Editing Assembly language (section)

==Assembly language syntax==
Assembly language uses a [[mnemonic]] to represent, e.g., each low-level [[machine code|machine instruction]] or [[opcode]], each [[directive (programming)|directive]], typically also each [[register (computing)#ARCHITECTURAL|architectural register]], [[bit field|flag]], etc. Some of the mnemonics may be built-in and some user-defined. Many operations require one or more [[Operand#Computer science|operands]] in order to form a complete instruction. Most assemblers permit named constants, registers, and [[Label (computer science)|labels]] for program and memory locations, and can calculate [[Expression (computer science)|expressions]] for operands. Thus, programmers are freed from tedious repetitive calculations and assembler programs are much more readable than machine code. Depending on the architecture, these elements may also be combined for specific instructions or [[addressing mode]]s using [[offset (computer science)|offset]]s or other data as well as fixed addresses. Many assemblers offer additional mechanisms to facilitate program development, to control the assembly process, and to aid [[debugging]].

Some are column oriented, with specific fields in specific columns; this was very common for machines using [[punched card]]s in the 1950s and early 1960s. Some assemblers have free-form syntax, with fields separated by delimiters, e.g., punctuation, [[Whitespace character|white space]]. Some assemblers are hybrid, with, e.g., labels, in a specific column and other fields separated by delimiters; this became more common than column-oriented syntax in the 1960s.