonsky, culminating in SNOBOL4. It was one of a ...
the source statements being executed are elements of a text array. Other languages, such as
Perl Perl is a high-level, general-purpose, interpreted, dynamic programming language. Though Perl is not officially an acronym, there are various backronyms in use, including "Practical Extraction and Reporting Language". Perl was developed ...
and Python, allow programs to create new code at run-time and execute it using an eval function, but do not allow existing code to be mutated. The illusion of modification (even though no machine code is really being overwritten) is achieved by modifying function pointers, as in this JavaScript example: var f = function (x) ; // assign a new definition to f: f = new Function('x', 'return x + 2'); Lisp macros also allow runtime code generation without parsing a string containing program code. The Push programming language is a
genetic programming Genetic programming (GP) is an evolutionary algorithm, an artificial intelligence technique mimicking natural evolution, which operates on a population of programs. It applies the genetic operators selection (evolutionary algorithm), selection a ...
system that is explicitly designed for creating self-modifying programs. While not a high level language, it is not as low level as assembly language.


Compound modification

Prior to the advent of multiple windows, command-line systems might offer a menu system involving the modification of a running command script. Suppose a DOS script (or "batch") file MENU.BAT contains the following: :start SHOWMENU.EXE Upon initiation of MENU.BAT from the command line, SHOWMENU presents an on-screen menu, with possible help information, example usages and so forth. Eventually the user makes a selection that requires a command ''SOMENAME'' to be performed: SHOWMENU exits after rewriting the file MENU.BAT to contain :start SHOWMENU.EXE CALL ''SOMENAME''.BAT GOTO start Because the DOS command interpreter does not compile a script file and then execute it, nor does it read the entire file into memory before starting execution, nor yet rely on the content of a record buffer, when SHOWMENU exits, the command interpreter finds a new command to execute (it is to invoke the script file ''SOMENAME'', in a directory location and via a protocol known to SHOWMENU), and after that command completes, it goes back to the start of the script file and reactivates SHOWMENU ready for the next selection. Should the menu choice be to quit, the file would be rewritten back to its original state. Although this starting state has no use for the label, it, or an equivalent amount of text is required, because the DOS command interpreter recalls the byte position of the next command when it is to start the next command, thus the re-written file must maintain alignment for the next command start point to indeed be the start of the next command. Aside from the convenience of a menu system (and possible auxiliary features), this scheme means that the SHOWMENU.EXE system is not in memory when the selected command is activated, a significant advantage when memory is limited.


Control tables

Control table interpreters can be considered to be, in one sense, 'self-modified' by data values extracted from the table entries (rather than specifically hand coded in conditional statements of the form "IF inputx = 'yyy'").


Channel programs

Some IBM
access method An access method is a function of a mainframe computer, mainframe operating system that enables access to data on disk, tape or other external devices. Access methods were present in several mainframe operating systems since the late 1950s, under ...
s traditionally used self-modifying channel programs, where a value, such as a disk address, is read into an area referenced by a channel program, where it is used by a later channel command to access the disk.


History

The IBM SSEC, demonstrated in January 1948, had the ability to modify its instructions or otherwise treat them exactly like data. However, the capability was rarely used in practice. In the early days of computers, self-modifying code was often used to reduce use of limited memory, or improve performance, or both. It was also sometimes used to implement subroutine calls and returns when the instruction set only provided simple branching or skipping instructions to vary the
control flow In computer science, control flow (or flow of control) is the order in which individual statements, instructions or function calls of an imperative program are executed or evaluated. The emphasis on explicit control flow distinguishes an '' ...
. This use is still relevant in certain ultra-
RISC In electronics and computer science, a reduced instruction set computer (RISC) is a computer architecture designed to simplify the individual instructions given to the computer to accomplish tasks. Compared to the instructions given to a comp ...
architectures, at least theoretically; see for example one-instruction set computer.
Donald Knuth Donald Ervin Knuth ( ; born January 10, 1938) is an American computer scientist and mathematician. He is a professor emeritus at Stanford University. He is the 1974 recipient of the ACM Turing Award, informally considered the Nobel Prize of comp ...
's MIX architecture also used self-modifying code to implement subroutine calls.


