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 binary[nb 1] representation of a computer program that is actually read and interpreted by the computer. A program in machine code consists of a sequence of machine instructions (possibly interspersed with data).[1]
Each machine code instruction causes the CPU to perform a specific task. Examples of such tasks include:
- Load a word from memory to a CPU register
- Execute an arithmetic logic unit (ALU) operation on one or more registers or memory locations
- Jump or skip to an instruction that is not the next one
In general, each architecture family (e.g., x86, ARM) has its own instruction set architecture (ISA), and hence its own specific machine code language. There are exceptions, such as the VAX architecture, which includes optional support of the PDP-11 instruction set; the IA-64 architecture, which includes optional support of the IA-32 instruction set; and the PowerPC 615 microprocessor, which can natively process both PowerPC and x86 instruction sets.
Machine code is a strictly numerical language, and it is the lowest-level interface to the CPU intended for a programmer. Assembly language provides a direct map between the numerical machine code and a human-readable mnemonic. In assembly, numerical machine code opcodes and operands are replaced with mnemonics and labels. For example, the x86 architecture has available the 0x90 opcode; it is represented as NOP in the assembly source code. While it is possible to write programs directly in machine code, managing individual bits and calculating numerical addresses is tedious and error-prone. Therefore, programs are rarely written directly in machine code. However, an existing machine code program may be edited if the assembly source code is not available.
The majority of programs today are written in a high-level language. A high-level program may be translated into machine code by a compiler.
Instruction set
editEvery processor or processor family has its own instruction set. Machine instructions are patterns of bits[nb 2] that specify some particular action.[2] An instruction set is described by its instruction format. Some ways in which instruction formats may differ:[2]
- all instructions may have the same length or instructions may have different lengths;
- the number of instructions may be small or large;
- instructions may or may not align with the architecture's word length.
A processor's instruction set needs to execute the circuits of a computer's digital logic level. At the digital level, the program needs to control the computer's registers, bus, memory, ALU, and other hardware components.[3] To control a computer's architectural features, machine instructions are created. Examples of features that are controlled using machine instructions:
- segment registers[4]
- protected address mode[5]
- binary-coded decimal (BCD) arithmetic[6]
The criteria for instruction formats include:
- Instructions most commonly used should be shorter than instructions rarely used.[2]
- The memory transfer rate of the underlying hardware determines the flexibility of the memory fetch instructions.
- The number of bits in the address field requires special consideration.[7]
Determining the size of the address field is a choice between space and speed.[7] On some computers, the number of bits in the address field may be too small to access all of the physical memory. Also, virtual address space needs to be considered. Another constraint may be a limitation on the size of registers used to construct the address. Whereas a shorter address field allows the instructions to execute more quickly, other physical properties need to be considered when designing the instruction format.
Instructions can be separated into two types: general-purpose and special-purpose. Special-purpose instructions exploit architectural features that are unique to a computer. General-purpose instructions control architectural features common to all computers.[8]
General-purpose instructions control:
- Data movement from one place to another
- Monadic operations that have one operand to produce a result
- Dyadic operations that have two operands to produce a result
- Comparisons and conditional jumps
- Procedure calls
- Loop control
- Input/output
Assembly languages
edit
A much more human-friendly rendition of machine language, named assembly language, uses mnemonic codes to refer to machine code instructions, rather than using the instructions' numeric values directly, and uses symbolic names to refer to storage locations and sometimes registers.[9] For example, on the Zilog Z80 processor, the machine code 00000101, which causes the CPU to decrement the B general-purpose register, would be represented in assembly language as DEC B.[10]
Examples
editIBM 709x
editThe IBM 704, 709, 704x and 709x store one instruction in each instruction word; IBM numbers the bit from the left as S, 1, ..., 35. Most instructions have one of two formats:
- Generic
- S,1-11
- 12-13 Flag, ignored in some instructions
- 14-17 unused
- 18-20 Tag
- 21-35 Y
- Index register control, other than TSX
- S,1-2 Opcode
- 3-17 Decrement
- 18-20 Tag
- 21-35 Y
For all but the IBM 7094 and 7094 II, there are three index registers designated A, B and C; indexing with multiple 1 bits in the tag subtracts the logical or of the selected index registers and loading with multiple 1 bits in the tag loads all of the selected index registers. The 7094 and 7094 II have seven index registers, but when they are powered on they are in multiple tag mode, in which they use only the three of the index registers in a fashion compatible with earlier machines, and require a Leave Multiple Tag Mode (LMTM) instruction in order to access the other four index registers.
