IBM Basic Assembly Language - Assembler Statement Format

Assembler Statement Format

The format of assembler language statements reflects the layout of an 80-column punched card, though successive versions have relaxed most of the restrictions.

• The optional statement label or name was one to six alphanumeric characters beginning in column 1. The first character had to be alphabetic.
• The operation code or mnemonic could begin in any column to the right of column 1, separated from the statement label by a blank.
• The operand field could begin in any column to the right of the operation code, separated from the operation code by at least one blank. Blanks were invalid in operands except in character constants. The operand field, consisting of one or more operands, was optional depending on the operation code.
• Optional comments could appear to the right of the operand field, separated by at least one blank.
• Basic Assembly Language did not allow statement continuation. Later versions of the assembler indicated continuation by the appearance of any character in column 72 of the statement being continued. Basic Assembly Language required that column 72 be blank.
• A "full-card comment" was indicated by an asterisk (*) in column 1.
• Card columns 73-80, called the identification-sequence field could be used by the programmer for any purpose, but usually contained sequence numbers for resorting a jumbled card deck.

Basic Assembly language also permitted an alternate statement format with the statement starting in column 25, allowing the assembled instruction to be punched into the same card beginning in column 1. This option was not continued in later versions of the assembler.