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Syntax of source lines in assembly language

The assembler parses and assembles assembly language to produce object code.


Each line of assembly language source code has this general form:

{symbol} {instruction|directive|pseudo-instruction} {;comment}

All three sections of the source line are optional.

symbol is usually a label. In instructions and pseudo-instructions it is always a label. In some directives it is a symbol for a variable or a constant. The description of the directive makes this clear in each case.

symbol must begin in the first column. It cannot contain any white space character such as a space or a tab unless it is enclosed by bars (|).

Labels are symbolic representations of addresses. You can use labels to mark specific addresses that you want to refer to from other parts of the code. Numeric local labels are a subclass of labels that begin with a number in the range 0-99. Unlike other labels, a numeric local label can be defined many times. This makes them useful when generating labels with a macro.

Directives provide important information to the assembler that either affects the assembly process or affects the final output image.

Instructions and pseudo-instructions make up the code a processor uses to perform tasks.


Instructions, pseudo-instructions, and directives must be preceded by white space, such as a space or a tab, irrespective of whether there is a preceding label or not.

Some directives do not allow the use of a label.

A comment is the final part of a source line. The first semicolon on a line marks the beginning of a comment except where the semicolon appears inside a string literal. The end of the line is the end of the comment. A comment alone is a valid line. The assembler ignores all comments. You can use blank lines to make your code more readable.

Considerations when writing assembly language source code

You must write instruction mnemonics, pseudo-instructions, directives, and symbolic register names (except a1-a4 and v1-v8 in A32 or T32 instructions) in either all uppercase or all lowercase. You must not use mixed case. Labels and comments can be in uppercase, lowercase, or mixed case.

        AREA     A32ex, CODE, READONLY
        ; Name this block of code A32ex
        ENTRY                   ; Mark first instruction to execute
        MOV      r0, #10        ; Set up parameters
        MOV      r1, #3
        ADD      r0, r0, r1     ; r0 = r0 + r1
        MOV      r0, #0x18      ; angel_SWIreason_ReportException
        LDR      r1, =0x20026   ; ADP_Stopped_ApplicationExit
        SVC      #0x123456      ; AArch32 semihosting (formerly SWI)
        END                     ; Mark end of file

To make source files easier to read, you can split a long line of source into several lines by placing a backslash character (\) at the end of the line. The backslash must not be followed by any other characters, including spaces and tabs. The assembler treats the backslash followed by end-of-line sequence as white space. You can also use blank lines to make your code more readable.


Do not use the backslash followed by end-of-line sequence within quoted strings.

The limit on the length of lines, including any extensions using backslashes, is 4095 characters.