CSC233 Assembly Language - Formatting Assembler Programs

Formatting Assembler Programs

Assembler Language Statements

All assembler statements have the same form. Each statement contains an opcode and operands. Some statements also contain a label and some statements also contain a comment. Some statements with just opcode and operand are:

         B     LOOP
         PACK  PNO(3),ZDNO
         MVC   MSG(5),=C'ERROR'

Some statements which also contain a label are:

BLANK    DC    CL133' '
LOOP     GET   FILE1,RECORD
NEXTTEST CLC   WEIGHT,LOW

Some statements which also contain a comment are:

         PUT   FILE2,TITLE          WRITE TITLE
         LA    R4,1(R4)             POINT TO NEXT LETTER INPUT
RECORD   DC    C' '                 RECORD FOR INPUT AND OUTPUT

You may see that the parts of a statement are determined by position. The first part is the (optional) label. The second part is the opcode. The third part is the operand. The fourth part is the (optional) comment. Each of these parts is separated by at least one blank. The label, opcode, and operand parts cannot contain any blanks. It is almost impossible to read an assembler program if the statement parts don't line up, so we use extra blanks between the parts to make the parts of all statements line up, as in the examples above.

A comment statement in an assembler program is created by placing an asterisk (*) in position 1.

Formatting Assembler Programs

There are very strict rules for formatting assembler programs. In C++ we have a lot of flexibility in how we format our code. We use indentation to make the program more readable, but the indentation is optional. We can continue statements on multiple lines, use blanks within statements or leave them out, and leave blank lines in our code. In assembler, we must be very careful about exactly how we format our code. Since blanks are used to separate the parts of a statement, we cannot have any blanks within a part. There are other rules (stated below) which must be followed exactly. Failure to follow the formatting rules will result in errors during assembly or execution.

Do not leave any blank lines in your code.
All code should be typed in uppercase. The easiest way to do this is to leave CAPS LOCK on.
Labels must start in position 1.
Start all opcodes in position 10.
Start all operands in position 16.
Comments do not need to start in a specific position. For each program, choose the same position for all comments so they line up evenly.
Do not leave any blanks within the operand part.
The rules for creating names for labels and operands are:
1. Names can be from 1 to 8 characters in length.
2. The first character must be a letter or @, #, or $.
3. The remaining characters must be letters, digits, @, #, or $. Notice that blanks are not allowed within a name.
If a statement is continued on a second line, place an X in position 72 on the first line, and start the new line in position 16. If one statement is continued on more than two lines, place an X in position 72 on all but the last line of the statement.
Do not continue any statement beyond position 72. Even blanks beyond position 72 will cause a problem. Only use position 72 when a statement is being continued on the next line.