HLASM Language Reference

Data Attributes
Defined Attribute (D')
The defined attribute shows whether or not the ordinary symbol or literal referenced
has been defined prior to the attribute reference. A symbol is defined if it has been
encountered in the operand field of an EXTRN or WXTRN statement, or in the
name field of any other statement except a TITLE statement or a macro instruction.
A literal is defined if it has been encountered in the operand field of a machine
instruction. The value of the defined attribute is an arithmetic value that can be
assigned to a SETA symbol, and is equal to 1 if the symbol has been defined, or 0
if the symbol has not been defined.
The defined attribute can reference:
Ordinary symbols not constructed by substitution
Macro instruction operands
SETC symbols whose value is an ordinary symbol
System variable symbols whose value is an ordinary symbol
Literals
The following is an example of how you can use the defined attribute:
Name Operation Operand
AIF
(D'A).AROUND
A LA 1,4
.AROUND ANOP
In this example, assuming there has been no previous definition of the symbol A,
the statement labeled A would be assembled, since the conditional-assembly
branch around it would not be taken. However, if by an AGO or AIF
conditional-assembly branch the same statement were processed again, the
statement at A would not be assembled:
Name Operation Operand
.UP AIF (D'A).AROUND
A LA 1,4
.AROUND ANOP
.
.
AGO
.UP
You can save assembly time using the defined attribute which avoids lookahead
mode (see “Lookahead” on page 340 for more information. You can use the
defined attribute in your program to prevent the assembler from making this
time-consuming forward scan. This attribute reference can be used in the operand
field of a SETA instruction or as one of the values in the operand field of a SETB or
AIF instruction.
Operation Code Attribute (O')
The operation code attribute shows whether a given operation code has been
defined prior to the attribute reference. The operation code can be represented by
a character string or by a variable symbol containing a character string. The
variable must be set using a SETC assembler instruction prior to being referenced
by the operation code (O') attribute.
Chapter 9. How to Write Conditional Assembly Instructions 337