Usage

Self-modifying code can be used for various purposes: * Semi-automatic optimizing of a state-dependent loop. * Dynamic in-place code optimization for speed depending on load environment. * Run-time code generation, or specialization of an algorithm in runtime or loadtime (which is popular, for example, in the domain of real-time graphics) such as a general sort utility – preparing code to perform the key comparison described in a specific invocation. * Altering of inlined state of an object, or simulating the high-level construction of closures. * Patching of
subroutine In computer programming, a function (also procedure, method, subroutine, routine, or subprogram) is a callable unit of software logic that has a well-defined interface and behavior and can be invoked multiple times. Callable units provide a ...
( pointer) address calling, usually as performed at load/initialization time of dynamic libraries, or else on each invocation, patching the subroutine's internal references to its parameters so as to use their actual addresses (i.e. indirect self-modification). * Evolutionary computing systems such as neuroevolution,
genetic programming Genetic programming (GP) is an evolutionary algorithm, an artificial intelligence technique mimicking natural evolution, which operates on a population of programs. It applies the genetic operators selection (evolutionary algorithm), selection a ...
and other
evolutionary algorithm Evolutionary algorithms (EA) reproduce essential elements of the biological evolution in a computer algorithm in order to solve "difficult" problems, at least Approximation, approximately, for which no exact or satisfactory solution methods are k ...
s. * Hiding of code to prevent
reverse engineering Reverse engineering (also known as backwards engineering or back engineering) is a process or method through which one attempts to understand through deductive reasoning how a previously made device, process, system, or piece of software accompl ...
(by use of a disassembler or
debugger A debugger is a computer program used to test and debug other programs (the "target" programs). Common features of debuggers include the ability to run or halt the target program using breakpoints, step through code line by line, and display ...
) or to evade detection by virus/spyware scanning software and the like. * Filling 100% of memory (in some architectures) with a rolling pattern of repeating
opcode In computing, an opcode (abbreviated from operation code) is an enumerated value that specifies the operation to be performed. Opcodes are employed in hardware devices such as arithmetic logic units (ALUs), central processing units (CPUs), and ...
s, to erase all programs and data, or to burn-in hardware or perform RAM tests. * Compressing code to be decompressed and executed at runtime, e.g., when memory or disk space is limited. * Some very limited
instruction set In computer science, an instruction set architecture (ISA) is an abstract model that generally defines how software controls the CPU in a computer or a family of computers. A device or program that executes instructions described by that ISA, s ...
s leave no option but to use self-modifying code to perform certain functions. For example, a one-instruction set computer (OISC) machine that uses only the subtract-and-branch-if-negative "instruction" cannot do an indirect copy (something like the equivalent of "*a = **b" in the
C language C (''pronounced'' '' – like the letter c'') is a general-purpose programming language. It was created in the 1970s by Dennis Ritchie and remains very widely used and influential. By design, C's features cleanly reflect the capabilities o ...
) without using self-modifying code. *
Booting In computing, booting is the process of starting a computer as initiated via Computer hardware, hardware such as a physical button on the computer or by a software command. After it is switched on, a computer's central processing unit (CPU) h ...
. Early microcomputers often used self-modifying code in their bootloaders. Since the bootloader was keyed in via the front panel at every power-on, it did not matter if the bootloader modified itself. However, even today many bootstrap loaders are self-relocating, and a few are even self-modifying. * Altering instructions for fault-tolerance.


Optimizing a state-dependent loop

Pseudocode example: repeat ''N'' times Self-modifying code, in this case, would simply be a matter of rewriting the loop like this: repeat ''N'' times Note that two-state replacement of the
opcode In computing, an opcode (abbreviated from operation code) is an enumerated value that specifies the operation to be performed. Opcodes are employed in hardware devices such as arithmetic logic units (ALUs), central processing units (CPUs), and ...
can be easily written as 'xor var at address with the value "opcodeOf(Inc) xor opcodeOf(dec)"'. Choosing this solution must depend on the value of and the frequency of state changing.


