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Concept
X86 assembly language
Summary
x86 assembly language is the name for the family of assembly languages which provide some level of backward compatibility with CPUs back to the Intel 8008 microprocessor, which was launched in April 1972. It is used to produce object code for the x86 class of processors.
Regarded as a programming language, assembly is machine-specific and low-level. Like all assembly languages, x86 assembly uses mnemonics to represent fundamental CPU instructions, or machine code. Assembly languages are most often used for detailed and time-critical applications such as small real-time embedded systems, operating-system kernels, and device drivers, but can also be used for other applications. A compiler will sometimes produce assembly code as an intermediate step when translating a high-level program into machine code.
Reserved keywords of x86 assembly language
Each x86 assembly instruction is represented by a mnemonic which, often combined with one or more operands, translates to one or more bytes called an opcode; the NOP instruction translates to 0x90, for instance, and the HLT instruction translates to 0xF4. There are potential opcodes with no documented mnemonic which different processors may interpret differently, making a program using them behave inconsistently or even generate an exception on some processors. These opcodes often turn up in code writing competitions as a way to make the code smaller, faster, more elegant or just show off the author's prowess.
x86 assembly language has two main syntax branches: Intel syntax and AT&T syntax. Intel syntax is dominant in the DOS and Windows world, and AT&T syntax is dominant in the Unix world, since Unix was created at AT&T Bell Labs.
Here is a summary of the main differences between Intel syntax and AT&T syntax:
Many x86 assemblers use Intel syntax, including FASM, MASM, NASM, TASM, and YASM. GAS, which originally used AT&T syntax, has supported both syntaxes since version 2.10 via the .intel_syntax directive. A quirk in the AT&T syntax for x86 is that x87 operands are reversed, an inherited bug from the original AT&T assembler.
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