Machine instruction

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 nonbinary machines it is, e.g., a decimal representation. 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).

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 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

Every processor or processor family has its own instruction set. Machine instructions are patterns of bitsOn early decimal machines, patterns of characters, digits and digit sign that specify some particular action. An instruction set is described by its instruction format. Some ways in which instruction formats may differ:
* 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. To control a computer's architectural features, machine instructions are created. Examples of features that are controlled using machine instructions:
* segment registers
* protected address mode
* binary-coded decimal (BCD) arithmetic

The criteria for instruction formats include:
* Instructions most commonly used should be shorter than instructions rarely used.
* 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.

Determining the size of the address field is a choice between space and speed. 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.

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

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. 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.

Examples

IBM 709x

The 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

The MIPS architecture provides a specific example for a machine code whose instructions are always 32 bits long. 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
rs , rt , rd , shamt, funct R-type
rs , rt , address/immediate I-type
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.

For example, adding the registers 1 and 2 and placing the result in register 6 is encoded:

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:

rs , rt , address/immediate 35 3 8 68 decimal
100011 00011 01000 00000 00001 000100 binary

Jumping to the address 1024:

target address 2 1024 decimal
000010 00000 00000 00000 10000 000000 binary

Overlapping instructions

On processor architectures with variable-length instruction sets (such as Intel's x86 processor family) it is, within the limits of the control-flow resynchronizing phenomenon known as the Kruskal count, sometimes possible through opcode-level programming to deliberately arrange the resulting code so that two code paths share a common fragment of opcode sequences. These are called ''overlapping instructions'', ''overlapping opcodes'', ''overlapping code'', ''overlapped code'', ''instruction scission'', or ''jump into the middle of an instruction''.

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. 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.

It is also sometimes used as a code obfuscation technique as a measure against disassembly and tampering.

The principle is also used in shared code sequences of fat binaries which must run on multiple instruction-set-incompatible processor platforms.

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.

Relationship to microcode

In 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. An example of this use is the IBM System/360 family of computers and their successors.

Relationship to bytecode

Machine 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.

Storing in memory

From 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.

The CPU knows what machine code to execute, based on its internal program counter. The program counter points to a memory address and is changed based on special instructions which may cause programmatic branches. 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.

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. This will typically trigger an architecture specific protection fault.

The CPU is oftentimes told, by page permissions in a paging based system, 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. E.g. on Unix-like systems memory pages can be toggled to be executable with the system call, and on Windows, can be used to achieve a similar result. If an attempt is made to execute machine code on a non-executable page, an architecture specific fault will typically occur. Treating data as machine code, or finding new ways to use existing machine code, by various techniques, is the basis of some security vulnerabilities.

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.

From the point of view of a process, the ''code space'' is the part of its address space where the code in execution is stored. In multitasking systems this comprises the program's code segment and usually shared libraries. In 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.

Readability by humans

Machine code can be seen as a set of electrical pulses that make the instructions readable to the computer; it is not readable by humans, with Douglas Hofstadter comparing it to examining the atoms of a DNA molecule. 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.

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), IBM's COBOL compiler, and IBM's PL/I compiler, either as a separate SYSADATA file or as ADATA records in a Generalized object output file (GOFF).
* Microsoft Windows has available a symbol table that is stored in a program database (.pdb) file.
* 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 .dSYM file.

See also

* Assembly language
* Endianness
* List of machine languages
* Machine code monitor
* Overhead code
* P-code machine
* Reduced instruction set computer (RISC)
* Very long instruction word
* Teaching Machine Code: Micro-Professor MPF-I

Notes

References

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Low-level programming languages