Specialization

Suppose a set of statistics such as average, extrema, location of extrema, standard deviation, etc. are to be calculated for some large data set. In a general situation, there may be an option of associating weights with the data, so each xi is associated with a wi and rather than test for the presence of weights at every index value, there could be two versions of the calculation, one for use with weights and one not, with one test at the start. Now consider a further option, that each value may have associated with it a Boolean to signify whether that value is to be skipped or not. This could be handled by producing four batches of code, one for each permutation and code bloat results. Alternatively, the weight and the skip arrays could be merged into a temporary array (with zero weights for values to be skipped), at the cost of processing and still there is bloat. However, with code modification, to the template for calculating the statistics could be added as appropriate the code for skipping unwanted values, and for applying weights. There would be no repeated testing of the options and the data array would be accessed once, as also would the weight and skip arrays, if involved.


Use as camouflage

Self-modifying code is more complex to analyze than standard code and can therefore be used as a protection against
reverse engineering Reverse engineering (also known as backwards engineering or back engineering) is a process or method through which one attempts to understand through deductive reasoning how a previously made device, process, system, or piece of software accompl ...
and
software cracking Software cracking (known as "breaking" mostly in the 1980s) is an act of removing copy protection from a software. Copy protection can be removed by applying a specific ''crack''. A ''crack'' can mean any tool that enables breaking software p ...
. Self-modifying code was used to hide copy protection instructions in 1980s disk-based programs for systems such as
IBM PC compatible An IBM PC compatible is any personal computer that is hardware- and software-compatible with the IBM Personal Computer (IBM PC) and its subsequent models. Like the original IBM PC, an IBM PC–compatible computer uses an x86-based central p ...
s and
Apple II Apple II ("apple Roman numerals, two", stylized as Apple ][) is a series of microcomputers manufactured by Apple Computer, Inc. from 1977 to 1993. The Apple II (original), original Apple II model, which gave the series its name, was designed ...
. For example, on an IBM PC, the floppy disk drive access instruction int 0x13 would not appear in the executable program's image but it would be written into the executable's memory image after the program started executing. Self-modifying code is also sometimes used by programs that do not want to reveal their presence, such as
computer virus A computer virus is a type of malware that, when executed, replicates itself by modifying other computer programs and Code injection, inserting its own Computer language, code into those programs. If this replication succeeds, the affected areas ...
es and some shellcodes. Viruses and shellcodes that use self-modifying code mostly do this in combination with polymorphic code. Modifying a piece of running code is also used in certain attacks, such as buffer overflows.


Self-referential machine learning systems

Traditional
machine learning Machine learning (ML) is a field of study in artificial intelligence concerned with the development and study of Computational statistics, statistical algorithms that can learn from data and generalise to unseen data, and thus perform Task ( ...
systems have a fixed, pre-programmed learning
algorithm In mathematics and computer science, an algorithm () is a finite sequence of Rigour#Mathematics, mathematically rigorous instructions, typically used to solve a class of specific Computational problem, problems or to perform a computation. Algo ...
to adjust their
parameter A parameter (), generally, is any characteristic that can help in defining or classifying a particular system (meaning an event, project, object, situation, etc.). That is, a parameter is an element of a system that is useful, or critical, when ...
s. However, since the 1980s Jürgen Schmidhuber has published several self-modifying systems with the ability to change their own learning algorithm. They avoid the danger of catastrophic self-rewrites by making sure that self-modifications will survive only if they are useful according to a user-given fitness,
error An error (from the Latin , meaning 'to wander'Oxford English Dictionary, s.v. “error (n.), Etymology,” September 2023, .) is an inaccurate or incorrect action, thought, or judgement. In statistics, "error" refers to the difference between t ...
or reward function.


Operating systems

The
Linux kernel The Linux kernel is a Free and open-source software, free and open source Unix-like kernel (operating system), kernel that is used in many computer systems worldwide. The kernel was created by Linus Torvalds in 1991 and was soon adopted as the k ...
notably makes wide use of self-modifying code; it does so to be able to distribute a single binary image for each major architecture (e.g.
IA-32 IA-32 (short for "Intel Architecture, 32-bit", commonly called ''i386'') is the 32-bit version of the x86 instruction set architecture, designed by Intel and first implemented in the i386, 80386 microprocessor in 1985. IA-32 is the first incarn ...
,
x86-64 x86-64 (also known as x64, x86_64, AMD64, and Intel 64) is a 64-bit extension of the x86 instruction set architecture, instruction set. It was announced in 1999 and first available in the AMD Opteron family in 2003. It introduces two new ope ...
, 32-bit ARM, ARM64...) while adapting the kernel code in memory during boot depending on the specific CPU model detected, e.g. to be able to take advantage of new CPU instructions or to work around hardware bugs. To a lesser extent, the DR-DOS kernel also optimizes speed-critical sections of itself at loadtime depending on the underlying processor generation. Regardless, at a meta-level, programs can still modify their own behavior by changing data stored elsewhere (see
metaprogramming Metaprogramming is a computer programming technique in which computer programs have the ability to treat other programs as their data. It means that a program can be designed to read, generate, analyse, or transform other programs, and even modi ...
) or via use of polymorphism.


