HLASM Language Reference

Lookahead
lookahead are saved on an internal file, and are fully processed when the
lookahead scan ends.
If a COPY instruction is encountered during lookahead, it is fully processed at that
time, the assembler copies the statements from the library, scans them, and saves
them on the lookahead file. When lookahead mode has ended any COPY
instructions saved to the lookahead file are ignored, as the statements from the
copy member have already been read and saved to the lookahead file.
If a variable symbol is used for the member name of a COPY that is expanded
during lookahead, the value of the variable symbol at the time the COPY is
expanded is used.
For purposes of attribute definition, a symbol is considered partially defined if it
depends in any way upon a symbol not yet defined. For example, if the symbol is
defined by a forward EQU that is not yet resolved, that symbol is assigned a type
attribute of U.
In this case, it is quite possible that, by the end of the assembly, the type attribute
has changed to some other value.
Generating END statements: Because no variable symbol substitution is carried
out during lookahead, you should consider the following effects of using macro,
AINSERT or open code substitution to generate END statements that separate
source modules assembled in one job step (BATCH assembler option). If a symbol
is undefined within a module, lookahead might read statements past the point
where the END statement is to be generated. Lookahead stops when:
1. It finds the symbol
2. It finds an END statement
3. It reaches the end of the source input data set
In the first two cases, the assembler begins the next module at the statement after
lookahead stopped, which could be after the point where you wanted to generate
the END statement.
Lookahead Restrictions
The assembler analyzes the statements it processes during lookahead, only to
establish attributes of symbols in their name fields.
Variable symbols are not replaced. Modifier expressions are evaluated only if all
symbols involved were defined prior to lookahead. Possible multiple or inconsistent
definition of the same symbol is not diagnosed during lookahead because
conditional assembly may eliminate one (or more) of the definitions.
Lookahead does not check undefined operation codes against library macro names.
If the name field contains an ordinary symbol and the operation code cannot be
matched with one in the current operation code table, then the ordinary symbol is
assigned the type attribute of M. If the operation code contains special characters
or is a variable symbol, a type attribute of U is assumed. This may be wrong if the
undefined operation code is later substituted with a known operation code or is later
defined by OPSYN. OPSYN statements are not processed; thus, labels are treated
in accordance with the operation code definitions in effect at the time of entry to
lookahead.
Chapter 9. How to Write Conditional Assembly Instructions 341