Reference Guide

19
2 Program Structure
An assembly language program is a sequence of statements. There are
three classes of statements:
• Instructions
• Pseudo-operations
• Directives
Instructions represent a single machine instruction in symbolic form.
Pseudo-operations cause the Assembler to initialize or reserve one or
more words of storage for data, rather than machine instructions.
Directives communicate information about the program to the
Assembler, but do not generally cause the Assembler to output any
machine instructions.
An assembly statement contains four fields:
• Label
• Opcode
• Operands
• Comments
Each of these fields is optional. However the operands field cannot
appear without an opcode field.The label field is used to associate a
symbolic address with an instruction or data location, or to define a
symbolic constant using the .EQU, .REG, or .MACRO directives. This field
is optional for all but a few statement types; if present, the label must
begin in column one of a source program line. If a label appears on a line
by itself, or with a comment only, the label is associated with the next
address within the same subspace and location counter.
When the label field begins with the pound sign (#) character, it is not
treated as a label. If # is followed by white space and an integer, the
Assembler's line number counter, used when reporting errors, is reset to
the value of the integer. Otherwise, the line beginning with # is ignored.
This feature is for the use of the C language preprocessor cpp.
The opcode field contains either a mnemonic machine instruction, a
pseudo-operation code, or the name of an Assembler directive. It must be
separated from the label field by a blank or tab. For certain machine
instructions, the opcode field can also contain completers, separated from
the instruction mnemonic by commas.