The effective address is normally Y-C(T), where C(T) is either 0 for a tag of 0, the logical or of the selected index registers in multiple tag mode or the selected index register if not in multiple tag mode. However, the effective address for index register control instructions is just Y.
A flag with both bits 1 selects indirect addressing; the indirect address word has both a tag and a Y field.
In addition to transfer (branch) instructions, these machines have skip instruction that conditionally skip one or two words, e.g., Compare Accumulator with Storage (CAS) does a three way compare and conditionally skips to NSI, NSI+1 or NSI+2, depending on the result.
MIPS
editThe MIPS architecture provides a specific example for a machine code whose instructions are always 32 bits long.[11]:?299? The general type of instruction is given by the op (operation) field, the highest 6 bits. J-type (jump) and I-type (immediate) instructions are fully specified by op. R-type (register) instructions include an additional field funct to determine the exact operation. The fields used in these types are:
6 5 5 5 5 6 bits [ op | rs | rt | rd |shamt| funct] R-type [ op | rs | rt | address/immediate] I-type [ op | target address ] J-type
rs, rt, and rd indicate register operands; shamt gives a shift amount; and the address or immediate fields contain an operand directly.[11]:?299–301?
For example, adding the registers 1 and 2 and placing the result in register 6 is encoded:[11]:?554?
[ op | rs | rt | rd |shamt| funct]
0 1 2 6 0 32 decimal
000000 00001 00010 00110 00000 100000 binary
Load a value into register 8, taken from the memory cell 68 cells after the location listed in register 3:[11]:?552?
[ op | rs | rt | address/immediate] 35 3 8 68 decimal 100011 00011 01000 00000 00001 000100 binary
Jumping to the address 1024:[11]:?552?
[ op | target address ]
2 1024 decimal
000010 00000 00000 00000 10000 000000 binary
Overlapping instructions
editOn processor architectures with variable-length instruction sets[12] (such as Intel's x86 processor family) it is, within the limits of the control-flow resynchronizing phenomenon known as the Kruskal count,[13][12][14][15][16] sometimes possible through opcode-level programming to deliberately arrange the resulting code so that two code paths share a common fragment of opcode sequences.[nb 3] These are called overlapping instructions, overlapping opcodes, overlapping code, overlapped code, instruction scission, or jump into the middle of an instruction.[17][18][19]
In the 1970s and 1980s, overlapping instructions were sometimes used to preserve memory space. One example were in the implementation of error tables in Microsoft's Altair BASIC, where interleaved instructions mutually shared their instruction bytes.[20][12][17] The technique is rarely used today, but might still be necessary to resort to in areas where extreme optimization for size is necessary on byte-level such as in the implementation of boot loaders which have to fit into boot sectors.[nb 4]
It is also sometimes used as a code obfuscation technique as a measure against disassembly and tampering.[12][15]
The principle is also used in shared code sequences of fat binaries which must run on multiple instruction-set-incompatible processor platforms.[nb 3]
This property is also used to find unintended instructions called gadgets in existing code repositories and is used in return-oriented programming as alternative to code injection for exploits such as return-to-libc attacks.[21][12]
Relationship to microcode
editIn some computers, the machine code of the architecture is implemented by an even more fundamental underlying layer called microcode, providing a common machine language interface across a line or family of different models of computer with widely different underlying dataflows. This is done to facilitate porting of machine language programs between different models.[22] An example of this use is the IBM System/360 family of computers and their successors.[23]
Relationship to bytecode
editMachine code is generally different from bytecode (also known as p-code), which is either executed by an interpreter or itself compiled into machine code for faster (direct) execution. An exception is when a processor is designed to use a particular bytecode directly as its machine code, such as is the case with Java processors.
