Assembly language allows programmers to develop programs that interact with specific hardware components. It provides direct access to CPU and memory for systems programming or low-level software development projects. Programmers write mnemonic instructions that an assembler program translates into machine code. Each processor family and instruction set architecture has different syntax for its assembler program to produce machine code output.
Basic Instructions
Assembler language provides a middle ground between machine code and higher-level programming languages. It enables programmers to write human-readable code that resembles the machine instructions executed by processors, while giving direct control over hardware components like CPU, memory and I/O devices - and can even be optimized for maximum performance.
Assemblers convert combinations of mnemonic codes representing basic machine commands and operations into binary form that a CPU recognizes. An assembler program assigns numeric addresses for labels as well as resolve symbolic names for memory locations, constant expressions, and GOTO destinations in its code.
Assembly code assembly takes on different forms depending on the processor and instruction set architecture of its target platform. An Intel x86 processor may use Intel Hexadecimal Notation while AT&T systems utilize AT&T Hexadecimal Syntax; there are also one-pass and multi-pass assemblers, each capable of taking multiple passes over source code to generate an object file.
Variables
Assembly language variables are memory locations used to store data. Variables may store any type of information (byte, word, int, float or long). Variables must first be initialized prior to use and declared in the bss section of your source code as assembler variables must have unique names within the program. Each assembly statement starts off with a label field, followed by an opcode field that contains one of the mnemonic instructions, and operand fields with all the values required by that instruction.
Many assemblers support macroinstructions, which enable you to specify a sequence of commands and have them translated into individual instructions by the assembler. This feature can reduce GOTO statements in your code which can lead to spaghetti code; alternatively you could also use macros for specifying IF/ELSE and ENDIF control flow blocks in your program.
Loops
Assembly language is used for writing programs that run on low-level hardware components, such as a computer processor. Although more error-prone and time consuming than higher-level languages, assembly language can be useful when performing certain tasks which require direct control over a computer's architecture or access to specific processor instructions.
Programmers utilize an assembler program to convert assembly code to machine language. An assembler recognizes combinations of mnemonics and syntax for operations and addressing modes, translating them to their numerical equivalents. Furthermore, an assembler resolves symbolic names of data structures such as registers or entities so as to allow programmers to avoid manual address calculations.
Some assemblers are single-pass, analyzing and producing code in one pass; others employ multiple passes in order to produce an object file. The number of passes an assembler needs in order to analyze and produce final code depends on how complex its symbols are; these could include literal values, constant expressions or relocatable addresses - with some assembly languages even supporting short macros like other programming languages.
Functions
Commands are at the core of every assembly program, instructing an assembler on what action should be taken on particular lines of code. They may consist of machine instruction opcode mnemonics or directives which give guidance for other fields within code. An opcode is the symbolic name of a machine instruction, followed by one or more operands which depend on its underlying processor architecture - these could include immediate values, registers specified in the opcode or implied registers or addresses pointed to by values in registers or direct addresses (also called immediate addresses).
An assembler program generates object code by translating combinations of opcode mnemonics, syntax and addressing modes into their numerical equivalents. Additionally, this type of compiler calculates constant expressions and resolves symbolic names for memory locations or other entities. Assemblers may be single or multi-pass, with different modes tailored specifically towards different CPU and instruction set architectures.