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Concept
Machine code
Summary
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). Although decimal computers were once common, the contemporary marketplace is dominated by binary computers; for those computers, machine code is "the binary representation of a computer program which 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 instruction causes the CPU to perform a very specific task, such as a load, a store, a jump, or an arithmetic logic unit (ALU) operation on one or more units of data in the CPU's registers or memory.
Early CPUs had specific machine code that might break backwards compatibility with each new CPU released. The notion of an instruction set architecture (ISA) defines and specifies the behavior and encoding in memory of the instruction set of the system, without specifying its exact implementation. This acts as an abstraction layer, enabling compatibility within the same family of CPUs, so that machine code written or generated according to the ISA for the family will run on all CPUs in the family, including future CPUs.
In general, each architecture family (e.g. x86, ARM) has its own ISA, and hence its own specific machine code language. There are exceptions, e.g. the IA-64 can emulate x86.
Machine code is a strictly numerical language, and is the lowest-level interface to the CPU intended for a programmer. Assembly language provides a direct mapping between the numerical machine code and a human-readable version where numerical opcodes and operands are replaced by readable strings (e.g. 0x90 is the NOP instruction on x86). While it is possible to write programs directly in machine code, managing individual bits and calculating numerical addresses and constants manually is tedious and error-prone.
Official source
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