Machine code and assembly code are sometimes called native code when referring to platform-dependent parts of language features or libraries.[24]
Storing in memory
editFrom the point of view of the CPU, machine code is stored in RAM, but is typically also kept in a set of caches for performance reasons. There may be different caches for instructions and data, depending on the architecture.[25]
In some systems, from the point of view of a process, the machine code lives in code space, a designated part of its address space. In a multi-threading environment, different threads of one process share code space along with data space, which reduces the overhead of context switching considerably as compared to process switching.[26]
The CPU knows what machine code to execute, based on its internal program counter.[27] The program counter points to a memory address and is changed based on special instructions which may cause programmatic branches.[28] The program counter is typically set to a hard coded value when the CPU is first powered on, and will hence execute whatever machine code happens to be at this address.[29]
Similarly, the program counter can be set to execute whatever machine code is at some arbitrary address, even if this is not valid machine code.[28] This will typically trigger an architecture specific protection fault.[30]
In a paging based system, the CPU is oftentimes told if the current page actually holds machine code by an execute bit — pages have multiple such permission bits (readable, writable, etc.) for various housekeeping functionality.[30] E.g. on Unix-like systems memory pages can be toggled to be executable with the mprotect() system call,[31] and on Windows, VirtualProtect() can be used to achieve a similar result.[32] If an attempt is made to execute machine code on a non-executable page, an architecture specific fault will typically occur.[30]
Similarly, in a segment based system, segment descriptors can indicate whether a segment can contain executable code and in what rings that code can run.[33]
In systems that lack this kind of protection, treating data as machine code, or finding new ways to use existing machine code, by various techniques, is the basis of some software exploits. For example, in Super Mario Bros. 3, manipulating the game's memory to resemble a jump instruction then forcing the game to execute that memory as code allows the player to win the game instantly.[34]
Readability by humans
editMachine code can be seen as a set of electrical pulses that make the instructions readable to the computer; it is not readable by humans,[35] with Douglas Hofstadter comparing it to examining the atoms of a DNA molecule.[36] However, various tools and methods exist to decode machine code to human-readable source code. One such method is disassembly, which easily decodes it back to its corresponding assembly language source code because assembly language forms a one-to-one mapping to machine code.[37]
Machine code may also be decoded to high-level language under two conditions. The first condition is to accept an obfuscated reading of the source code. An obfuscated version of source code is displayed if the machine code is sent to a decompiler of the source language. The second condition requires the machine code to have information about the source code encoded within. The information includes a symbol table that contains debug symbols. The symbol table may be stored within the executable, or it may exist in separate files. A debugger can then read the symbol table to help the programmer interactively debug the machine code in execution.
- The SHARE Operating System (1959) for the IBM 709, IBM 7090, and IBM 7094 computers allowed for an loadable code format named SQUOZE. SQUOZE was a compressed binary form of assembly language code and included a symbol table.
- Modern IBM mainframe operating systems, such as z/OS, have available a symbol table named Associated data (ADATA). The table is stored in a file that can be produced by the IBM High-Level Assembler (HLASM),[38][39] IBM's COBOL compiler,[40] and IBM's PL/I compiler,[41] either as a separate SYSADATA file or as ADATA records in a Generalized object output file (GOFF).[42] This obsoletes the TEST records from OS/360, although it is still possible to request them and to use them in the TSO TEST command.
- Microsoft Windows has available a symbol table[43] that is stored in a program database (
.pdb) file.[44] - Most Unix-like operating systems have available symbol table formats named stabs and DWARF. In macOS and other Darwin-based operating systems, the debug symbols are stored in DWARF format in a separate
.dSYMfile.
See also
edit
Notes
edit- ^ On nonbinary machines it is, e.g., a decimal representation.