Massalin's Synthesis kernel

The Synthesis kernel presented in Alexia Massalin's Ph.D. thesis is a tiny
Unix Unix (, ; trademarked as UNIX) is a family of multitasking, multi-user computer operating systems that derive from the original AT&T Unix, whose development started in 1969 at the Bell Labs research center by Ken Thompson, Dennis Ritchie, a ...
kernel that takes a
structured Structuring, also known as smurfing in banking jargon, is the practice of executing financial transactions such as making bank deposits in a specific pattern, calculated to avoid triggering financial institutions to file reports required by law ...
, or even object oriented, approach to self-modifying code, where code is created for individual quajects, like filehandles. Generating code for specific tasks allows the Synthesis kernel to (as a JIT interpreter might) apply a number of
optimization Mathematical optimization (alternatively spelled ''optimisation'') or mathematical programming is the selection of a best element, with regard to some criteria, from some set of available alternatives. It is generally divided into two subfiel ...
s such as constant folding or common subexpression elimination. The Synthesis kernel was very fast, but was written entirely in assembly. The resulting lack of portability has prevented Massalin's optimization ideas from being adopted by any production kernel. However, the structure of the techniques suggests that they could be captured by a higher level
language Language is a structured system of communication that consists of grammar and vocabulary. It is the primary means by which humans convey meaning, both in spoken and signed language, signed forms, and may also be conveyed through writing syste ...
, albeit one more complex than existing mid-level languages. Such a language and compiler could allow development of faster operating systems and applications. Paul Haeberli and Bruce Karsh have objected to the "marginalization" of self-modifying code, and optimization in general, in favor of reduced development costs.


Interaction of cache and self-modifying code

On architectures without coupled data and instruction cache (for example, some SPARC, ARM, and MIPS cores) the cache synchronization must be explicitly performed by the modifying code (flush data cache and invalidate instruction cache for the modified memory area). In some cases short sections of self-modifying code execute more slowly on modern processors. This is because a modern processor will usually try to keep blocks of code in its cache memory. Each time the program rewrites a part of itself, the rewritten part must be loaded into the cache again, which results in a slight delay, if the modified codelet shares the same cache line with the modifying code, as is the case when the modified memory address is located within a few bytes to the one of the modifying code. The cache invalidation issue on modern processors usually means that self-modifying code would still be faster only when the modification will occur rarely, such as in the case of a state switching inside an inner loop. Most modern processors load the machine code before they execute it, which means that if an instruction that is too near the instruction pointer is modified, the processor will not notice, but instead execute the code as it was ''before'' it was modified. See prefetch input queue (PIQ). PC processors must handle self-modifying code correctly for backwards compatibility reasons but they are far from efficient at doing so.


Security issues

Because of the security implications of self-modifying code, all of the major
operating system An operating system (OS) is system software that manages computer hardware and software resources, and provides common daemon (computing), services for computer programs. Time-sharing operating systems scheduler (computing), schedule tasks for ...
s are careful to remove such vulnerabilities as they become known. The concern is typically not that programs will intentionally modify themselves, but that they could be maliciously changed by an exploit. One mechanism for preventing malicious code modification is an operating system feature called W^X (for "write xor execute"). This mechanism prohibits a program from making any page of memory both writable and executable. Some systems prevent a writable page from ever being changed to be executable, even if write permission is removed. Other systems provide a ' back door' of sorts, allowing multiple mappings of a page of memory to have different permissions. A relatively portable way to bypass W^X is to create a file with all permissions, then map the file into memory twice. On Linux, one may use an undocumented SysV shared memory flag to get executable shared memory without needing to create a file.