- ^ On early decimal machines, patterns of characters, digits and digit sign
- ^ a b While overlapping instructions on processor architectures with variable-length instruction sets can sometimes be arranged to merge different code paths back into one through control-flow resynchronization, overlapping code for different processor architectures can sometimes also be crafted to cause execution paths to branch into different directions depending on the underlying processor, as is sometimes used in fat binaries.
- ^ For example, the DR-DOS master boot records (MBRs) and boot sectors (which also hold the partition table and BIOS Parameter Block, leaving less than 446 respectively 423 bytes for the code) were traditionally able to locate the boot file in the FAT12 or FAT16 file system by themselves and load it into memory as a whole, in contrast to their counterparts in MS-DOS and PC DOS, which instead rely on the system files to occupy the first two directory entry locations in the file system and the first three sectors of IBMBIO.COM to be stored at the start of the data area in contiguous sectors containing a secondary loader to load the remainder of the file into memory (requiring SYS to take care of all these conditions). When FAT32 and logical block addressing (LBA) support was added, Microsoft even switched to require i386 instructions and split the boot code over two sectors for code size reasons, which was no option to follow for DR-DOS as it would have broken backward- and cross-compatibility with other operating systems in multi-boot and chain load scenarios, and as with older IBM PC–compatible PCs. Instead, the DR-DOS 7.07 boot sectors resorted to self-modifying code, opcode-level programming in machine language, controlled utilization of (documented) side effects, multi-level data/code overlapping and algorithmic folding techniques to still fit everything into a physical sector of only 512 bytes without giving up any of their extended functions.
References
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- ^ Tanenbaum 1990, p. 162
- ^ Tanenbaum 1990, p. 231
- ^ Tanenbaum 1990, p. 237
- ^ Tanenbaum 1990, p. 236
- ^ a b Tanenbaum 1990, p. 253
- ^ Tanenbaum 1990, p. 283
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- ^ a b c d e Jacob, Matthias; Jakubowski, Mariusz H.; Venkatesan, Ramarathnam [at Wikidata] (20–21 September 2007). Towards Integral Binary Execution: Implementing Oblivious Hashing Using Overlapped Instruction Encodings (PDF). Proceedings of the 9th workshop on Multimedia & Security (MM&Sec '07). Dallas, Texas, US: Association for Computing Machinery. pp. 129–140. CiteSeerX 10.1.1.69.5258. doi:10.1145/1288869.1288887. ISBN 978-1-59593-857-2. S2CID 14174680. Archived (PDF) from the original on 2025-08-07. Retrieved 2025-08-07. (12 pages)
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- ^ "Associated Data Architecture". High Level Assembler and Toolkit Feature.
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- ^ "Querying the .Pdb File". Microsoft Learn. 2025-08-07.
Sources
edit- Hofstadter, Douglas R. (1979). G?del, Escher, Bach: An Eternal Golden Braid. Basic Books. ISBN 0-465-02685-0. Retrieved 2025-08-07.
- Samuelson, Pamela (1984). "CONTU Revisited: The Case Against Copyright Protection for Computer Programs in Machine-Readable Form". Duke Law Journal. 33 (4): 663–769. doi:10.2307/1372418. hdl:hein.journals/duklr1984. JSTOR 1372418. Retrieved 2025-08-07.
- Tanenbaum, Andrew S. (1990). Structured Computer Organization, Third Edition. Prentice Hall. p. 398. ISBN 978-0-13-854662-5.
Further reading
edit- Hennessy, John L.; Patterson, David A. (1994). Computer Organization and Design. The Hardware/Software Interface. Morgan Kaufmann Publishers. ISBN 1-55860-281-X.
- Tanenbaum, Andrew S. (1999). Structured Computer Organization (Fourth ed.). Prentice Hall. ISBN 0-13-020435-8. Retrieved 2025-08-07.
- Brookshear, J. Glenn (2007). Computer Science: An Overview (Ninth ed.). Addison Wesley. ISBN 978-0-321-38701-1. Retrieved 2025-08-07.