Advantages

* Fast paths can be established for a program's execution, reducing some otherwise repetitive
conditional branch A branch, jump or transfer is an instruction in a computer program that can cause a computer to begin executing a different instruction sequence and thus deviate from its default behavior of executing instructions in order. ''Branch'' (or ''br ...
es. * Self-modifying code can improve algorithmic efficiency.


Disadvantages

Self-modifying code is harder to read and maintain because the instructions in the source program listing are not necessarily the instructions that will be executed. Self-modification that consists of substitution of
function pointer A function pointer, also called a subroutine pointer or procedure pointer, is a pointer referencing executable code, rather than data. Dereferencing the function pointer yields the referenced function, which can be invoked and passed arguments ...
s might not be as cryptic, if it is clear that the names of functions to be called are placeholders for functions to be identified later. Self-modifying code can be rewritten as code that tests a
flag A flag is a piece of textile, fabric (most often rectangular) with distinctive colours and design. It is used as a symbol, a signalling device, or for decoration. The term ''flag'' is also used to refer to the graphic design employed, and fla ...
and branches to alternative sequences based on the outcome of the test, but self-modifying code typically runs faster. Self-modifying code conflicts with authentication of the code and may require exceptions to policies requiring that all code running on a system be signed. Modified code must be stored separately from its original form, conflicting with memory management solutions that normally discard the code in RAM and reload it from the executable file as needed. On modern processors with an
instruction pipeline In computer engineering, instruction pipelining is a technique for implementing instruction-level parallelism within a single processor. Pipelining attempts to keep every part of the processor busy with some instruction by dividing incoming Mac ...
, code that modifies itself frequently may run more slowly, if it modifies instructions that the processor has already read from memory into the pipeline. On some such processors, the only way to ensure that the modified instructions are executed correctly is to flush the pipeline and reread many instructions. Self-modifying code cannot be used at all in some environments, such as the following: * Application software running under an operating system with strict W^X security cannot execute instructions in pages it is allowed to write to—only the operating system is allowed to both write instructions to memory and later execute those instructions. * Many
Harvard architecture The Harvard architecture is a computer architecture with separate computer storage, storage and signal pathways for Machine code, instructions and data. It is often contrasted with the von Neumann architecture, where program instructions and d ...
microcontroller A microcontroller (MC, uC, or μC) or microcontroller unit (MCU) is a small computer on a single integrated circuit. A microcontroller contains one or more CPUs (processor cores) along with memory and programmable input/output peripherals. Pro ...
s cannot execute instructions in read-write memory, but only instructions in memory that it cannot write to, ROM or non-self-programmable
flash memory Flash memory is an Integrated circuit, electronic Non-volatile memory, non-volatile computer memory storage medium that can be electrically erased and reprogrammed. The two main types of flash memory, NOR flash and NAND flash, are named for t ...
. * A multithreaded application may have several threads executing the same section of self-modifying code, possibly resulting in computation errors and application failures.


See also

* Overlapping code * Polymorphic code * Polymorphic engine * Persistent data structure * AARD code * Algorithmic efficiency * Data as code * eval statement * IBM 1130 (Example) * Just-in-time compilation: This technique can often give users many of the benefits of self-modifying code (except memory size) without the disadvantages. * Dynamic dead code elimination *
Homoiconicity In computer programming, homoiconicity (from the Greek words ''homo-'' meaning "the same" and ''icon'' meaning "representation") is an informal property of some programming languages. A language is homoiconic if a program written in it can be mani ...
* PCASTL *
Quine (computing) A quine is a computer program that takes no input and produces a copy of its own source code as its only output. The standard terms for these programs in the computability theory and computer science literature are "self-replicating programs", "s ...
* Self-replication *
Reflective programming In computer science, reflective programming or reflection is the ability of a process to examine, introspect, and modify its own structure and behavior. Historical background The earliest computers were programmed in their native assembly lang ...
* Monkey patch: a modification to runtime code that does not affect a program's original source code * Extensible programming: a programming paradigm in which a programming language can modify its own syntax * Self-modifying computer virus * Self-hosting * Synthetic programming * Compiler bootstrapping * Patchable microcode


Notes


References


Further reading

* * (80 pages)


External links


Using self-modifying code under Linux




{{DEFAULTSORT:Self-Modifying Code Programming paradigms